CN1078795C - Improved motion compensation method for use in image encoding system - Google Patents

Abstract

To eliminate blocking phenomenon by dividing a search block into a center area and other peripheral parts and generating an optimum motion vector using a motion vector about peripheral parts. A motion detector 200 receives a present frame signal and the previous frame signal, detects a motion vector of the present frame with a block matching method and stores it in a memory 210. An area formation block 213 divides a search block which consists of 16&times 16 pixels into a center area of 12&times 12 pixels and an edge and corner areas other than the center area, and sends area information indicating position of each area to the optimum motion vector deciding block 216. The optimum motion vector deciding block 216 generates the optimum motion vector from a motion vector of the present frame and a motion vector of the search block adjacent to the other and sends it to a motion compensator 219. In the edge and corner areas, the mean value of the motion vector of adjacent search block on the basis of the area information is regarded as the optimum motion vector.

Description

An Improved Motion Compensation Method Used in Image Coding System

The present invention relates to a motion compensation method used in an image coding system; more particularly, to an improved method capable of eliminating blocking phenomena occurring in a decoded image signal.

It is well known that the transmission of digitized video signals can provide much higher quality video images than the transmission of analog signals. When an image signal comprising a sequence of image "frames" is represented in digital form, large amounts of data are generated for transmission, especially in the case of high definition television systems. However, since the available frequency bandwidth of a traditional transmission channel is limited, in order to transmit a large amount of digital data through the limited channel bandwidth, it is inevitable to compress or reduce the amount of transmitted data. Among various video compression techniques, a so-called hybrid coding technique that combines temporal and spatial compression techniques with statistical coding techniques is known to be the most effective.

Most hybrid coding techniques employ motion compensated DPCM9 (Differential Pulse Code Modulation), two-dimensional DCT (Discrete Cosine Transform), quantization of DCT coefficients, and VLC (Variable Length Coding). Motion-compensated DPCM is the process of determining the motion of an object between a current frame and its previous frame, and predicting the current frame from the motion flow of the object to generate a differential signal representing the difference between the current frame and its prediction.

Two-dimensional DCT, which reduces or eliminates spatial redundancy between image data such as motion compensated DPCM data, converts a block of digital image data (eg, a block of 8x8 pixels) into a set of transform coefficient data.

Specifically, in motion-compensated DPCM, current frame data is predicted from previous frame data based on a motion estimate between the current and previous frames. This estimated motion can be described by a two-dimensional motion vector representing the pixel displacement between the previous and current frames.

Several methods have been proposed for estimating the displacement of an object in a video sequence. In general, they can be divided into two categories: pixel recursive algorithms and block matching algorithms. The present invention primarily considers block matching algorithms.

According to the block matching algorithm, a current frame is divided into multiple search blocks. The size of a search block is usually between 8x8 and 32x32 pixels. To determine the motion vector for a search block in the current frame, a similarity is performed between the search block in the current frame and each of a plurality of candidate blocks of the same size contained in a usually larger search area in the previous frame. sexual calculation. The similarity measurement between the search block of the current frame and each candidate block in the search area is performed using an error function such as mean absolute error or mean square error. By definition, a motion vector represents the displacement between the search block and a candidate block that yields the smallest error function.

The encoded image data is transmitted through a conventional transmission channel to an image signal decoder included in an image signal decoding system, which performs the inverse process of the encoding operation, thereby reconstructing the original image data . However, reconstructed image data often exhibits an unpleasant artifact known as blocking, in which the boundary lines of a block become visible at the receiving end. This blocking effect occurs because a frame is coded in blocks.

Therefore, a main object of the present invention is to provide a motion compensation method used in an image coding system, which can eliminate the blocking effect occurring on the boundary of a block of the image signal, thereby improving the performance generated by the system. image quality.

According to the present invention, there is provided a method of determining the best motion vector between a current frame of a video signal and a previous frame, wherein the current frame is divided into a plurality of search blocks of equal size, while the previous frame contains A corresponding number of search areas, each search area has a plurality of candidate blocks of equal size, the method comprises the following steps:

(a) using a block matching algorithm to detect a motion vector for each search block in the current frame;

(b) dividing a search block into a central area located in the center of the search block and a border area located outside the central area;

(c) determining the motion vector of the search block as an optimal motion vector for the central area,

(d) Determining a plurality of optimal motion vectors for the border region of the search block based on the motion vectors of the search block and one or more adjacent search blocks.

From the following description of the preferred embodiment given in conjunction with the accompanying drawings, the above-mentioned and other objects and features of the present invention will be apparent, in the accompanying drawings:

Fig. 1 is a block diagram providing an image signal encoding system employing the motion compensation device of the present invention;

Figure 2 shows a detailed block diagram of the motion compensation device of Figure 1; and

FIG. 3 is a diagram illustrating formation of a margin region performed in the motion compensation device of the present invention.

Referring to FIG. 1, there is shown a block diagram of an encoding apparatus 10 for compressing a digital video signal comprising a plurality of frames of video signal, which includes the motion compensation apparatus 150 of the present invention.

Encoding device 10 comprises a first frame memory 100, a subtractor 102, an image signal encoder 105, an entropy encoder 107, an image signal decoder 113, an adder 115, a second frame memory 124 and The motion compensation device 150 .

A current frame contained in an input video signal is stored in a first frame memory 100 which is connected via line L9 to subtractor 102 and via line L10 to motion compensation means 150 . The stored current frame is read on a block-by-block basis, where block sizes are typically in the range of 8x8 and 32x32 pixels.

The motion compensation device 150 of the present invention firstly detects a motion vector for each search block of the current frame by using a conventional block matching algorithm, and the motion vector represents the relationship between each search block in the current frame and the previous frame from the second frame memory 124. The spatial displacement between the most similar candidate blocks in a corresponding search area of a frame; and according to the motion vectors of the search block and its adjacent search blocks, determine how many best motion vector. Thereafter, the motion compensation means 150 uses the determined optimal motion vector to retrieve the corresponding pixel values of the previous frame from the second frame memory 124, thereby providing a predicted current frame. The motion vector of the search block and the predicted current frame signal are respectively fed to the entropy encoder 107 , the subtractor 102 and the adder 113 . Details of the motion compensation device 150 are described below with reference to FIGS. 2 and 3 .

The predicted current frame signal from the motion compensation device 150 is subtracted from the current frame signal provided by the line L9 at the subtractor 102, and the result of the difference between the pixel values representing the current frame and the predicted current frame is obtained The output data, that is, the error signal, is sent to the image signal encoder 105, where the error signal is coded into sets of quantized transform coefficients using, for example, DCT and any known quantization method.

Thereafter, the quantized transform coefficients are transmitted to the entropy encoder 107 and the image signal decoder 113 . The image signal decoder 113 utilizes inverse quantization and inverse discrete cosine transform to convert the quantized transform coefficient from the image signal encoder 105 back into a reconstructed error signal. The received reconstructed error signal and the predicted current frame signal provided via line L30 from the motion compensation means 150 are combined to provide a reconstructed current frame signal for storage in the second frame memory 124 as a previous frame signal .

In the entropy encoder 107, the quantized transform coefficient supplied from the image signal encoder 105 and the motion vector sent from the motion compensation device 150 through the line L20 are coded together by a variable length coding technique, for example. Thereafter, the encoded signal is provided to a transmitter (not shown) for transmission.

Referring now to FIG. 2 , details of the motion compensation device 150 shown in FIG. 1 are shown. The motion compensation device 150 includes a motion estimator 209 , a memory 210 , a region forming unit 213 , an optimal motion vector determining unit 216 and a motion compensator 219 .

First, the motion estimator 209 retrieves the current frame signal from the first frame memory 100 and the previous frame signal from the second frame memory 124, and detects a signal representing the current frame by using conventional block matching techniques well known in the art. The motion vector of the spatial displacement between each search block and the most similar block in the previous frame. The motion vectors of the respective search areas from the motion estimator 209 are supplied to the entropy encoder 107 shown in FIG. 1 and the memory 210 in which the motion vectors of the search blocks of the current frame are stored.

At the same time, the current frame signal is supplied to the area forming unit 213, in which each search block is divided into a border area and a center area. In FIG. 3, a region forming scheme performed on a region forming unit 213 according to a preferred embodiment of the present invention is shown. As shown in FIG. 3, each search block such as SB5 of 16*16 pixels is divided into a central area CR5 located in the center of the search block SB5 of, for example, 12*12 pixels, and a central area CR5 located outside the central area CR5. A border region composed of pixels, wherein the border region includes four border regions ER5-1 to ER5-4 each consisting of 2×12 pixels and four corner regions CR5-4 each consisting of 2×2 pixels 1 to CR5-4. The region information representing the positions of the center, side and corner regions in the current frame is supplied to the optimum motion vector determination unit 216 .

The optimal motion vector determining unit 216 retrieves the motion vectors of the search blocks in the current frame from the memory 210, and responds to the area information provided by the area forming unit 213, and uses the motion vectors of each search block and its adjacent search blocks for each search block. The region of the block determines the best motion vector. Specifically, an edge region ER5-1 located along a border between two search blocks, SB5 and SB2, is obtained by averaging the motion vectors of the two search blocks such as SB5 and SB2. Best motion vector. Similarly, the optimum motion vectors of the border regions ER5-2 to ER5-4 are determined by averaging the motion vectors of the two search blocks SB5 and SB4, SB5 and SB6, and SB5 and SB8, respectively. On the other hand, an optimum motion vector for a corner area is obtained by obtaining an average value of a motion vector of a search block including a corner area and a vector of a search block connected to the search block on the corner area. For example, the average value of the motion vectors of the search blocks SB1, SB2, SB4 and SB5 is obtained to obtain the optimal motion vector of the corner region CR5-1. Likewise, the optimum motion vectors of the corner regions CR5-2 to CR5-4 are determined based on the motion vectors of the search blocks SB2 to SB9. As for the optimum motion vector of a central area such as CR5, the motion vector of the search block SB5 containing this central area CR5 is designated as its optimum motion vector.

In this way and according to the present invention, any corner regions interconnected such as CR1-1, CR2-2, CR4-1, and CR5-1 have The same optimal motion vector is determined by the average of the motion vectors of SB5), while any connected border regions such as ER2-1 and ER5-1 share a search block (such as SB2 and SB5) that contains these border regions. ) The best motion vector calculated by the average of the motion vectors.

The best motion vectors of the search blocks of the current frame are then supplied to a motion compensator 219 .

Motion compensator 219 utilizes the optimal motion vector provided by optimal motion vector determination unit 216 to retrieve each pixel value from second frame memory 124 shown in Fig. 1, thereby provides the current frame signal of prediction by line L30 to the subtractor 102 and adder 115 shown in FIG. 1 .

In a digital video signal decoding device corresponding to the encoding device 10 of the present invention, based on the motion vector transmitted from the encoding device 10, in a manner similar to that described with respect to the region forming unit 213 and the optimal motion vector determining unit, reconstruction An optimal motion vector is used, whereby the current frame signal can be reconstructed according to the transmitted coded signal representing the error signal and the motion vector.

Although the invention has been described with respect to particular embodiments, it will be apparent to those skilled in the art that various changes and modifications can be made without departing from the scope of the invention as defined in the following claims.

Claims (5)

1. A method for determining an optimal motion vector between a current frame of a video signal and a previous frame, wherein the current frame is divided into a plurality of search blocks of equal size, and the previous frame contains corresponding number of search areas, each search area has a plurality of candidate blocks of equal size, the method comprises the steps of: (a) using a block matching algorithm to detect a motion vector for each search block in the current frame; (b) dividing a search block into a central area located in the center of the search block and an edge area located outside the central area; (c) determining the motion vector of the search block as the best motion vector for the central region; and (d) Determining a plurality of optimal motion vectors for the border region of the search block based on the motion vectors of the search block and one or more adjacent search blocks. 2. A method according to claim 1, wherein the optimum motion vector for the border region is determined by averaging the motion vectors of the search block and one or more adjacent search blocks. 3. The method according to claim 1, wherein the border region of the search block comprises four border regions and four corner regions, wherein each corner region is located at a corner of the search block, and each border region is located between two corner regions Each corner area has three adjacent search blocks, and the edge area has one adjacent search block. 4. According to the method of claim 3, an optimal motion vector of each corner area is determined according to its three adjacent search blocks and four motion vectors of the search block, and an optimal motion vector of each side area The best motion vector is determined according to its adjacent search block and the two motion vectors of the search block. 5. According to the method of claim 4, the optimal motion vectors of said corner regions are determined by finding the average value of said four motion vectors, and the optimal motion vectors of said edge regions are determined by obtaining the average value of said four motion vectors. determined by taking the average of the two motion vectors.

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