HK1196734B - Method and device for encoding and decoding video data - Google Patents

Description

Video data coding and decoding method and device

This application is claimed in united states provisional application No. 61/556,029, filed on 2011, 11/4, the entire contents of which are incorporated herein by reference.

Technical Field

The present disclosure relates to video coding, and more particularly, to methods and apparatus for encoding and decoding video data.

Background

Digital video capabilities can be incorporated into a wide range of devices, including digital televisions, digital direct broadcast systems, wireless broadcast systems, Personal Digital Assistants (PDAs), laptop or desktop computers, tablet computers, e-book readers, digital cameras, digital recording devices, digital media players, video gaming devices, video gaming consoles, cellular or satellite radio telephones, so-called "smart phones," video teleconferencing devices, video streaming devices, and the like. Digital video devices implement video compression techniques such as those described in the standards defined by MPEG-2, MPEG-4, ITU-T H.263, ITU-T H.264/MPEG-4 part 10 (advanced video coding (AVC)), the High Efficiency Video Coding (HEVC) standard currently under development, and extensions of these standards. Video devices may transmit, receive, encode, decode, and/or store digital video information more efficiently by implementing these video compression techniques.

Video compression techniques perform spatial (intra-picture) prediction and/or temporal (inter-picture) prediction to reduce or remove redundancy inherent in video sequences. For block-based video coding, a video slice (e.g., a video picture or a portion of a video picture) may be partitioned into video blocks, which may also be referred to as treeblocks, Coding Units (CUs), and/or coding nodes. Video blocks in an intra-coded (I) slice of a picture are encoded using spatial prediction with respect to reference samples in neighboring blocks in the same picture. Video blocks in inter-coded (P or B) slices of a picture may use spatial prediction with respect to reference samples in neighboring blocks in the same picture or temporal prediction with respect to reference samples in other reference pictures. A picture may be referred to as a frame and a reference picture may be referred to as a reference frame.

Spatial or temporal prediction generates a predictive block for the block to be coded. The residual data represents pixel differences between the original block to be coded and the predictive block. An inter-coded block is encoded according to a motion vector that points to a block of reference samples that forms a predictive block and residual data that indicates a difference between the coded block and the predictive block. The intra-coded block is encoded according to an intra-coding mode and the residual data. For further compression, the residual data may be transformed from the pixel domain to a transform domain, producing residual transform coefficients, which may then be quantized. Quantized transform coefficients, initially arranged in a two-dimensional array, may be scanned in order to generate a one-dimensional vector of transform coefficients, and entropy coding may be applied to achieve even more compression.

Disclosure of Invention

In general, this disclosure relates to techniques for video coding. The techniques of this disclosure generally relate to encoding and decoding video data. In some examples, the techniques involve ordering of most probable intra prediction modes (MPMs). That is, certain aspects of this disclosure relate to avoiding ordering of MPMs, which may reduce video coder complexity. Other aspects of the invention relate to default MPMs, orders for determining MPMs, and other concepts related to MPMs.

In one example, this disclosure describes a method of encoding video data. The method comprises the following steps: determining an intra-mode for predicting a current block of video data; determining a Most Probable Mode (MPM) for predicting the current block of video data; determining an index for each of the MPMs based on an order in which the intra-mode for predicting the current block is compared to the MPMs; and signaling the index of the matching MPM when one of the MPMs used to predict the current block matches the intra-mode used to predict the current block.

In another example, this disclosure describes an apparatus for encoding video data, the apparatus comprising one or more processors configured to: determining an intra-mode for predicting a current block of video data; determining an MPM for predicting the current block of video data; determining an index for each of the MPMs based on an order in which the intra-mode for predicting the current block is compared to the MPMs; and signaling the index of the matching MPM when one of the MPMs used to predict the current block matches the intra-mode used to predict the current block.

In another example, this disclosure describes a computer-readable storage medium. The computer-readable storage medium stores instructions that, when executed, cause one or more processors of a device to: determining an intra-mode for predicting a current block of video data; determining an MPM for predicting the current block of video data; determining an index for each of the MPMs based on an order in which the intra-mode for predicting the current block is compared to the MPMs; and signaling the index of the matching MPM when one of the MPMs used to predict the current block matches the intra-mode used to predict the current block.

In another example, this disclosure describes an apparatus for encoding video data, the apparatus comprising: means for determining an intra-mode for predicting a current block of video data; means for determining an MPM for predicting the current block of video data; means for determining an index for each of the MPMs based on an order in which the intra-mode for predicting the current block is compared to the MPMs; and means for signaling the index of the matching MPM when one of the MPMs used to predict the current block matches the intra-mode used to predict the current block.

In another example, this disclosure describes a method of decoding video data, the method comprising: generating a list of MPMs for a current block of video data when intra-mode for the current block comprises MPMs, wherein the list of MPMs is arranged in an order that compares the intra-mode for the current block of video data to one or more intra-modes associated with one or more reference blocks of video data; determining an MPM index that identifies the intra-mode for the current block in the list of MPMs; identifying the intra-mode for the current block using the MPM index; and decoding the current block at the identified intra-mode for the current block.

In another example, this disclosure describes an apparatus comprising one or more processors configured to: generating a list of MPMs for a current block of video data when intra-mode for the current block comprises MPMs, wherein the list of MPMs is arranged in an order that compares the intra-mode for the current block of video data to one or more intra-modes associated with one or more reference blocks of video data; determining an MPM index that identifies the intra-mode for the current block in the list of MPMs; identifying the intra-mode for the current block using the MPM index; and decoding the current block at the identified intra-mode for the current block.

In another example, this disclosure describes a computer-readable storage medium. The computer-readable storage medium stores instructions that, when executed, cause one or more processors of a device to: generating a list of MPMs for a current block of video data when intra-mode for the current block comprises MPMs, wherein the list of MPMs is arranged in an order that compares the intra-mode for the current block of video data to one or more intra-modes associated with one or more reference blocks of video data; determining an MPM index that identifies the intra-mode for the current block in the list of MPMs; identifying the intra-mode for the current block using the MPM index; and decoding the current block at the identified intra-mode for the current block.

In another example, this disclosure describes an apparatus for decoding video data, the apparatus comprising: means for generating a list of MPMs for a current block of video data when intra-mode for the current block comprises MPMs, wherein the list of MPMs is arranged in an order that compares the intra-mode for the current block of video data to one or more intra-modes associated with one or more reference blocks of video data; means for determining an MPM index that identifies the intra-mode for the current block in the list of MPMs; means for identifying the intra-mode for the current block using the MPM index; and means for decoding the current block at the identified intra-mode for the current block.

The details of one or more examples of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages will be apparent from the description and drawings, and from the claims.

Drawings

FIG. 1 is a block diagram illustrating an example video encoding and decoding system that may utilize the techniques described in this disclosure.

FIG. 2 is a block diagram illustrating an example video encoder that may implement the techniques described in this disclosure.

FIG. 3 is a block diagram illustrating an example video decoder that may implement the techniques described in this disclosure.

Fig. 4 is a block diagram illustrating blocks of video data that may be considered during intra-mode prediction.

Fig. 5 is a conceptual diagram illustrating intra mode prediction.

Fig. 6 is another conceptual diagram illustrating intra mode prediction.

Fig. 7 is a block diagram illustrating most probable intra mode candidates according to aspects of this disclosure.

Fig. 8 is a flow diagram illustrating an example method of encoding video data in accordance with one or more examples described in this disclosure.

Fig. 9 is a flow diagram illustrating an example method of decoding video data in accordance with one or more examples described in this disclosure.

Fig. 10 is a flow diagram illustrating an example method of coding video data in accordance with one or more examples described in this disclosure.

Detailed Description

In one example, aspects of this disclosure are directed to ordering of most probable intra prediction modes (MPMs). For example, according to some video coding techniques, a video coder (e.g., a video encoder or a video decoder) may order MPMs of a block currently being coded prior to determining and signaling the MPMs. Aspects of this disclosure relate to removing this ordering, which may reduce video coder complexity.

In one example, a video encoder may generate a list of MPMs from a set of MPMs in an order in which the MPMs occur in a picture or slice of video data (e.g., a coding order), the list including intra-prediction modes. In another example, the video encoder may generate the list of MPMs in an order in which the video encoder checks whether the intra-modes of neighboring blocks are the same as the block currently being coded (referred to herein as "check order"). The video encoder may signal the MPMs according to the indices of the generated list and without ordering or reordering the MPMs in the list. The video decoder may perform the same process to generate a list of MPMs, obtain indices for the list from the encoded bitstream, and select an MPM from the list according to the indices without ordering or reordering the MPMs in the list.

In one example, for purposes of illustration, a video coder may first check whether the intra-mode of a block located to the left of the block currently being coded (referred to herein as a "left-neighboring block") is the same as the intra-mode of the current block. The video coder may then check whether the intra-mode of a block positioned above the block currently being coded (referred to herein as an "above-neighboring block") is the same as the intra-mode of the current block. In this example, according to aspects of this disclosure, the intra-mode of the left-neighboring block may have an index of zero in the list of MPMs maintained by the video coder, and the intra-mode of the above-neighboring block may have an index of one in the list. Thus, the video encoder may signal that the index of the intra-mode of the left-neighboring block is zero and the index of the above-neighboring block is one, regardless of whether the actual intra-mode number (e.g., the predefined mode number as specified by the video coding standard) of the left-neighboring block is greater than the above-neighboring block. Alternatively, if the video coder checks the above-neighboring block for intra-mode before the left-neighboring block, the video coder may signal that the index of the above-neighboring block is zero and the index of the left-neighboring block is one. In any case, according to these examples and aspects of this disclosure, a video encoder may signal an index of an intra-mode without reordering or ordering the intra-modes in a list. In some examples, if the intra-mode is not one of MPMs, then ordering may be applied to intra-mode coding. That is, the video encoder may sort or otherwise modify the list of intra-modes when an intra-mode that is not an MPM is signaled. According to aspects of this disclosure, the order in which a video coder checks intra-modes of neighboring blocks (referred to herein as "checking order") may be implicitly derived for intra-modes from collected statistics of intra-modes of previously coded blocks. In other examples, the video coder may derive the checking order based on the availability of neighboring blocks. In still other examples, the video encoder may signal (and the video decoder may obtain from the encoded bitstream) an explicit indication of the checking order.

FIG. 1 is a block diagram illustrating an example video encoding and decoding system 10 that may utilize the techniques described in this disclosure. As shown in fig. 1, system 10 includes a source device 12, source device 12 generating encoded video data to be decoded by a destination device 14 at a later time. Source device 12 and destination device 14 may comprise any of a wide range of devices, including desktop computers, notebook (e.g., laptop) computers, tablet computers, set-top boxes, telephone handsets such as so-called "smart" phones, so-called "smart" touch pads, televisions, cameras, display devices, digital media players, video game consoles, video streaming devices, or the like. In some cases, source device 12 and destination device 14 may be equipped for wireless communication.

Destination device 14 may receive encoded video data to be decoded over link 16. Link 16 may comprise any type of media or device capable of moving encoded video data from source device 12 to destination device 14. In one example, link 16 may comprise a communication medium that enables source device 12 to transmit encoded video data directly to destination device 14 in real-time. The encoded video data may be modulated according to a communication standard, such as a wireless communication protocol, and transmitted to destination device 14. The communication medium may comprise any wireless or wired communication medium, such as a Radio Frequency (RF) spectrum or one or more physical transmission lines. The communication media may form part of a packet-based network, such as a local area network, a wide area network, or a global network such as the internet. The communication medium may include routers, switches, base stations, or any other equipment that may be useful for facilitating communication from source device 12 to destination device 14.

Alternatively, the encoded data may be output from output interface 22 to storage device 24. Similarly, encoded data may be accessed from storage device 24 by input interface. Storage device 24 may include any of a variety of distributed or locally accessed data storage media such as a hard drive, blu-ray discs, DVDs, CD-ROMs, flash memory, volatile or non-volatile memory, or any other suitable digital storage media for storing encoded video data. In another example, storage device 24 may correspond to a file server or another intermediate storage device that may hold the encoded video generated by source device 12. Destination device 14 may access the stored video data from storage device 24 via streaming or download. The file server may be any type of server capable of storing encoded video data and transmitting that encoded video data to destination device 14. Example file servers include a website server (e.g., for a website), a File Transfer Protocol (FTP) server, a Network Attached Storage (NAS) device, or a local disk drive. Destination device 14 may access the encoded video data over any standard data connection, including an internet connection. Such a data connection may include a wireless channel (e.g., a Wi-Fi connection), a wired connection (e.g., DSL, cable modem, etc.), or a combination of both, suitable for accessing encoded video data stored on a file server. The transmission of the encoded video data from storage device 24 may be a streaming transmission, a download transmission, or a combination of both.

The techniques of this disclosure are not necessarily limited to wireless applications or settings. The techniques may be applied to video coding to support any of a variety of multimedia applications, such as over-the-air television broadcasts, cable television transmissions, satellite television transmissions, streaming video transmissions (e.g., via the internet), encoding digital video for storage on a data storage medium, decoding digital video stored on a data storage medium, or other applications. In some examples, system 10 may be configured to support one-way or two-way video transmission to support applications such as video streaming, video playback, video broadcasting, and/or video telephony.

In the example of fig. 1, source device 12 includes a video source 18, a video encoder 20, and an output interface 22. In some cases, output interface 22 may include a modulator/demodulator (modem) and/or a transmitter. In source device 12, video source 18 may include sources such as: a video capture device (e.g., a video camera), a video archive containing previously captured video, a video feed interface to receive video from a video content provider, and/or a computer graphics system for generating computer graphics data as source video, or a combination of these sources. As one example, if video source 18 is a video camera, source device 12 and destination device 14 may form so-called camera phones or video phones. However, the techniques described in this disclosure may be generally applicable to video coding and may be applicable to wireless and/or wired applications.

Captured, pre-captured, or computer-generated video may be encoded by video encoder 20. Encoded video data may be transmitted directly to destination device 14 via output interface 22 of source device 12. The encoded video data may also (or alternatively) be stored onto storage device 24 for later access by destination device 14 or other devices for decoding and/or playback.

Destination device 14 includes input interface 28, video decoder 30, and display device 32. In some cases, input interface 28 may include a receiver and/or a modem. Input interface 28 of destination device 14 receives the encoded video data over link 16. Encoded video data communicated over link 16 or provided on storage device 24 may include a variety of syntax elements generated by video encoder 20 for use by a video decoder, such as video decoder 30, to decode the video data. These syntax elements may be included with the encoded video data transmitted on a communication medium, stored on a storage medium, or stored on a file server.

The display device 32 may be integrated with the destination device 14 or external to the destination device 14. In some examples, destination device 14 may include an integrated display device and also be configured to interface with an external display device. In other examples, destination device 14 may be a display device. In general, display device 32 displays the decoded video data to a user, and may comprise any of a variety of display devices, such as a Liquid Crystal Display (LCD), a plasma display, an Organic Light Emitting Diode (OLED) display, or another type of display device.

Video encoder 20 and video decoder 30 may operate according to a video compression standard, such as the High Efficiency Video Coding (HEVC) standard currently under development, and may comply with the HEVC test model (HM). Alternatively, video encoder 20 and video decoder 30 may operate according to other proprietary or industrial standards such as the ITU-T H.264 standard, alternatively referred to as MPEG-4 part 10 (advanced video coding (AVC)), or extensions of these standards. However, the techniques of this disclosure are not limited to any particular coding standard. Other examples of video compression standards include MPEG-2 and ITU-T H.263.

Although not shown in fig. 1, in some aspects, video encoder 20 and video decoder 30 may each be integrated with an audio encoder and decoder, and may include appropriate multiplexer-demultiplexer (MUX-DEMUX) units or other hardware and software to handle encoding of both audio and video in a common data stream or separate data streams. If applicable, in some examples, the MUX-DEMUX unit may conform to the ITU H.223 multiplexer protocol or other protocols such as the User Datagram Protocol (UDP).

Video encoder 20 and video decoder 30 may each be implemented as any of a variety of suitable encoder circuits, such as one or more microprocessors, Digital Signal Processors (DSPs), Application Specific Integrated Circuits (ASICs), Field Programmable Gate Arrays (FPGAs), discrete logic, software, hardware, firmware or any combinations thereof. When the techniques are implemented in part in software, the device may store the instructions of the software in a suitable non-transitory computer-readable medium and execute the instructions in hardware using one or more processors to perform the techniques of this disclosure. Each of video encoder 20 and video decoder 30 may be included in one or more encoders or decoders, either of which may be integrated as part of a combined encoder/decoder (CODEC) in the respective device.

This disclosure may generally relate to video encoder 20 "signaling" particular information to another device, such as video decoder 30. It should be understood, however, that video encoder 20 may signal information by associating particular syntax elements with various encoded portions of video data. That is, video encoder 20 may "signal" the data by storing particular syntax elements to headers of various encoded portions of the video data. In some cases, these syntax elements may be encoded and stored (e.g., to storage system 34 or file server 36) prior to being received and decoded by video decoder 30. Thus, the term "signaling" may generally refer to the conveyance of syntax or other data for decoding compressed video data, regardless of whether such conveyance occurs in real-time or near real-time or over a span of time, such as may occur when syntax elements are stored to media at the time of encoding, which may then be retrieved by a decoding device at any time after being stored to such media.

JCT-VC is dedicated to the development of the HEVC standard. HEVC standardization efforts are based on an evolution model of the video coding device called the HEVC test model (HM). The latest Working Draft (WD) of HEVC (and referred to hereinafter as HEVC WD7) may be derived from http: int-evry. fr/jct/doc end user/documents/9_ Geneva/wg11/JCTVC-I1003-v5.zip, where more recent versions are available from http: int-evry, fr/jct/doc end user/documents/9 Geneva/wg11/JCTVC-I1003-v6.zip, both versions being hereby incorporated by reference as if set forth herein in their entirety. The HM assumes several additional capabilities of video coding devices relative to existing devices in accordance with, for example, ITU-T h.264/AVC. For example, h.264 provides nine intra-prediction encoding modes, while HM may provide up to thirty-three intra-prediction encoding modes.

In general, the working model description for HM may divide a video frame or picture into a sequence of treeblocks or Largest Coding Units (LCUs) that include both luma and chroma samples. Treeblocks have a similar purpose as macroblocks of the h.264 standard. A slice includes a number of consecutive treeblocks in coding order. A video frame or picture may be partitioned into one or more slices. Each treeblock may be split into Coding Units (CUs) according to a quadtree. For example, a treeblock that is the root node of a quadtree may be split into four child nodes, and each child node may in turn be a parent node and split into four other child nodes. The final non-fragmentable child node, which is a leaf node of the quadtree, comprises a coding node, e.g., a coded video block. Syntax data associated with a coded bitstream may define a maximum number of times a treeblock may be split, and may also define a minimum size of a coding node.

A Coding Unit (CU) includes a coding node and a Prediction Unit (PU) and a Transform Unit (TU) associated with the coding node. The size of a CU corresponds to the size of the coding node and must be square in shape. The size of a CU may range from 8 x 8 pixels up to a maximum treeblock size of 64 x 64 pixels or more. Each CU may contain one or more PUs and one or more TUs. For example, syntax data associated with a CU may describe a situation in which the CU is partitioned into one or more PUs. The partition mode may be different between cases where a CU is skipped or is directly mode encoded, intra prediction mode encoded, or inter prediction mode encoded. The PU may be partitioned into shapes other than square. For example, syntax data associated with a CU may also describe a situation in which the CU is partitioned into one or more TUs according to a quadtree. The shape of a Transform Unit (TU) may be square or non-square.

The HEVC standard allows for transform according to TUs, which may be different for different CUs. A TU is typically sized based on the size of a PU within a given CU defined for a partitioned LCU, although this may not always be the case. The size of a TU is typically the same as or smaller than a PU. In some examples, residual samples corresponding to a CU may be subdivided into smaller units using a quadtree structure referred to as a "residual quadtree" (RQT). The leaf nodes of the RQT may be referred to as Transform Units (TUs). The pixel difference values associated with the TUs may be transformed to produce transform coefficients, which may be quantized.

Generally, a PU includes data related to a prediction process. For example, when the PU is intra-mode encoded, the PU may include data describing an intra-prediction mode of the PU. As another example, when the PU is inter-mode encoded, the PU may include data defining a motion vector for the PU. For example, the data defining the motion vector for the PU may describe a horizontal component of the motion vector, a vertical component of the motion vector, a resolution of the motion vector (e.g., one-quarter pixel precision or one-eighth pixel precision), a reference picture to which the motion vector points, and/or a reference picture list (e.g., list 0, list 1, or list C) of the motion vector.

Generally, TUs use transform and quantization processes. A given CU with one or more PUs may also include one or more Transform Units (TUs). After prediction, video encoder 20 may calculate residual values corresponding to the PUs. The residual values comprise pixel difference values that may be transformed into transform coefficients, quantized, and scanned using TUs to produce serialized transform coefficients for entropy coding. This disclosure generally uses the term "video block" to refer to a coding node of a CU. In some particular cases, this disclosure may also use the term "video block" to refer to a treeblock that includes a coding node as well as PUs and TUs, e.g., an LCU or CU.

A video sequence typically comprises a series of video frames or pictures. A group of pictures (GOP) generally includes a series of one or more video pictures. The GOP may include syntax data in a header of the GOP, in a header of one or more of the pictures, or elsewhere, the syntax data describing a number of pictures included in the GOP. Each slice of a picture may include slice syntax data that describes an encoding mode for the respective slice. Video encoder 20 typically operates on video blocks within individual video slices in order to encode the video data. The video block may correspond to a coding node within a CU. Video blocks may have fixed or varying sizes, and may differ in size according to a specified coding standard.

As one example, the HM supports prediction of various PU sizes. Assuming that the size of a particular CU is 2N × 2N, the HM supports intra prediction of PU sizes of 2N × 2N or N × N, and inter prediction of symmetric PU sizes of 2N × 2N, 2N × N, N × 2N, or N × N. The HM also supports asymmetric partitioning for inter prediction for PU sizes of 2 nxnu, 2 nxnd, nlx 2N and nR x 2N. In asymmetric partitioning, one direction of a CU is not partitioned, while the other direction is partitioned into 25% and 75%. The portion of the CU corresponding to the 25% section is indicated by an indication of "n" followed by "Up", "Down", "Left", or "Right". Thus, for example, "2N × nU" refers to a horizontally partitioned 2N × 2NCU with 2N × 0.5N PU on top and 2N × 1.5N PU on the bottom.

In this disclosure, "nxn" and "N by N" are used interchangeably to refer to the pixel size of a video block in both the vertical and horizontal dimensions, e.g., 16 x 16 pixels or 16 by 16 pixels. In general, a 16 x 16 block will have 16 pixels in the vertical direction (y-16) and 16 pixels in the horizontal direction (x-16). Likewise, an nxn block generally has N pixels in the vertical direction and N pixels in the horizontal direction, where N represents a non-negative integer value. The pixels in a block may be arranged in rows and columns. Furthermore, the block does not necessarily need to have the same number of pixels in the horizontal direction as in the vertical direction. For example, a block may comprise N × M pixels, where M is not necessarily equal to N.

After using intra-predictive or inter-predictive coding of PUs of the CU, video encoder 20 may calculate residual data for the TUs of the CU. A PU may comprise pixel data in a spatial domain (also referred to as a pixel domain), and a TU may comprise coefficients in a transform domain after applying a transform (e.g., a Discrete Cosine Transform (DCT), an integer transform, a wavelet transform, or a conceptually similar transform) to residual video data. The residual data may correspond to pixel differences between pixels of the unencoded picture and prediction values corresponding to the PU. Video encoder 20 may form TUs that include the residual data of the CU, and then transform the TUs to generate transform coefficients for the CU.

After any transform to generate transform coefficients, video encoder 20 may perform quantization of the transform coefficients. Quantization generally refers to the process of quantizing coefficients to potentially reduce the amount of data used to represent the coefficients, providing further compression. The quantization process may reduce the bit depth associated with some or all of the coefficients. For example, an n-bit value may be reduced to an m-bit value during quantization, where n is greater than m.

In some examples, video encoder 20 may utilize a predefined scan order to scan the quantized transform coefficients to generate a serialized vector that may be entropy encoded. In other examples, video encoder 20 may perform adaptive scanning. After scanning the quantized transform coefficients to form a one-dimensional vector, video encoder 20 may entropy encode the one-dimensional vector, e.g., according to Context Adaptive Variable Length Coding (CAVLC), Context Adaptive Binary Arithmetic Coding (CABAC), syntax-based context adaptive binary arithmetic coding (SBAC), Probability Interval Partitioning Entropy (PIPE) coding, or another entropy encoding method. Video encoder 20 may also entropy encode syntax elements associated with the encoded video data for use by video decoder 30 in decoding the video data.

To perform CABAC, video encoder 20 may assign a context within the context model to a symbol to be transmitted. A context may relate to, for example, whether adjacent values of a symbol are non-zero. To perform CAVLC, video encoder 20 may select a variable length code of the symbol to be transmitted. Codewords in Variable Length Coding (VLC) may be constructed such that relatively shorter codes correspond to more likely symbols, while longer codes correspond to less likely symbols. In this way, the use of VLC may achieve bit savings relative to, for example, using equal length codewords for each symbol to be transmitted. The probability determination may be based on the context assigned to the symbol.

In some examples, video encoder 20 and/or video decoder 30 may identify a so-called "most probable" intra-prediction mode during intra-prediction coding. That is, for example, video encoder 20 and/or video decoder 30 may identify intra-prediction modes of previously coded blocks (referred to as "reference blocks") that neighbor the block currently being coded, and compare these intra-prediction modes to the intra-prediction mode of the block currently being coded (referred to as "current block"). Due to the spatial proximity of neighboring blocks to the current block, the probability that the intra-modes of these reference blocks are the same or similar to the current block may be relatively high. As described in more detail below, intra-prediction modes for multiple reference blocks may be considered in identifying MPMs.

In addition, according to some examples, video encoder 20 and/or video decoder 30 may signal an index identifying the MPM. That is, each intra-mode may have an associated (original) intra-mode index that identifies the intra-mode as one of a plurality of possible intra-modes, as defined according to a coding standard. For example, the proposed HEVC standard may support up to 35 intra-modes, where each intra-mode is assigned an index value (e.g., of a lookup table) that may be used to identify the intra-mode.

According to some video coding standards, video encoder 20 and/or video decoder 30 may order MPMs according to the original intra-mode index values. Video encoder 20 and/or video decoder 30 may then assign a new index value of "0" to the intra-mode with the smaller original intra-mode index (e.g., the lower original index value), assign an index value of "1" to the intra-mode with the next larger original intra-mode index value (e.g., the higher original intra-mode index value), and so on. In this way, video encoder 20 may signal (and video decoder 30 may receive from the encoded bitstream) the MPM using fewer bits than the actual intra-mode index is sent. However, although gains are achieved by indicating intra-modes using MPM, intra-mode ordering may increase the computational complexity of the coding process.

According to aspects of this disclosure, rather than ordering MPMs based on their index values, video encoder 20 and/or video decoder 30 may generate a list of MPMs that do not require ordering. That is, for example, video encoder 20 and/or video decoder 30 may assign index values to the MPMs in the order in which the MPMs are compared to the intra-mode of the current block. In other examples, video encoder 20 and/or video decoder 30 may assign index values according to the order in which the MPMs are coded. In this way, if the coded intra-mode is equal to one of the MPMs, the MPMs need not be ordered according to their original intra-mode index values, and video coder complexity may be reduced. In some examples, intra-mode ordering may be applied to code an intra-mode if the intra-mode is not equal to one of the MPMs.

In some examples, video encoder 20 determines an intra-mode for predicting a current block of video data, determines MPMs for predicting the current block of video data, compares the intra-mode to each of the MPMs in a comparison order, determines an index for each of the MPMs based on the comparison order, and signals an index of an MPM that matches the intra-mode for predicting the current block of data in the bitstream.

Additionally, in one example, video encoder 20 may signal the generated list of MPMs in order of comparison. Video encoder 20 may also determine an index for each of the MPMs, including assigning index values to each of the MPMs in the list in ascending order. In another example, video encoder 20 may signal an MPM flag prior to the index of the MPM to indicate that the index of the MPM has been signaled.

The MPMs may be associated with a left-neighboring video block of the current block and an above-neighboring video block of the current block, and the comparison order may include comparing intra-modes associated with the left-neighboring video block before comparing the above-neighboring blocks. The MPMs may be associated with a left-neighboring video block of the current block and an above-neighboring video block of the current block, and the order of comparison may include comparing intra-modes associated with the above-neighboring blocks prior to comparing intra-modes associated with the left-neighboring video block.

In another example, video decoder 30 may generate a list of MPMs for a current block of video data. The list of MPMs may then be arranged in order of comparison. Such a comparison order may indicate an order in which an intra-mode of a current block of video data is compared to one or more intra-modes associated with one or more reference blocks of video data during encoding of the current block of video data. Video decoder 30 may determine an MPM index that identifies an intra-mode for the current block in the list of MPMs. Video decoder 30 may then identify the intra-mode for the current block using the MPM index and decode the current block at the identified intra-mode for the current block.

In one example, the two or more reference blocks include one or more blocks positioned above and adjacent to the current block. The more than two reference blocks may include one or more blocks positioned to the left of the current video block and adjacent to the current block. In one example, the list of MPMs may be arranged in a comparison order that indicates an order in which an intra-mode of a current block of video data is compared to one or more intra-modes associated with one or more reference blocks of video data during encoding of the current block of video data.

In another example, MPMs may be associated with a left-neighboring video block of the current block and an above-neighboring video block of the current block, and the order of comparison may include comparing the intra-mode associated with the left-neighboring block before comparing the intra-mode associated with the above-neighboring block. In another example, the MPMs may be associated with a left-neighboring video block of the current block and an above-neighboring video block of the current block, and the order of comparison includes comparing the intra-mode associated with the above-neighboring block before comparing the intra-mode associated with the left-neighboring video block.

Certain aspects of this disclosure relate to assigning a default mode in instances in which block a or block B may not be used for intra-mode coding. For example, a video coder, such as video encoder 20 or video decoder 30, may identify a current block of video data. The coder may make a determination as to whether a block is unavailable for use as a reference block for determining an MPM for a current block of video data. The video coder may assign a default intra-mode to any block that is not available for use as a reference block. In some examples, the default intra mode may be planar mode, DC mode, or the like. The coder may determine an intra-mode for a current block of video data based on intra-modes of one or more blocks of video data. In addition, the coder may code the current block using the determined intra-mode.

FIG. 2 is a block diagram illustrating an example video encoder 20 that may implement the techniques described in this disclosure. Video encoder 20 may perform intra-coding and inter-coding of video blocks within a video slice. Intra-coding relies on spatial prediction to reduce or remove spatial redundancy of video within a given video frame or picture. Inter-coding relies on temporal prediction to reduce or remove temporal redundancy of video within adjacent frames or pictures of a video sequence. Intra mode (I-mode) may refer to any of a number of space-based compression modes. An inter mode, such as uni-directional prediction (P-mode) or bi-directional prediction (B-mode), may refer to any of a number of time-based compression modes.

In the example of fig. 2, video encoder 20 includes partition unit 35, prediction unit 41, reference picture memory 64, summer 50, transform processing unit 52, quantization unit 54, and entropy encoding unit 56. Prediction unit 41 includes a motion estimation unit 42, a motion compensation unit 44, and an intra-prediction module 46. For video block reconstruction, video encoder 20 also includes an inverse quantization unit 58, an inverse transform unit 60, and a summer 62. A deblocking filter (not shown in fig. 2) may also be included to filter block boundaries to remove blockiness artifacts from reconstructed video. The deblocking filter will typically filter the output of summer 62, if desired. In addition to deblocking filters, additional loop filters (in-loop or post-loop) may also be used.

As shown in fig. 2, video encoder 20 receives video data and partition unit 35 partitions the data into video blocks. Such partitioning may also include partitioning into slices, tiles, or other larger units, as well as video block partitioning, e.g., according to the quadtree structure of the LCUs and CUs. Video encoder 20 generally illustrates components of video blocks encoded within a video slice to be encoded. In general, a slice may be divided into a plurality of video blocks (and possibly into a set of video blocks referred to as an image block).

Prediction unit 41 may select one of a plurality of possible coding modes, such as one of a plurality of intra coding modes or one of a plurality of inter coding modes, for the current video block based on the error results (e.g., coding rate and degree of distortion). Prediction unit 41 may provide the resulting intra-coded or inter-coded block to summer 50 to generate residual block data and provide the resulting intra-coded or inter-coded block to summer 62 to reconstruct the encoded block for use as a reference picture.

Motion estimation unit 42 and motion compensation unit 44 within prediction unit 41 perform inter-predictive coding of the current video block relative to one or more predictive blocks in one or more reference pictures to provide temporal compression. Motion estimation unit 42 may be configured to determine an inter-prediction mode for a video slice according to a predetermined pattern of a video sequence. The predetermined pattern may designate video slices in the sequence as P slices, B slices, or GPB slices. Motion estimation unit 42 and motion compensation unit 44 may be highly integrated, but are illustrated separately for conceptual purposes. Motion estimation, performed by motion estimation unit 42, is the process of generating motion vectors that estimate the motion of video blocks. For example, a motion vector may indicate the displacement of a PU of a video block within a current video frame or picture relative to a predictive block within a reference picture.

A predictive block is a block that is found to closely match a PU of a video block to be coded in terms of pixel differences, which may be determined by Sum of Absolute Differences (SAD), Sum of Squared Differences (SSD), or other difference metrics. In some examples, video encoder 20 may calculate values for sub-integer (sub-integer) pixel positions of reference pictures stored in reference picture memory 64. For example, video encoder 20 may interpolate values for a quarter-pixel position, an eighth-pixel position, or other fractional-pixel positions of a reference picture. Thus, motion estimation unit 42 may perform a motion search relative to full pixel positions and fractional pixel positions and output motion vectors with fractional pixel precision.

Motion estimation unit 42 calculates motion vectors for PUs of video blocks in inter-coded slices by comparing the locations of the PUs to the locations of predictive blocks of the reference picture. A reference picture may be selected from a first reference picture list (list 0) or a second reference picture list (list 1), each of the lists identifying one or more reference pictures stored in reference picture memory 64. Motion estimation unit 42 sends the calculated motion vectors to entropy encoding unit 56 and motion compensation unit 44.

The motion compensation performed by motion compensation unit 44 may involve extracting or generating a predictive block based on motion vectors determined by motion estimation, possibly performing interpolation to sub-pixel precision. Upon receiving the motion vector for the PU of the current video block, motion compensation unit 44 may locate the predictive block in one of the reference picture lists to which the motion vector points. Video encoder 20 forms a residual video block by subtracting the pixel values of the predictive block from the pixel values of the current video block being coded, forming pixel difference values. The pixel difference values form residual data for the block and may include both luminance and chrominance difference components. Summer 50 represents one or more components that perform this subtraction operation. Motion compensation unit 44 may also generate syntax elements associated with the video blocks and the video slice for use by video decoder 30 in decoding the video blocks of the video slice.

Intra-prediction unit 46 within prediction unit 41 may perform intra-predictive coding of the current video block relative to one or more neighboring blocks in the same picture or slice as the current block to be coded to provide spatial compression. Accordingly, in lieu of inter-prediction performed by motion estimation unit 42 and motion compensation unit 44 (as described above), intra-prediction unit 46 may intra-predict the current block. In particular, intra-prediction unit 46 may determine the intra-prediction mode used to encode the current block. In some examples, intra-prediction unit 46 may encode the current block using various intra-prediction modes, e.g., during separate encoding passes, and intra-prediction unit 46 (or mode select unit 40 in some examples) may select an appropriate intra-prediction mode to use from the tested modes.

For example, intra-prediction unit 46 may calculate rate-distortion values using rate-distortion analysis for various tested intra-prediction modes, and select the intra-prediction mode having the best rate-distortion characteristics among the tested modes. Rate-distortion analysis generally determines the amount of distortion (or error) between an encoded block and an original, unencoded block that was encoded to produce the encoded block, as well as the bit rate (i.e., number of bits) used to produce the encoded block. Intra-prediction unit 46 may calculate the ratio of distortion and rate for various encoded blocks to determine which intra-prediction mode exhibits the best rate-distortion value for the block. According to the proposed HEVC standard, there may be up to 35 intra-prediction modes, and each intra-prediction mode may be associated with an index.

Aspects of this disclosure relate generally to intra coding. Thus, certain techniques of this disclosure may be performed by intra-prediction unit 46. That is, for example, intra-prediction unit 46 may perform the techniques of this disclosure described below with respect to fig. 4-10. In other examples, one or more other units of video encoder 20 may additionally or alternatively be responsible for performing the techniques of this disclosure.

For example, intra-prediction unit 46 may determine the intra-mode of the block currently being encoded (e.g., according to a rate-distortion analysis as described above). Intra-prediction unit 46 may also determine the intra-prediction mode (referred to as MPM) for one or more previously-coded blocks that neighbor the block currently being intra-coded. Intra-prediction unit 46 may indicate the determined intra-mode of the current block based on the determined intra-modes of the neighboring blocks, e.g., by comparing the MPM to the intra-mode of the current block, as described in more detail below.

According to aspects of this disclosure, intra-prediction unit 46 may generate a list of MPMs in the order in which they are compared to the intra-mode of the current block. Intra-prediction unit 46 may then assign index values to the MPMs in the order in which the MPMs are compared to the intra-mode of the current block. In this way, intra-prediction unit 46 may indicate a particular MPM without ordering the MPMs according to their original intra-mode index values (e.g., according to a video coding standard).

After prediction unit 41 generates the predictive block for the current video block via inter-prediction or intra-prediction, video encoder 20 forms a residual video block by subtracting the predictive block from the current video block. The residual video data in the residual block may be included in one or more TUs and applied to transform processing unit 52. Transform processing unit 52 transforms the residual video data into residual transform coefficients using a transform, such as a Discrete Cosine Transform (DCT) or a conceptually similar transform. Transform processing unit 52 may convert the residual video data from the pixel domain to a transform domain (e.g., the frequency domain).

Transform processing unit 52 may send the resulting transform coefficients to quantization unit 54. Quantization unit 54 quantizes the transform coefficients to further reduce the bit rate. The quantization process may reduce the bit depth associated with some or all of the coefficients. The degree of quantization may be modified by adjusting the quantization parameter. In some examples, quantization unit 54 may then perform a scan of a matrix including quantized transform coefficients. Alternatively, entropy encoding unit 56 may perform scanning.

After quantization, entropy encoding unit 56 may entropy encode the quantized transform coefficients. For example, entropy encoding unit 56 may perform Context Adaptive Variable Length Coding (CAVLC), Context Adaptive Binary Arithmetic Coding (CABAC), syntax-based context adaptive binary arithmetic coding (SBAC), Probability Interval Partition Entropy (PIPE) coding, or another entropy encoding method or technique. Entropy encoding unit 56 may also entropy encode the motion vectors and other syntax elements of the current video slice being coded. After entropy encoding by entropy encoding unit 56, the encoded bitstream may be transmitted to video decoder 30 or archived for later transmission or retrieval by video decoder 30.

Entropy coding unit 56 may encode information that indicates the selected intra-prediction mode in accordance with the techniques of this disclosure. Video encoder 20 may include, in transmitted bitstream configuration data that may include a plurality of intra-prediction mode index tables and a plurality of modified intra-prediction mode index tables (also referred to as codeword mapping tables), definitions of encoding contexts for various blocks and indications of MPMs, intra-prediction mode index tables, and modified intra-prediction mode index tables for each of the contexts.

Inverse quantization unit 58 and inverse transform unit 60 apply inverse quantization and inverse transform, respectively, to reconstruct the residual block in the pixel domain for later use as a reference block for a reference picture. Motion compensation unit 44 may calculate the reference block by adding the residual block to a predictive block of one of the reference pictures within one of the reference picture lists. Motion compensation unit 44 may also apply one or more interpolation filters to the reconstructed residual block to calculate sub-integer pixel values for motion estimation. Summer 62 adds the reconstructed residual block to the motion compensated prediction block generated by motion compensation unit 44 to generate a reference block for storage in reference picture memory 64. The reference block may be used by motion estimation unit 42 and motion compensation unit 44 as a reference block to inter-predict a block in a subsequent video frame or picture.

FIG. 3 is a block diagram illustrating an example video decoder 30 that may implement the techniques described in this disclosure. In the example of fig. 3, video decoder 30 includes an entropy decoding unit 80, a prediction unit 81, an inverse quantization unit 86, an inverse transform unit 88, a summer 90, and a reference picture memory 92. Prediction unit 81 includes motion compensation unit 82 and intra prediction unit 84. In some examples, video decoder 30 may perform a decoding pass that is substantially reciprocal to the encoding pass described with respect to video encoder 20 from fig. 4.

During the decoding process, video decoder 30 receives an encoded video bitstream representing video blocks of an encoded video slice and associated syntax elements from video encoder 20. Entropy decoding unit 80 of video decoder 30 entropy decodes the bitstream to generate quantized coefficients, motion vectors, and other syntax elements. Entropy decoding unit 80 forwards the motion vectors and other syntax elements to prediction unit 81. Video decoder 30 may receive syntax elements at the video slice level and/or the video block level.

When a video slice is coded as an intra-coded (I) slice, intra-prediction unit 84 of prediction unit 81 may generate prediction data for a video block of the current video slice based on the signaled intra-prediction mode and data from previously decoded blocks of the current frame or picture.

As noted above, aspects of this disclosure relate generally to intra coding. Thus, certain techniques of this disclosure may be performed by intra-prediction unit 84. That is, for example, intra-prediction unit 84 may perform the techniques of this disclosure described below with respect to fig. 4-7. In other examples, one or more other units of video decoder 30 may additionally or alternatively be responsible for performing the techniques of this disclosure.

For example, intra-prediction unit 84 may obtain, from entropy decoding unit 80, an index for a list of MPMs for decoding a current block of video data. Intra-prediction unit 84 may generate the list to which the index belongs by including the MPMs in the list in the same manner as video encoder 20 (e.g., in the order in which the MPMs are compared to the intra-mode of the current block). Intra-prediction unit 84 may then determine an appropriate intra-mode for decoding the current block of video data based on the obtained index. In this way, intra-prediction unit 84 may determine an appropriate MPM for decoding the current block without ordering the MPMs according to their original intra-mode index values (e.g., according to a video coding standard).

When a video picture is coded as an inter-coded (e.g., B, P or GPB) slice, motion compensation unit 82 of prediction unit 81 generates predictive blocks for video blocks of the current video slice based on motion vectors and other syntax elements received from entropy decoding unit 80. The predictive block may be generated from one of the reference pictures within one of the reference picture lists. Video decoder 30 may use a default construction technique to construct reference picture lists (list 0 and list 1) based on the reference pictures stored in reference picture memory 92.

Motion compensation unit 82 determines prediction information for video blocks of the current video slice by parsing the motion vectors and other syntax elements, and uses the prediction information to generate predictive blocks for the current video block being decoded. For example, motion compensation unit 82 uses some of the received syntax elements to determine the prediction mode (e.g., intra-prediction or inter-prediction) used to code the video blocks of the video slice, the inter-prediction slice type (e.g., B-slice, P-slice, or GPB-slice), the construction information for one or more of the reference picture lists of the slice, the motion vector of each inter-coded video block of the slice, the inter-prediction state of each inter-coded video block of the slice, and other information used to decode the video blocks in the current video slice.

The motion compensation unit 82 may also perform interpolation based on the interpolation filter. Motion compensation unit 82 may calculate interpolated values for sub-integer pixels of the reference block using interpolation filters as used by video encoder 20 during encoding of the video block. In this case, motion compensation unit 82 may determine the interpolation filter used by video encoder 20 from the received syntax element and use the interpolation filter to generate the predictive block.

Inverse quantization unit 86 inverse quantizes (e.g., dequantizes) the quantized transform coefficients provided in the bitstream and decoded by entropy decoding unit 80. The inverse quantization process may include determining a degree of quantization using a quantization parameter calculated by video encoder 20 for each video block in the video slice, and likewise determining a degree of inverse quantization that should be applied. The inverse transform unit 88 applies an inverse transform (e.g., an inverse DCT, an inverse integer transform, or a conceptually similar inverse transform process) to the transform coefficients in order to generate a residual block in the pixel domain.

After motion compensation unit 82 generates the predictive block for the current video block based on the motion vector and other syntax elements, video decoder 30 forms a decoded video block by summing the residual block from inverse transform unit 88 with the corresponding predictive block generated by motion compensation unit 82. Summer 90 represents one or more components that perform this summation operation. When needed, deblocking filters may also be applied to filter the decoded blocks in order to remove blockiness artifacts. Other loop filters (in or after the coding loop) may also be used to smooth pixel transitions, or otherwise improve video quality. The decoded video blocks in a given frame or picture are then stored in reference picture memory 92, reference picture memory 92 storing reference pictures for use in subsequent motion compensation. Reference picture memory 92 also stores decoded video for later presentation on a display device, such as display device 32 of fig. 1.

As noted above, the techniques of this disclosure generally relate to intra coding. It should be understood that the techniques of this disclosure may be performed by any of the video coders described in this disclosure, including, for example, video encoder 20 and video decoder 30 as shown and described with respect to fig. 1-3. That is, in one example, intra-prediction unit 46 described with respect to fig. 2 may perform certain techniques described below when performing intra-prediction during encoding of a block of video data. In another example, intra-prediction unit 84 described with respect to fig. 3 may perform certain techniques described below when performing intra-prediction during decoding of a block of video data. Thus, reference to a general "video coder" may include video encoder 20, video decoder 30, or another video encoding or decoding unit.

In some examples, a video coder may identify a so-called "most probable" intra-prediction mode during intra-prediction coding. That is, for example, a video encoder (e.g., video encoder 20) may identify intra-prediction modes of previously encoded blocks (e.g., reference blocks) and compare these intra-prediction modes to the intra-prediction mode of the current block. Due to the spatial proximity of these reference blocks to the current block, the probability that the intra-modes of these reference blocks are the same or similar to the current block may be relatively high. As described in more detail below, intra-prediction modes for multiple reference blocks may be considered in identifying MPMs.

If the intra-prediction mode for the current block is the same as MPM, video encoder 20 may signal the intra-prediction mode using a one-bit MPM flag. That is, video encoder 20 may signal that the intra-prediction mode of the current block is the same as the MPM without explicitly identifying the intra-prediction mode of the current block. Video decoder 30 may receive a flag indicating that the intra-mode of the current block is the same as the MPM, and repeat the process used by video encoder 20 to determine that MPM. That is, video decoder 30 may identify the MPM using the same blocks considered by video encoder 20 during encoding.

Fig. 4 shows an example of a current block (e.g., a coding unit) ("current CU") and two reference blocks (e.g., "a" and "B") that may be considered during intra coding. For example, a video encoder (e.g., video encoder 20) may consider intra-modes associated with reference block a (located to the left of the current block) and reference block B (located above the current block) as the MPM for the current block. In some examples, if any of the MPM candidates (e.g., block a or block B) do not use intra-mode, or are otherwise unavailable (e.g., blocks that have not yet been coded), video encoder 20 may assign a default intra-mode, such as DC mode, to the block. Also, in some examples, the number of MPMs may be greater than two. For example, video encoder 20 may generate additional MPMs based on the intra-modes of more than two reference blocks.

If the actual intra-mode of the current block (e.g., as calculated, for example, by intra-prediction unit 46) is the same as reference block a or reference block B, video encoder 20 may signal a one-bit flag indicating that the current block is encoded using MPM (e.g., set the MPM flag equal to one).

Additionally, according to some examples, video encoder 20 may signal an index identifying the MPM. That is, each intra-mode may have an associated (original) intra-mode index that identifies the intra-mode as one of a plurality of possible intra-modes as defined according to a coding standard. For example, the proposed HEVC standard may support up to 35 intra-modes, with each intra-mode being assigned an original standard-specified index value, as demonstrated in table 1 below:

table 1: intra prediction mode and index number

Intra prediction mode	Associated name
		0	Intra_Planar
1	Intra_DC
		(2-34)	Intra_Angular

In the example of table 1, the planar intra mode has an original index value of 0, the DC intra mode has an original index value of 1, and the various angular intra modes have original index values between 2 and 34.

Conventionally, when coding a current block intra based on MPM, video encoder 20 orders the MPM according to the original intra-mode index value. In an example with two MPMs, video encoder 20 may then assign a new index value of "0" to the intra-mode with the smaller original intra-mode index (e.g., the lower original index value). In addition, video encoder 20 assigns a new index value of "1" to the intra-mode with the larger original intra-mode index value (e.g., the higher original intra-mode index value). In this way, video encoder 20 may signal the MPM using fewer bits than the actual intra-mode index. In some examples, video encoder 20 does not send the additional index value if the intra-mode of reference block a is the same as the intra-mode of reference block B. In some examples, similar index assignments may be made if the number of MPMs is greater than two.

If the intra-mode of the current block is not equal to the MPM, video encoder 20 may intra-code the current block, for example, using a fixed length coding table or other methods.

A video decoder, such as video decoder 30, may receive the MPM flag and MPM index. Video decoder 30 may then perform a similar process as described with respect to video encoder 20 to determine to which intra-mode the MPM index refers. That is, video decoder 30 may order the intra modes in ascending original index order. Video decoder 30 may assign an index value of "0" to the smaller original intra-mode index and an index value of "1" to the larger original intra-mode index. Video decoder 30 may then select one of the ordered intra-modes using the signaled MPM index. Video decoder 30 may then decode the intra-coded blocks of video data.

According to some video coding techniques, a video coder (e.g., a video encoder or a video decoder) may order MPMs for a block currently being coded prior to determining and signaling the MPMs. This may increase video coder complexity. As described in more detail below, certain aspects of this disclosure relate to removing the ordering of MPMs to reduce video coder complexity. For example, video encoder 20, video decoder 30, or both may generate a list of MPMs that does not require sorting. Video encoder 20 and/or video decoder 30 may assign index values to the MPMs in the order in which the MPMs are compared to the intra-mode of the current block. In other examples, video encoder 20 and/or video decoder 30 may assign index values according to the order in which the MPMs are coded. In this way, the MPMs do not need to be ordered according to their original intra-mode index values, and video coder complexity may be reduced.

In addition, certain aspects of this disclosure also relate to assigning a default mode in instances where block a or block B may not be used for intra-mode coding. For example, video encoder 20 may assign a pre-selected mode, such as planar mode or DC mode, as a default mode.

Fig. 5 is a conceptual diagram of intra-mode coding as described above with respect to fig. 4. For example, as shown in the example of fig. 5, if the intra-mode of the current block matches one of the MPMs (e.g., the "yes" branch), video encoder 20 may set the MPM flag equal to one ("1"). In addition, video encoder 20 may order the MPMs according to their original index values ("order MPM modes"). That is, video encoder 20 assigns a value of zero to an MPM having a smaller original intra-mode index ("0" "" smaller MPM "), and a value of one to an MPM having a larger original intra-mode index (" 1 "" "other MPM"). Video encoder 20 may then signal the intra-mode for the current block based on the MPM. That is, video encoder 20 may send a "1" flag to indicate that MPM is used, and send a "0" flag or a "1" flag to identify the appropriate MPM.

If the intra-mode of the current block does not match one of the MPMs (e.g., the "no" branch), video encoder 20 may set the MPM flag to zero ("0"). In addition, video encoder 20 may code the intra modes ("the remainder of intra mode coding"), e.g., using a fixed length table or other methods.

Fig. 6 is a conceptual diagram of intra-mode coding according to aspects of this disclosure. For example, as shown in the example of fig. 6, if the intra-mode of the current block matches one of the MPMs (e.g., the "yes" branch), video encoder 20 may set the MPM flag equal to one ("1"). According to aspects of this disclosure, rather than ordering MPMs based on their index values, video encoder 20 may generate a list of MPMs that does not require ordering ("generate list of MPMs"). That is, for example, certain techniques of this disclosure involve assigning index values to MPMs in the order in which they are compared to intra-modes of a current block. In this way, the MPMs do not need to be ordered according to their original intra-mode index values. If the intra-mode of the current block is not one of the MPMs, video encoder 20 may set the MPM flag equal to zero ("0"), and may apply additional MPM ordering before performing the rest of the intra-mode coding. That is, for example, video encoder 20 may order the list of remaining intra-modes (the list does not include MPMs) before identifying one of the intra-modes of the current block in the list.

In one example, for purposes of explanation, video encoder 20 may compare an intra-mode associated with a current block of video data to intra-modes associated with one or more reference blocks, such as intra-modes of one or more neighboring blocks (e.g., blocks that are spatially neighboring the current block), to determine that one of the MPMs matches the actual intra-mode of the current block. As noted above with respect to fig. 4 and 5, video encoder 20 may compare the intra-mode of the current block to two neighboring blocks, but may consider the intra-modes of more or fewer reference blocks (e.g., one, three, five, and the like), in addition, MPMs may be generated based on neighboring intra-modes.

According to aspects of this disclosure, video encoder 20 may assign index values to the MPMs in the order in which they are compared to the current intra-mode. In an example with two MPMs, video encoder 20 may first compare the intra-mode of the current block to the intra-mode of the left-neighboring block, followed by comparing the intra-mode of the current block to the intra-mode of the above-neighboring block (see, e.g., the arrangement shown in fig. 4). Thus, video encoder 20 may assign an index value of zero to the intra mode of the left-neighboring block ("0" "" first comparison ") and an index value of one to the intra mode of the above-neighboring block (" 1 "" "second comparison"). Although the example shown in fig. 6 includes only two MPMs, additional index values may be assigned to other MPMs if these MPMs are considered.

If the intra-mode of the current block does not match one of the MPMs (e.g., the "no" branch), video encoder 20 may set the MPM flag to zero ("0"). In addition, video encoder 20 may code the intra modes ("the remainder of intra mode coding"), e.g., using a fixed length table or other methods. Additionally, in some examples, and as noted above, video encoder 20 may order the remaining intra-modes according to an MPM ordering process.

Assume for purposes of explanation that 35 intra modes are available for intra coding a block of video data. Further assuming that 35 intra-modes can be identified by a mode number, the mode number can be included in a table identifying available intra-modes. In examples where the current mode is not an MPM, where, for example, the MPMs are 15, 2, and 31, and where the current mode is 16, a video coder (e.g., video decoder 30 or video encoder 20) may initially order the MPMs into an ascending order. Thus, the list of MPMs 15, 2, and 31 becomes 2, 15, and 31 after sorting. Second, the video coder may eliminate MPMs from the remaining intra-modes, since the current mode is known not to be an MPM. The remaining 32 modes can then be remapped to 32 5-bit codewords. Some examples may use a table of the remaining 32 modes, eliminating modes 2, 15, 31. However, other examples do not use a table.

With regard to mapping, for example, because the current mode (e.g., mode 16) is greater than or equal to the first of the ordered MPMs (mode 2), the video coder may subtract 1 from the current mode (16-1-15). The value (15) after the first subtraction is also greater than or equal to the second (15) of the ordered MPMs, so the video coder subtracts 1 again (15-1-14). The value (14) after the second subtraction is less than 31, so the video coder does not perform another subtraction. Thus, the current mode maps to the fourteenth entry in the new table as calculated by performing two subtractions (16-2-14) on the current mode 16. In other instances, the patterns may be mapped differently.

In another example where the current mode is not an MPM, where, for example, the MPMs are 5, 4, and 6 and the current mode is 15, the video coder may order the MPMs into ascending order. Therefore, 5, 4, 6 becomes 4, 5, 6. The video coder may then generate a list or table of the remaining 32 modes, eliminating modes 4, 5, and 6. As discussed above, it should be understood that not all examples use a table.

In the above example, it is assumed that the current mode is not MPM. The video coder may set the MPM flag equal to "1" as compared to the case when the current mode is MPM (e.g., MPMs 5, 4, and 6 and current mode 4). In addition, the video coder may set the index of the MPM equal to "1". For example, the index may map the MPMs based on the order in the list. That is, in the above example, the video coder may map 5 to index 0, 4 to index 1, and 6 to index 2.

In some examples, as noted above, video encoder 20 may generate a list of MPMs in the order in which they are compared to the intra-mode for the current block. Thus, the index may identify matching MPMs based on the comparison order. Video encoder 20 may then assign an index value to each of the MPMs in the list. For example, video encoder 20 may assign index values to each of the MPMs in the list in ascending order such that the first MPM compared to the current intra-mode has the lowest relative index value and the last MPM compared to the current intra-mode has the highest relative index value. In contrast, each intra-mode may have an originally associated intra-mode index that identifies the intra-mode as one of a plurality of possible intra-modes as defined according to a coding standard (e.g., one of 35 intra-modes). According to aspects of the invention, this original index may be different from the index signaled in the bitstream.

Video encoder 20 may then signal the MPM flag and MPM index to a video decoder (e.g., video decoder 30). Video decoder 30 may perform a similar process as described with respect to video encoder 20 to identify the appropriate intra-mode from the received index value. That is, for example, video decoder 30 may generate a list including MPMs in the order in which the intra-mode is compared to the current intra-mode. Video decoder 30 may then apply an index value to each of the MPMs and use the received index values to select the appropriate intra-mode for the current block.

In some examples, the order in which the MPMs are compared to the current intra-mode may be implicitly derived from collected statistics of previously coded intra-modes. That is, for example, if an intra-mode associated with a certain reference block matches a current intra-mode more frequently than other MPMs of other blocks, the intra-mode associated with the particular reference block may be compared earlier than other MPMs of other blocks.

In other examples, the comparison order may be derived based on availability, or may be explicitly signaled. For example, if one or more reference blocks containing MPMs are frequently unavailable, these reference blocks may be compared to the current block relatively late compared to more common reference blocks. In other examples, video encoder 20 may determine and explicitly signal a particular comparison order in the encoded bitstream.

Eliminating the sorting step (e.g., sorting the MPMs according to their index values) may reduce the complexity of the coding process. That is, rather than having to explicitly order the MPMs, the video coder may generate a list of MPMs while determining whether the current intra-mode is the same as any of the MPMs. In this manner, certain techniques of this disclosure may be used to increase the computational efficiency of a video coder. However, in some examples, MPM ordering may be preserved for intra-mode coding if the intra-mode is not equal to one of the MPMs. For example, when an intra-mode does not match one of the MPMs, the remaining modes may be ordered.

The techniques of this disclosure also involve assigning a default intra-mode to a block of video data that is not available to be considered a reference block during MPM coding. That is, a reference block may be considered "unavailable" if, for example, the reference block has not been coded (and thus its prediction mode is unknown), if the reference block is coded using inter-prediction (described above), or if the reference block is not present (a block positioned in the upper left corner of a picture or slice may not have neighboring blocks to the left and/or above).

According to aspects of this disclosure, upon identifying blocks that cannot be used for reference during intra coding (e.g., during the MPM derivation process), default intra modes may be assigned to these blocks. That is, for example, these blocks may be assigned a planar intra mode, which may also be referred to as a planar intra mode. Plane intra modes may include linear plane functions that fit blocks for prediction purposes. Planar intra modes can work well (provide accurate prediction) in areas where the luminance varies smoothly.

In some examples, the planar intra mode may be selected relatively frequently as an intra mode for coding video data. That is, the chance of selecting a planar mode for coding the current block may be relatively high compared to other coding modes. Thus, setting the default mode to planar mode may increase the likelihood that the video encoder is able to code the current mode based on MPM when a block is not available.

The techniques of this disclosure also involve considering more than two MPMs. For example, as shown in the example of fig. 7, a video coder may consider intra-modes associated with multiple neighboring blocks as MPMs. That is, the video coder may consider the intra-mode associated with any reference block that neighbors the block currently being coded as the MPM for the current block. The video coder may generate a list of MPMs, and assign an index value to each MPM in the list. The video coder may then intra code the current block based on the MPM, as described above.

In the example shown in fig. 7, the video coder considers the intra-mode associated with each block that neighbors the current block when determining MPM, including an "upper-left" (AL) neighboring block, as well as a "first upper" neighboring block (a1), "second upper" neighboring block a2 (not shown), and "nth upper" neighboring block a_NAnd so on until the "upper right" (AR) neighboring block. Additionally, in the example shown in fig. 7, the video coder may consider a block L adjacent to a "first left" (L1), a "second left" neighboring block L2 (not shown), an "nth left" neighboring block L_NAnd so on until the "lower-left" (BL) neighboring block's associated intra mode.

According to some aspects of this disclosure, a video coder may consider only intra-coded reference blocks in determining MPMs. In another example, a video coder may consider all blocks (e.g., including inter-coded blocks and/or blocks that are otherwise unavailable). In this example, the video coder may assign a default intra-mode (e.g., planar mode or DC mode) to the unavailable block prior to determining the MPM, as described above.

In some examples, the order in which the comparisons are made by the encoder or decoder (e.g., the order in which the current intra mode is compared to the intra mode of the reference block) may be from left to right, followed by top to bottom. For example, as illustrated in fig. 7, the comparison order may be from left to right in the direction from AL to AR, followed by top to bottom in the direction from AL to BL. In another example, the comparison order may be from right to left, followed by bottom to top. For example, as also illustrated in fig. 7, the comparison order may be from right to left in the direction from AR to AL, followed by from bottom to top in the direction from BL to AL. In other examples, any other combination of comparison orders may be used (e.g., from bottom to top, followed by left to right, right to left, followed by top to bottom, and the like).

In still other examples, the comparison order may be defined by a predetermined rule. In these examples, video encoder 20 may signal rules regarding comparison order, and video decoder 30 may receive rules regarding comparison order from the encoded bitstream. In still other examples, according to aspects of this disclosure, a video coder may consider a subset of neighboring blocks, such as every other block, every third block, or a different subset of blocks, when comparing a current intra-mode to intra-modes of the neighboring blocks.

The reference blocks considered in determining the MPM may be fixed or signaled. That is, for example, both video encoder 20 and video decoder 30 may be configured to determine the MPM by comparing the intra-modes of the same neighboring reference blocks. In addition, both video encoder 20 and video decoder 30 may be configured to determine the same comparison order (e.g., the order in which the current intra-mode is compared to the intra-mode of the reference block).

Alternatively, video encoder 20 may signal which reference blocks are to be considered during MPM intra coding, and/or the order of comparison. In this example, video decoder 30 may perform MPM coding based on the received signaling provided by video encoder 20.

According to aspects of this disclosure, the number of reference blocks considered in identifying an MPM may be related to the number of different MPMs that may be used for selection. That is, for example, each reference block under consideration may be associated with a single MPM. The number of MPMs may be fixed, derived, or signaled. For example, MPM may depend on a number of different factors, such as neighboring blocks, slice types, block sizes, and so on. This information may be used to determine the number of MPMs, for example, by a known formula. This known formula can then be used to derive the number of MPMs.

It should be understood that although the reference blocks in the example of fig. 7 are shown to be similar or identical in size, a video coder may consider different sized reference blocks when identifying MPMs. Further, more or fewer candidates than shown in the example of fig. 7 may be considered.

Fig. 8 is a flow diagram illustrating an example method of encoding video data in accordance with one or more examples described in this disclosure. In the example method of encoding video data of fig. 8, video encoder 20 determines an intra-mode for predicting a current block of video data (800). For example, video encoder 20 may perform intra-coding of video blocks within a video slice. Intra-coding may rely on spatial prediction to reduce or remove spatial redundancy of video within a given video frame or picture. Intra mode may refer to any of a number of spatial compression modes.

Video encoder 20 determines candidate MPMs for predicting a current block of video data (802). That is, for example, video encoder 20 may identify intra-prediction modes of previously encoded blocks (e.g., reference blocks), and compare these intra-prediction modes to the intra-prediction mode of the current block (e.g., the actual intra-mode used to code the current block as selected, for example, using the rate-distortion analysis described above with respect to fig. 2). Due to the spatial proximity of these reference blocks to the current block, the probability that the intra-modes of these reference blocks are the same or similar to the current block may be relatively high. Intra prediction modes for multiple reference blocks may be considered in identifying MPMs.

In some examples, video encoder 20 compares the intra-mode to each of the MPMs in order of comparison. As discussed above, the comparison order is the order in which the current intra-mode is compared to the intra-mode of the reference block, e.g., by video encoder 20. The order of comparison may be from left to right, followed by top to bottom. In another example, the comparison order may be from right to left, followed by bottom to top. In other examples, any other combination of comparison orders may be used (e.g., from bottom to top, followed by left to right, right to left, followed by top to bottom, and the like).

As discussed above, in still other examples, the comparison order may be defined by some rule, and the rule may be signaled (e.g., signaled by video encoder 20 for use by video decoder 30). In still other examples, video encoder 20 may consider a subset of neighboring blocks, such as every other block, every third block, or a different subset of blocks, when comparing the current intra mode to the intra modes of the neighboring blocks, according to aspects of this disclosure.

Video encoder 20 determines an index for each of the MPMs based on the comparison order (804). For example, as discussed above with respect to fig. 6, video encoder 20 may assign an index value of zero to the intra mode of the left-neighboring block ("0" "" first comparison ") and an index value of one to the intra mode of the above-neighboring block (" 1 "" "second comparison"). Although the example shown in fig. 6 includes only two MPMs, additional index values may be assigned to other MPMs if these MPMs are considered.

Video encoder 20 signals an index of the MPM in the bitstream that matches the intra-mode for the current block of prediction data (806). For example, as noted above, the index may identify matching MPMs based on the order of comparison. In contrast, each intra-mode may have an originally associated intra-mode index that identifies the intra-mode as one of a plurality of possible intra-modes as defined according to a coding standard (e.g., one of 35 intra-modes). According to aspects of the invention, this original index may be different from the index signaled in the bitstream. For example, as noted above, video encoder 20 may assign MPM index values based on an ascending order of comparison, such MPMs that are compared to the current intra-mode relatively earlier in the comparison process may have lower index values than MPMs that are compared to the current intra-mode relatively later in the comparison process. Thus, an MPM compared earlier may have a lower index value than an MPM compared later, regardless of the original intra-mode index of the MPM compared earlier. That is, in some examples, an MPM that is compared earlier may have a higher original index value than an MPM that is compared later.

Fig. 9 is a flow diagram illustrating an example method of decoding video data in accordance with one or more examples described in this disclosure. In the example method of decoding video data of fig. 9, video decoder 30 generates a list of MPMs for a current block of video data (900). For example, video coder 30 may generate a list of MPMs, and assign an index value to each MPM in the list. The list of MPMs may be arranged in a comparison order that indicates an order in which an intra-mode of a current block of video data is compared to one or more intra-modes associated with one or more reference blocks of video data during encoding of the current block of video data.

Video decoder 30 determines an MPM index that identifies the intra-mode for the current block in the list of MPMs (902). Video decoder 30 may determine an MPM index that identifies an intra-mode for the current block in the list of MPMs. Video decoder 30 may then identify the intra-mode for the current block using the MPM index and decode the current block with the identified intra-mode for the current block. In one example, video decoder 30 may determine the MPM index based on signaling included in the encoded bitstream. Video encoder 20 may have signaled the intra-prediction mode using a one-bit MPM flag. That is, video encoder 20 may signal that the intra-prediction mode of the current block is the same as the MPM without having to explicitly identify the intra-prediction mode of the current block. Video decoder 30 may then receive a flag indicating that the intra-mode of the current block is the same as the MPM.

Video decoder 30 identifies the intra-mode for the current block using the MPM index (904). For example, video decoder 30 may identify the MPM using the same blocks considered by video encoder 20 during encoding. Video decoder 30 may then select one of the ordered intra-modes using the signaled MPM index. Video decoder 30 decodes the current block with the identified intra-mode for the current block (906).

For example, as noted above, the index may identify matching MPMs based on the order of comparison. In contrast, each intra-mode may have an originally associated intra-mode index that identifies the intra-mode as one of a plurality of possible intra-modes as defined according to a coding standard (e.g., one of 35 intra-modes). According to aspects of the invention, this original index may be different from the index signaled in the bitstream. For example, video decoder 30 may determine the MPM index values based on the comparison order in ascending order.

Fig. 10 is a flow diagram illustrating an example method of coding video data in accordance with one or more examples described in this disclosure. In the example method of coding video data of fig. 10, a video coder, such as video encoder 20 or video decoder 30, may identify one or more blocks of video data for determining an MPM for a current block of video data (1000). The video coder may determine whether any of the one or more blocks are unavailable for use as a reference block for determining an MPM for a current block of video data (1002). For example, a reference block may be considered "unavailable" if it has not been coded (and thus its prediction mode is unknown), if it is coded using inter-prediction (described above), or if it does not exist (a block positioned in the upper left corner of a picture or slice may not have neighboring blocks to the left and/or above).

The video coder assigns a default intra-mode to any of one or more blocks that are unavailable for use as reference blocks. In one example, the default intra mode may be a planar mode (1004). Planar intra modes (also referred to as planar intra modes) may include linear plane functions that fit blocks for prediction purposes and may provide accurate prediction in areas where luminance varies smoothly. In other examples, the default intra-mode may be DC mode or another intra-mode.

The video coder determines an intra-mode for a current block of video data based on the intra-modes of the one or more blocks (1006). For example, with respect to video encoder 20, as described above with respect to the example of fig. 4, if the actual intra-mode of the current block (e.g., as calculated, for example, by intra-prediction unit 46) is the same as reference block a or reference block B, video encoder 20 may signal a one-bit flag indicating that the current block is encoded using MPM (e.g., setting the MPM flag equal to one). Alternatively, with respect to video decoder 30, as described above, video decoder 30 may obtain an MPM flag from the encoded bitstream and use the MPM flag to determine the intra-mode for decoding the current block.

The video coder codes the current block using the determined intra-mode (1008). For example, video encoder 20 encodes the current block by predicting the current block using the determined intra mode to generate a reference video block. Video encoder 20 may also determine a residual block that includes the difference between the reference block and the current block, and include the residual block in the bitstream. Alternatively, video decoder 30 decodes the current block with the identified MPM for the current block. For example, video decoder 30 may obtain a residual video block associated with the current block from the encoded bitstream. Video decoder 30 may generate a reference block by predicting the current block using the identified intra-mode for the current block. In addition, video decoder 30 may determine the value of the current block from a combination of the reference block and the received residual video block.

Although particular aspects of this disclosure have been described with respect to video encoder 20 and video decoder 30, it should be understood that the techniques of this disclosure may be applied by many other video encoding and/or decoding units, processors, processing units, hardware-based coding units such as encoders/decoders (CODECs), and the like. Further, it should be understood that the steps shown and described with respect to fig. 8-10 are provided as examples only. That is, the steps shown in the examples of fig. 8-10 need not necessarily be performed in the order shown in fig. 8-10, and fewer, additional, or alternative steps may be performed.

Further, it is to be understood that depending on the example, certain acts or events of any of the methods described herein can be performed in a different sequence, added, combined, or left out all together (e.g., not all described acts or events are necessary for the practice of the methods). Further, in particular instances, acts or events may be performed concurrently rather than sequentially, e.g., via multi-threaded processing, interrupt processing, or multiple processors. Additionally, although specific aspects of this disclosure are described as being performed by a single module or unit for purposes of clarity, it should be understood that the techniques of this disclosure may be performed by a combination of units or modules associated with a video coder.

In one or more examples, the functions described may be implemented in hardware, software, firmware, or any combination thereof. If implemented in software, the functions may be stored on or transmitted over as one or more instructions or code on a computer-readable medium and executed by a hardware-based processing unit. Computer-readable media may include computer-readable storage media, corresponding to tangible media such as data storage media, or communication media, including any medium that facilitates transfer of a computer program from one place to another, such as according to a communication protocol.

In this manner, the computer-readable medium may generally correspond to (1) a tangible, non-transitory computer-readable storage medium, or (2) a communication medium such as a signal or carrier wave. A data storage medium may be any available medium that can be accessed by one or more computers or one or more processors to retrieve instructions, code and/or data structures for implementing the techniques described in this disclosure. The computer program product may include a computer-readable medium.

By way of example, and not limitation, such computer-readable storage media can comprise RAM, ROM, EEPROM, CD-ROM or other optical disk storage, magnetic disk storage or other magnetic storage devices, flash memory, or any other medium that can be used to store desired code in the form of instructions or data structures and that can be accessed by a computer. Also, any connection is properly termed a computer-readable medium. For example, if the instructions are transmitted from a website, server, or other remote source using a coaxial cable, fiber optic cable, twisted pair, Digital Subscriber Line (DSL), or wireless technologies such as infrared, radio, and microwave, then the coaxial cable, fiber optic cable, twisted pair, DSL, or wireless technologies such as infrared, radio, and microwave are included in the definition of medium.

It should be understood, however, that computer-readable storage media and data storage media do not include connections, carrier waves, signals, or other transitory media, but are instead directed to non-transitory tangible storage media. Disk and disc, as used herein, includes Compact Disc (CD), laser disc, optical disc, Digital Versatile Disc (DVD), floppy disk and blu-ray disc where disks usually reproduce data magnetically, while discs reproduce data optically with lasers. Combinations of the above should also be included within the scope of computer-readable media.

The instructions may be executed by one or more processors, such as one or more Digital Signal Processors (DSPs), general purpose microprocessors, Application Specific Integrated Circuits (ASICs), Field Programmable Gate Arrays (FPGAs), or other equivalent integrated or discrete logic circuitry. Thus, as used herein, the term "processor" may refer to any of the foregoing structure or any other structure suitable for implementation of the techniques described herein. In addition, in some aspects, the functionality described herein may be provided within dedicated hardware and/or software modules configured for encoding and decoding, or incorporated in a combined codec. Likewise, the techniques may be fully implemented in one or more circuits or logic elements.

The techniques of this disclosure may be implemented in a wide variety of devices or apparatuses, including wireless handsets, Integrated Circuits (ICs), or a collection of ICs (e.g., a chipset). Various components, modules, or units are described in this disclosure to emphasize functional aspects of devices configured to perform the disclosed techniques, but do not necessarily require realization by different hardware units. Rather, as described above, the various units may be combined in a codec hardware unit or provided by an interoperating hardware unit (including one or more processors as described above) in conjunction with a collection of suitable software and/or firmware.

Various aspects of the present invention have been described. These and other aspects are within the scope of the following claims.

Claims (50)

1. A method of encoding video data, the method comprising:

determining an intra-mode for predicting a current block of video data;

determining a Most Probable Mode (MPM) for predicting the current block of video data to generate a list of MPMs, wherein the MPMs are intra-modes associated with respective neighboring blocks of video data encoded prior to the current block;

assigning an index to each of the MPMs in the list based on an order in which the intra-mode for predicting the current block is compared to the MPMs, without reordering the MPMs in the list based on the mode indices of the MPMs; and

encoding intra-mode data for the current block comprising encoding data representing the index of the matching MPM in the list in an encoded bitstream when one of the MPMs matches the intra-mode used to predict the current block.

2. The method of claim 1, wherein encoding intra-mode data of the current block comprises, when one of the MPMs does not match the intra-mode used to predict the current block,

encoding an MPM flag indicating that an intra-mode used to predict the current block is not an MPM;

generating an intra mode modified list; and

encoding data representing an index of the intra-mode for the current block in a modified list.

3. The method of claim 2, wherein generating a modified list comprises eliminating the MPM from an intra-mode modified list.

4. The method of claim 2, wherein generating a modified list comprises sorting intra modes of a modified list in ascending order according to mode values of the intra modes.

5. The method of claim 1, further comprising encoding an MPM flag prior to encoding the index of the matching MPM, the MPM flag indicating that data representing an index of the matching MPM is present in an encoded bitstream.

6. The method of claim 1, wherein determining the MPMs comprises determining an intra-mode associated with a left-neighboring video block of the current block and an above-neighboring video block of the current block, and wherein generating the list of MPMs comprises assigning an index to an intra-mode associated with the left-neighboring video block that is less than an index of an intra-mode associated with the above-neighboring block.

7. The method of claim 1, wherein determining the MPMs comprises determining an intra-mode associated with a left-neighboring video block of the current block and an above-neighboring video block of the current block, and wherein generating the list of MPMs comprises assigning an index to an intra-mode associated with the above-neighboring block that is less than an index of an intra-mode associated with the left-neighboring video block.

8. The method of claim 1, wherein assigning an index to each of the MPMs comprises assigning an index to an MPM based on statistics associated with a likelihood that the MPM matches the intra-mode used to predict the current block.

9. The method of claim 1, further comprising encoding data representing an order in which indices are assigned to the MPMs.

10. The method of claim 1, wherein the MPMs comprise more than two MPMs associated with more than two reference blocks.

11. The method of claim 1, wherein determining MPMs for predicting the current block of video data further comprises:

identifying one or more blocks that cannot be used for reference during intra coding; and

assigning a default intra mode to the one or more blocks.

12. The method of claim 11, wherein the default mode is a planar intra mode.

13. The method of claim 11, wherein the default mode is a DC intra mode.

14. A method of encoding video data, the method comprising:

determining an intra-mode for predicting a current block of video data;

determining a Most Probable Mode (MPM) for predicting the current block of video data to generate a list of MPMs, wherein the MPMs are intra-modes associated with respective neighboring blocks of video data encoded prior to the current block;

assigning an index to each of the MPMs in the list based on an order in which the neighboring blocks are encoded, without reordering the MPMs in the list based on the mode indices of the MPMs; and

encoding intra-mode data for the current block comprising encoding data representing the index of the matching MPM in the list in an encoded bitstream when one of the MPMs matches the intra-mode used to predict the current block.

15. An apparatus for encoding video data, the apparatus comprising:

a memory configured to store a current block of video data; and

one or more processors configured to perform the steps of:

determining an intra-mode for predicting a current block of video data;

determining a Most Probable Mode (MPM) for predicting the current block of video data to generate a list of MPMs, wherein the MPMs are intra-modes associated with respective neighboring blocks of video data encoded prior to the current block;

assigning an index to each of the MPMs in the list based on an order in which the intra-mode for predicting the current block is compared to the MPMs, without reordering the MPMs in the list based on the mode indices of the MPMs; and

encoding intra-mode data for the current block comprising encoding data representing the index of the matching MPM in the list in an encoded bitstream when one of the MPMs matches the intra-mode used to predict the current block.

16. The apparatus of claim 15, wherein to encode intra-mode data for the current block, when one of the MPMs does not match the intra-mode used to predict the current block, the one or more processors are further configured to:

encoding an MPM flag indicating that an intra-mode used to predict the current block is not an MPM;

generating an intra mode modified list; and

encoding data representing an index of the intra-mode for the current block in a modified list.

17. The apparatus of claim 16, wherein to generate a modified list, the one or more processors are configured to eliminate the MPM from a modified list of intra-modes.

18. The apparatus of claim 16, wherein to generate a modified list, the one or more processors are configured to sort intra-modes of a modified list in ascending order according to mode values of the intra-modes.

19. The apparatus of claim 15, the one or more processors further configured to encode an MPM flag prior to encoding the index of the matching MPM, the MPM flag indicating that data representing an index of the matching MPM is present in an encoded bitstream.

20. The apparatus of claim 15, wherein to determine the MPMs, the one or more processors are configured to determine intra-modes associated with a left-neighboring video block of the current block and an above-neighboring video block of the current block, and wherein to assign an index to each of the MPMs, the one or more processors are configured to assign an index to an intra-mode associated with the left-neighboring video block that is less than an index of an intra-mode associated with the above-neighboring block.

21. The apparatus of claim 15, wherein to determine the MPMs, the one or more processors are further configured to determine intra-modes associated with a left-neighboring video block of the current block and an above-neighboring video block of the current block, and wherein to assign an index to each of the MPMs, the one or more processors are configured to assign an index to an intra-mode associated with the above-neighboring block that is less than an index of an intra-mode associated with the left-neighboring video block.

22. The apparatus of claim 15, wherein to assign an index to each of the MPMs, the one or more processors are configured to assign an index to an MPM based on statistics associated with a likelihood that the MPM matches the intra-mode used to predict the current block.

23. The apparatus of claim 15, wherein the one or more processors are further configured to encode data representing an order in which indices are assigned to the MPMs.

24. The apparatus of claim 15, wherein the MPMs comprise more than two MPMs associated with more than two reference blocks.

25. The apparatus of claim 15, wherein to determine MPMs for predicting the current block of video data, the one or more processors are further configured to perform the steps of:

identifying one or more blocks that cannot be used for reference during intra coding; and

assigning a default intra-mode to the one or more blocks.

26. The apparatus of claim 25, wherein the default mode is a planar intra mode.

27. The apparatus of claim 25, wherein the default mode is a DC intra mode.

28. The apparatus of claim 15, further comprising a camera configured to capture the current block, wherein the apparatus comprises a video encoder, and wherein the one or more processors are further configured to:

predicting the current block using the determined intra mode to generate a reference video block;

determining a residual block comprising a difference between the reference block and the current block; and

encoding data representing the residual block in an encoded bitstream.

29. An apparatus for encoding video data, the apparatus comprising:

a memory configured to store a current block of video data; and

one or more processors configured to perform the steps of:

determining an intra-mode for predicting a current block of video data;

determining a Most Probable Mode (MPM) for predicting the current block of video data to generate a list of MPMs, wherein the MPMs are intra-modes associated with respective neighboring blocks of video data encoded prior to the current block;

assigning an index to each of the MPMs in the list based on an order in which the neighboring blocks are encoded, without reordering the MPMs in the list based on the mode indices of the MPMs; and

encoding intra-mode data for the current block comprising encoding data representing the index of the matching MPM in the list in an encoded bitstream when one of the MPMs matches the intra-mode used to predict the current block.

30. An apparatus for encoding video data, the apparatus comprising:

means for determining an intra-mode for predicting a current block of video data;

means for determining a Most Probable Mode (MPM) for predicting the current block of video data to generate a list of MPMs, wherein the MPMs are intra-modes associated with respective neighboring blocks of video data encoded prior to the current block;

means for assigning an index to each of the MPMs in the list based on an order in which the intra-mode for predicting the current block is compared to the MPMs, without reordering the MPMs in the list based on the mode indices of the MPMs; and

means for encoding data representing the index of the matching MPM in the list in an encoded bitstream when one of the MPMs matches the intra-mode used to predict the current block.

31. A method of decoding video data, the method comprising:

determining a Most Probable Mode (MPM) for predicting a current block of video data to generate a list of MPMs, wherein the MPMs are intra-modes associated with respective neighboring blocks of video data decoded prior to the current block;

assigning an index to each of the MPMs in the list based on an order in which the intra-mode for predicting the current block is compared to the MPMs, without reordering the MPMs in the list based on the mode indices of the MPMs;

decoding intra-mode data for the current block, including decoding data representing the index from the matching MPM in the list of encoded bitstreams when one of the MPMs matches an intra-mode for the current block;

identifying the intra-mode for predicting the current block using the index; and

decoding the current block at the identified intra-mode for the current block.

32. The method of claim 31, wherein decoding intra-mode data of the current block comprises, when one of the MPMs does not match the intra-mode used to predict the current block,

decoding an MPM flag indicating that an intra-mode used to predict the current block is not an MPM;

generating an intra mode modified list; and

decoding data representing an index of the intra-mode for the current block in a modified list.

33. The method of claim 32, wherein generating a modified list comprises eliminating the MPM from a modified list of intra-modes.

34. The method of claim 32, wherein generating a modified list comprises sorting intra-modes in a modified list in ascending order according to mode values of the intra-modes.

35. The method of claim 31, wherein determining the MPMs comprises determining intra-modes associated with a left-neighboring video block of the current block and an above-neighboring video block of the current block, and wherein assigning an index to each of the MPMs comprises assigning an index to an intra-mode associated with the left-neighboring video block that is less than an index of an intra-mode associated with the above-neighboring block.

36. The method of claim 31, wherein determining the MPMs comprises determining intra-modes associated with a left-neighboring video block of the current block and an above-neighboring video block of the current block, and wherein assigning an index to each of the MPMs comprises assigning an index to an intra-mode associated with the above-neighboring block that is less than an index of an intra-mode associated with the left-neighboring block.

37. The method of claim 31, wherein assigning an index to each of the MPMs comprises assigning an index to an MPM based on statistics associated with a likelihood that the MPM matches the intra-mode used to predict the current block.

38. The method of claim 31, further comprising decoding data representing an order in which indices are assigned to the MPMs.

39. A method of decoding video data, the method comprising:

determining a Most Probable Mode (MPM) for predicting a current block of video data to generate a list of MPMs, wherein the MPMs are intra-modes associated with respective neighboring blocks of video data decoded prior to the current block;

assigning an index to each of the MPMs in the list based on an order in which the neighboring blocks are decoded without reordering the MPMs in the list based on the mode indices of the MPMs;

decoding intra-mode data for the current block, including decoding data representing the index from the matching MPM in the list of encoded bitstreams when one of the MPMs matches an intra-mode for the current block;

identifying the intra-mode for predicting the current block using the index; and

decoding the current block at the identified intra-mode for the current block.

40. An apparatus for decoding video data, the apparatus comprising:

a memory configured to store a current block of video data; and

one or more processors configured to:

determining a Most Probable Mode (MPM) for predicting a current block of video data to generate a list of MPMs, wherein the MPMs are intra-modes associated with respective neighboring blocks of video data decoded prior to the current block;

assigning an index to each of the MPMs in the list based on an order in which the intra-mode for predicting the current block is compared to the MPMs, without reordering the MPMs in the list based on the mode indices of the MPMs;

decoding intra-mode data for the current block, including decoding data representing the index from the matching MPM in the list of encoded bitstreams when one of the MPMs matches an intra-mode for the current block;

identifying the intra-mode for predicting the current block using the index; and

decoding the current block at the identified intra-mode for the current block.

41. The apparatus of claim 40, wherein to decode intra-mode data, the one or more processors are further configured to, when one of the MPMs does not match the intra-mode used to predict the current block,

decoding an MPM flag indicating that an intra-mode used to predict the current block is not an MPM;

generating an intra mode modified list; and

decoding data representing an index of the intra-mode for the current block in a modified list.

42. The apparatus of claim 41, wherein to generate a modified list, the one or more processors are configured to eliminate the MPMs from a modified list of intra-modes.

43. The apparatus of claim 41, wherein to generate a modified list, the one or more processors are configured to sort intra-modes in a modified list in ascending order according to mode values of the intra-modes.

44. The apparatus of claim 40, wherein to determine the MPMs, the one or more processors are configured to determine intra-modes associated with a left-neighboring video block of the current block and an above-neighboring video block of the current block, and wherein to assign an index to each of the MPMs, the one or more processors are configured to assign an index to an intra-mode associated with the left-neighboring video block that is less than an index of an intra-mode associated with the above-neighboring video block.

45. The apparatus of claim 40, wherein to determine the MPMs, the one or more processors are configured to determine intra-modes associated with a left-neighboring video block of the current block and an above-neighboring video block of the current block, and wherein to assign an index to each of the MPMs, the one or more processors are configured to assign an index to an intra-mode associated with the above-neighboring block that is less than an index of an intra-mode associated with the left-neighboring video block.

46. The apparatus of claim 40, wherein to assign an index to each of the MPMs, the one or more processors are configured to assign an index to an MPM based on statistics associated with a likelihood that the MPM matches the intra-mode used for predicting the current block.

47. The apparatus of claim 40, wherein the one or more processors are further configured to decode data representing an order in which indices are assigned to the MPMs.

48. The apparatus of claim 40, wherein the apparatus comprises a video decoder, and wherein to decode the current block, the one or more processors are further configured to:

obtaining, from an encoded bitstream, a residual video block associated with the current block;

generating a reference block by predicting the current block using the identified intra-mode for the current block; and

determining a value for the current block from a combination of the reference block and a received residual video block,

wherein the apparatus further comprises a display configured to display the current block.

49. An apparatus for decoding video data, the apparatus comprising:

a memory configured to store a current block of video data; and

one or more processors configured to:

determining a Most Probable Mode (MPM) for predicting a current block of video data to generate a list of MPMs, wherein the MPMs are intra-modes associated with respective neighboring blocks of video data decoded prior to the current block;

assigning an index to each of the MPMs in the list based on an order in which the neighboring blocks are decoded without reordering the MPMs in the list based on the mode indices of the MPMs;

decoding intra-mode data for the current block, including decoding data representing the index from the matching MPM in the list of encoded bitstreams when one of the MPMs matches an intra-mode for the current block;

identifying the intra-mode for predicting the current block using the index; and

decoding the current block at the identified intra-mode for the current block.

50. An apparatus for decoding video data, the apparatus comprising:

means for determining a Most Probable Mode (MPM) for predicting a current block of video data to generate a list of MPMs, wherein the MPMs are intra-modes associated with respective neighboring blocks of video data decoded prior to the current block;

means for assigning an index to each of the MPMs in the list based on an order in which the intra-mode for predicting the current block is compared to the MPMs, without reordering the MPMs in the list based on the mode indices of the MPMs;

means for decoding intra-mode data for the current block when one of the MPMs matches an intra-mode of the current block, comprising decoding data representing the index from the matching MPM in the list of encoded bitstreams;

means for identifying the intra-mode for predicting the current block using the MPM index; and

means for decoding the current block at the identified intra-mode for the current block.