CN114387356A - A PNG format encoding and decoding method and device - Google Patents

Abstract

The invention belongs to the technical field of image processing, and particularly relates to a PNG format encoding and decoding method and device; reading target pixels in an original picture, and calculating the bit number of each target pixel and the bit numbers of different channels in each target pixel; identifying that the color type of the target pixel corresponds to a preset definition value; judging the size of a defined value of a color type to be coded, and calculating the bit depth of pixels of different color types according to different conditions; determining the bit depth of each pixel when encoding is needed, and calculating the size of the image; partitioning the image, and presetting a compression algorithm in an image header block and sampling the calculated image down by adopting a scanning line pair; and filtering the scanning line data, compressing the scanning line data, and encoding the PNG data stream. The invention adds the codes of true color images with unequal bit depths and can realize the conversion among various color types.

Description

A PNG format encoding and decoding method and device

technical field

The invention belongs to the technical field of image processing, and in particular relates to a PNG format encoding and decoding method and device.

Background technique

In recent years, MCU chips have been widely used in smart wear, industrial control, Internet of Things control, medical electronics, etc. Can be more beautiful and gorgeous. The MCU itself is limited by the main frequency, the image processing capability, the limited size of the Memory, and the interface rate. In particular, the MCU widely uses the SPI interface screen, which is limited by the SPI interface rate limit, which makes the pixel data transmitted between the MCU and the screen. Quantity has higher requirements. made a request. When the color screen is displayed in the MCU, the color mode of RGB565 can reduce the amount of data transmission between the MCU and the color screen by 1/3 compared with RGB888, so RGB565 is widely used in MCU products.

In MCU products, the PNG format is usually used for image storage. Due to its high compression rate for images, according to statistics, at least half of the memory space can be saved, or even higher. Of course, if the PNG format is used for image storage, the MCU needs to decode the PNG image, so the processing capability of the MCU is required. This bottleneck can generally be solved by designing a PNG hardware accelerator, which will not be discussed further here. .

PNG is a bitmap format that uses a lossless compression algorithm. Its design purpose is to try to replace the GIF and TIFF file formats, while adding some features that the GIF file format does not have. PNG uses a lossless data compression algorithm derived from LZ77. Due to its high compression ratio and small file size, PNG is generally used in JAVA programs, web pages or S60 programs. The PNG format mainly has three forms: 8-bit, 24-bit, and 32-bit. Among them, 8-bit PNG supports two different forms of transparency (index transparency and alpha transparency), 24-bit PNG does not support transparency, and 32-bit PNG is based on 24-bit. An 8-bit transparency channel has been added, so 256 levels of transparency can be displayed.

The encoding process of PNG includes three processes: filter (filtering), deflate (compression), and chunk (data block); the decoding process of PNG includes chunk (block) decoding, inflate (decompression), and un-filter (filtering restoration). ) 3 processes. Among them, the filtering is lossless, and the filtering is performed in units of bytes. The filtering process itself does not reduce the size of the data volume, but the compression capability of deflate can be improved through filtering; the deflate process includes LZ77 compression and Huffman encoding for compressed data. process, which also operates on the data itself. When PNG is defined by the standard, it only supports pixel images with the same color bit depth. For example, it can support several formats with color bit depths of 1, 2, 4, 8, and 16; The bit depth is inconsistent, so the PNG encoding process does not support RGB565 and RGB565A formats.

On the other hand, when PNG is used on the MCU, if RGB565 is used for sending the screen to the screen, it is required that PNG directly supports RGB565 and RGB565A. Then, in order to save the storage space of PNG images, it avoids one more decoding time. RGB888 (or RGB888A) to RGB565 (or RGB565A) mapping. Therefore, it is necessary to study the PNG image format and compression method to realize the PNG encoding and decoding method that supports the RGB format with inconsistent color bit depth.

SUMMARY OF THE INVENTION

Based on the problems existing in the prior art, the present invention proposes an encoding and decoding method and device in PNG format; considering that the color mode of RGB565 occupies 16 bits, compared with 24 bits of RGB888, the data transmission between the MCU and the color screen is Therefore, the present invention further studies the PNG image format and its compression method, and provides a coding and decoding scheme that enables PNG images to directly support RGB565 and other formats.

Specifically, the present invention is realized by the following scheme:

In a first aspect of the present invention, the present invention provides an encoding method in PNG format, the encoding method comprising the following steps:

Read out the target pixel in the original image, and calculate the number of bits of the current target pixel and the number of bits of different channels in each target pixel;

Identify the color type of the current target pixel and correspond to the preset defined value; set the allowed bit depth in the encoding process to point to the defined value; wherein the color type at least includes grayscale image, true color image, indexed color image, band Grayscale image with alpha channel data, true color image with alpha channel data, true color image with unequal bit depth, and true color image with unequal bit depth with alpha channel data;

Determine the size of the defined value of the color type to be encoded, and calculate the bit depth in the pixel according to the first condition for true color images with unequal bit depths and true color images with unequal bit depths with alpha channel data; The image of the color type calculates the bit depth in the pixel according to the second condition;

Determine the bit depth of each pixel that needs to be encoded, calculate the size of the image, divide the image into blocks, and preset the compression algorithm in the image header block to downsample the calculated image by scanning lines;

After filtering the scan line data, it is compressed according to the preset compression algorithm, the image data block is encoded, and the image end block is encoded at the end of the image data block, thereby completing the encoding of the PNG image.

Further, before judging the size of the definition value of the color type to be encoded, the target pixel in the read out original picture is also included to perform color type conversion, that is, from grayscale image, true color image, indexed color image, band alpha image. A grayscale image with channel data, a true color image with alpha channel data, a true color image with unequal bit depth, and a true color image with unequal bit depth with alpha channel data are converted from any color type to another with Different color types before; and redefine the definition value of the new color type.

The identifying the color type of the target pixel and corresponding to the preset definition value includes assigning a grayscale image, a true color image, an indexed color image, a grayscale image with alpha channel data, and a true color image with alpha channel data [ an integer value between 0, 6], assigning an integer value between [7, 12] for true color images of unequal bit depth and true color images of unequal bit depth with alpha channel data; and the defined value Occupies one byte of space.

Further, setting the allowable bit depth in the encoding process to point to the defined value includes setting multiple values for grayscale images, true color images, indexed color images, grayscale images with alpha channel data, and true color images with alpha channel data. an optional third bit depth, and the bit depth occupied by each channel is equal; set a fixed bit depth for true color images with unequal bit depths and true color images with unequal bit depths with alpha channel data, the The fixed bit depth includes a first bit depth and a second bit depth; the first bit depth is the bit depth occupied by one of the color channels in the color type, and the second bit depth is the bit depth occupied by a pixel in the color type the total bit depth.

Further, the preset first function is expressed as f(x)=3A+α, A represents the bit depth of the channel, and α represents the bit depth occupied by the α channel; the value of the adjustable parameter is ±1 or ±1. 2.

On the basis of the first aspect of the present invention, in the second aspect of the present invention, the present invention provides a decoding method in PNG format corresponding to the first aspect, the decoding method comprising the following steps:

Read out the PNG image, read out the image header block, the image data block and the image end block;

According to the compression algorithm read from the image header block, the image data block is decompressed and filtered to restore;

The color type is determined according to the defined value read from the image header block, and the color type includes grayscale image, true color image, indexed color image, grayscale image with alpha channel data, and true color with alpha channel data. images, true color images of unequal bit depth, and true color images of unequal bit depth with alpha channel data;

According to the allowable bit depth of the color type read from the image header block, for the true color image of unequal bit depth and the true color image of unequal bit depth with alpha channel data, the third condition is used to calculate the number of pixels in the pixel. Bit depth; for images of other color types, the bit depth in the pixel is calculated according to the second condition;

The image information data is decoded from the PNG image data block according to the bit depth of each pixel;

After the decoding of the information of the image data block is completed, the decoded information of the image header block and the image end block are respectively loaded into the information before and after the image block to complete the decoding of the PNG image.

On the basis of the first aspect of the present invention, in the third aspect of the present invention, the present invention also provides an encoding device implementing the PNG format of the first aspect, the encoding device comprising:

a first collection unit, used for collecting target pixels in the original picture;

The first identification unit is used for identifying the color type of the target pixel and corresponding to the preset definition value;

An encoding preparation unit, for setting the allowable bit depth in the encoding process to point to the defined value; and for determining the bit depth of each pixel when encoding is required, thereby calculating the image size and determining the content of the image header block;

The encoding unit is configured to perform PNG encoding processing according to the image blocks determined by the encoding preparation unit, and encode the image header block, the image data block and the image end block.

On the basis of the second aspect and the third aspect of the present invention, in the fourth aspect of the present aspect, the present invention also provides a PNG format decoding device corresponding to the third aspect, the encoding device comprising:

The second collection unit is used to collect PNG images;

The second identification unit is used to read out the image header block, the image data block and the image end block from the PNG image; and identify the color type and the allowable bit depth from the image header block;

The decoding preparation unit is used to determine the number of bits of pixels in the PNG data stream and the number of bits of different channels in the pixels according to the color type;

A decoding unit, configured to parse out the information of the PNG image according to the number of bits of the pixel and the number of bits of different channels in the pixel.

Beneficial effects of the present invention:

The present invention expands two color types on the IHDR of PNG to support encoding and decoding of true color images with unequal bit depths and true color images with unequal bit depths with alpha channel data; the present invention can effectively reduce PNG images It saves storage space and speeds up decoding; it is especially suitable for MCU chips and products that support color formats such as RGB565 on the display screen; the method of the present invention is also suitable for extending PNG’s support for other color types, and can be easily realized Exchange between various color types; meanwhile, the method of the present invention can also be compatible with the original PNG color type format.

Description of drawings

Fig. 1 is the encoding method flow chart of PNG format of the present invention;

Fig. 2 is the decoding method flow chart of PNG format of the present invention;

Fig. 3 is the structure diagram of the encoding device of PNG format of the present invention;

FIG. 4 is a structural diagram of a decoding apparatus in PNG format according to the present invention.

Detailed ways

The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, but not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the protection scope of the present invention.

1 is a flowchart of an encoding method in PNG format in an embodiment of the present invention, and the encoding method includes the following steps:

S101, read out the target pixels in the original picture, and calculate the number of bits of each target pixel and the number of bits of different channels in each target pixel;

In this embodiment, the original picture may be a natural picture or an unnatural picture, which is classified based on the way of generating the picture. , which are usually pictures generated by human participation or automatically generated by devices, such as screenshots of instant messaging interface, game pictures, etc.; screenshots and game pictures of instant messaging interface may include both the part that people participate in and the part that is automatically generated by the device; More specifically, take the game screen as an example: in real-time battle games, players will use text to communicate, and the text part is the part that people participate in; the other parts are drawn by the equipment according to computer program instructions or single-chip control instructions. image content. It can be understood that, in addition to the screenshots of the instant messaging interface, there can also be screenshots of the entire desktop or other application interfaces. In addition to the game screen, it can also be any other pictures drawn by the computer. The above examples of natural pictures and unnatural pictures are not. It should be understood as a unique limitation to the embodiments of the present invention. In addition, in this embodiment, the original picture is the picture input to the MCU, and the format of the picture can be any picture format that the MCU can support; for example, BMP, JPEG, PNG and GIF format, or other formats.

S102, identify the color type of the target pixel and correspond to the preset definition value; set the allowable bit depth in the encoding process to point to the definition value;

The color types include at least grayscale images, true color images, indexed color images, grayscale images with alpha channel data, true color images with alpha channel data, true color images with unequal bit depths, and alpha channel data. Truecolor images of unequal bit depths;

For a PNG file, the standard file structure includes PNG file logo and PNG data block; PNG defines two types of data blocks, one is called critical data block (critical chunk), which is a standard data block, There is also ancillary chunks called ancillary chunks, which are optional chunks. Important data blocks define 4 standard data blocks, each PNG file must include them, and PNG read and write software must also support these data blocks. For the sake of simplicity, in the PNG file we use, the description of the PLTE palette data block is omitted; the three data blocks of the IHDR file header data block, the IDAT image data block and the IEND image end block are set in the above order. , and both appear only once.

The present invention expands and improves the IHDR file header data block (image header block), so as to realize encoding and decoding of pictures, pixels and data streams of different color types; it includes the images stored in the PNG file. The basic information of the data, and appears as the first data block in the PNG data stream, and there can only be one file header data block in a PNG data stream. First explain the color types defined in IHDR in the prior art as shown in Table 1:

Table 1 IHDR color types in the definition of the PNG standard

PNG image type color type allowable bit depth Grayscale image 0 1,2,4,8,16 true color image 2 8,16 indexed color image 3 1,2,4,8 Grayscale image with alpha channel data 4 8,16 Truecolor image with alpha channel data 6 8,16

On the basis of the color types defined by the PNG standard, the present invention adds formats for defining two color types, which are used to support true color images with unequal bit depths and true color images with unequal bit depths with alpha channel data, As shown in table 2.

Table 2 IHDR color types in the definition of the PNG standard of the present invention

PNG image type color type allowable bit depth Truecolor images with unequal bit depths 8 2, 3, 5, 6, 8, 16 Truecolor images with unequal bit depths with alpha channel data 12 2, 3, 5, 6, 8, 24

Among them, the color type=8 indicates that the PNG is a true color image with unequal bit depth. Taking the RGB565 format as an example, each pixel occupies 16 bits, the Red channel occupies 5 bits, the Green channel occupies 6 bits, and the Blue channel occupies 5 bits. , the bit depth is set to 5bit; color type = 12 indicates that the PNG is a true color image with unequal bit depth with alpha channel data, especially taking the RGB565A format as an example, at this time, each pixel occupies 24bit, and the Red channel occupies 5bit, The Green channel occupies 6 bits, the Blue channel occupies 5 bits, and the alpha channel occupies 8 bits, and the bit depth can still be set to 5 bits.

S103, determine the size of the definition value of the color type that needs to be encoded, and calculate the bit depth in the pixel according to the first condition to the true color image of unequal bit depth and the true color image of unequal bit depth with alpha channel data; Calculate the bit depth in the pixel according to the second condition for images of other color types;

Since the traditional encoding of images of equal bit depth is to call functions for calculation, for example, in a true color image, when the allowable bit depth is 8bit, this type represents RGB888 format, in order to calculate the encoding byte used in compression Size, the traditional PNG encoding is to directly call the function 3A+α to calculate the pixel size in the RGB888 format, where A represents the allowable bit depth, and α represents the bit depth occupied by the alpha channel; this function is the allowable bit depth multiplied by the number of channels , and the allowed bit depth of the RGB888 format is 8, then the calculated pixel size of the RGB888 format is 3×8+0=24bit, and since the present invention adds a true color image with unequal bit depth and data with α channel For the setting of true color images with unequal bit depths, the pixel size calculated by the traditional function is different from its own size; for the convenience of explanation, RGB565 is used to represent the true color images of unequal bit depths, and RGB565A is used to represent the True color images with unequal bit depths of alpha channel data; it is understandable that true color images with unequal bit depths are not limited to RGB565, but can also be RGB233 and other methods. True color images are also not limited to RGB565A. In order to be compatible with the traditional PNG encoding method, the allowable bit depth set by the present invention for the RGB565 format is 5bit or 6bit; at this time, the pixel bit depth calculated by the calling function is 15bit or 18bit, which is obviously similar to the pixel in the RGB565 format. The bit depth is different; in order to solve the problem that the calling function cannot obtain the true pixel bit depth of a true color image with unequal bit depth or a true color image with unequal bit depth with alpha channel data; the embodiment of the present invention provides at least two ways to To solve the above problem:

In an achievable manner, in order to cooperate with the calling function, the present invention processes the allowable bit depth; when it is judged that the size of the defined value is 8 or 12, then refer to the records in Definition Table 2, RGB565 and RGB565A The allowable bit depth is 5bit; if the color type is equal to 8, the total bit depth of each pixel needs to be calculated by adding 1, that is, the three colors R, G, and B are (15+1)=16bit; if the color type If it is equal to 12, then the total bit depth of the pixel needs to be added by 1 when calculating, then the three colors R, G, B are (15+1)+alpha(8)=24bit; although the pixel encoding is RGB565 (or RGB565A), although The rule that each color is equal is broken, but the traditional filtering and compression standard process can still be used based on the characteristics of image pixel progressiveness and the characteristics of filtering and compression to operate on bytes.

In a preferred embodiment, before judging the size of the definition value of the color type to be encoded, the target pixel in the read original picture is also included to perform color type conversion, that is, from grayscale image, true color image, indexed color image. , grayscale image with alpha channel data, true color image with alpha channel data, true color image with unequal bit depth, and true color image with unequal bit depth with alpha channel data. Convert any color type to another. A different color type from the previous one; and redefines the definition value of the new color type. The conversion functions used for these color type conversions can use existing techniques,

In order to facilitate understanding, for example, in terms of RGB888->RGB565: the specific ideas of its conversion are as follows:

Take the upper 5 bits of the first byte in RGB888 as the upper 5 bits of the second byte of the converted RGB565; take the upper 3 bits of the second byte in RGB888 as the second byte of the converted RGB565 the lower 3 bits; take the 4th to 6th bits of the second byte in RGB888 as the upper 3 bits of the first byte of the converted RGB565; take the third byte of the second byte in RGB888 The upper 5 bits are used as the lower 5 bits of the first byte of the converted RGB565;

Conversely, for the conversion of RGB565->RGB888:

Take the lower 5 bits of the first byte of RGB565 as the upper 5 bits of RGB888; take the lower 3 bits of the second byte of RGB565, move it to the left by 5 bits, as the upper 5 bits of the second byte of RGB888; Take the upper 3 bits of the first byte of RGB565 and shift it to the right by 3 bits as the 4-6 bits of the second byte of RGB888; take the upper 5 bits of the second byte of RGB565 as the third byte of RGB888 .

In a preferred embodiment, the present invention can also use quantization compression and quantization supplement to linearly compensate for the loss of precision, for example, the original data can be filled to high bits; for low bits, the low bits of the original data can be used for compensation; The padded bits continue to use the lower bits of the original data for cyclic compensation.

S104, determine the bit depth of each pixel when encoding is required, calculate the size of the image, divide the image into blocks, and downsample the calculated image by using a scan line to preset a compression algorithm in the image header block;

A PNG image is a rectangular array of pixels, with pixels in each row from left to right, and scan lines from top to bottom. The size of each pixel is determined by the number of bits per pixel.

The pixels of a scanline are always packed into a sequence of bytes, with no extra bits between pixels. Scan lines always start on byte boundaries. The allowed bit depths and color types are limited so that packing is simple and efficient in all cases.

In PNG images, the color is of type 0 (gray) and there is only one single sample per pixel, which may have less than one byte of precision (1, 2, 4 bits). The samples are packed into bytes, with the leftmost sample in the high order of a byte, and the other samples immediately after.

In PNG images, colors are of type 3 (indexed color) and each pixel has only one index into the palette. This indicates that the index is packed into bytes in the same way that the color type is 0.

When a byte has multiple pixels, some low-order bits of the last byte of a scan line may not be used. The contents of these unused bits are not specified.

In the embodiment of the present invention, the non-indexed color image of the PNG image may have a bit depth of 16 bits in sample value. Such sample values are stored and transmitted in network byte order.

PNG allows multi-sampled pixels with only 8-bit or 16-bit sample depth, so multiple samples of a pixel are not packed into a single byte at all.

Specifically, taking the RGB565 format with a bit depth of 16bit as an example, the scan line scans the pixels of the original picture from left to right and from top to bottom. One pixel occupies two bytes, and one scan line includes multiple bytes. Packing and encoding according to the byte sequence.

S105 , compress the scan line data according to a preset compression algorithm after filtering, encode an image data block, and encode an image end block at the end of the image data block, thereby completing the encoding of the PNG image.

PNG allows the scanline's data to be filtered before it is compressed. Filtering can provide the compression ratio of the data.

There is only one filtering method in a filter, but there are multiple filtering types. The filtering method does not affect the size of the data, nor does it affect the loss of any information. The purpose of filtering is only one, to provide better compressed data for the compression method. The filtering step itself makes the resulting sequence of bytes the same size as the input sequence, but in a different representation, depending on the type of filtering performed. Filtering does not reduce the size of the actual scan line data. All filtering is strictly lossless.

In international standards, only compression method 0 is defined. Other values for the compression method are reserved for future standardization. The sequence of filtered scan lines is compressed and the resulting data stream is placed separately into multiple data blocks of the IDAT. The union of the contents of all IDAT data blocks forms a zlib data stream. This data stream is compressed into a filtered PNG image data stream.

On the basis of the above embodiment, FIG. 2 is a flowchart of a decoding method in PNG format of the present invention; the decoding method may include the following steps:

S201, read out the PNG image, read out the image header block, the image data block and the image end block;

First, we read out the PNG image and check whether it is the byte length of a valid PNG file. After the verification is completed, the image header block, image data block and image end block can be read out.

S202, according to the compression algorithm read out from the image header block, decompress the image data block and perform filter restoration;

The compression algorithm read from the image header block decompresses the image data block and performs filter restoration; in order to invert the effect of the filter, the decoding process may need to use the decoded value of the previous pixel on the same line, the previous line The pixel directly above the current pixel and the pixel to the left of the pixel above. This means that the decoding process always needs to store at least one scanline of image data. Even though some filter types do not reference previous scanlines, the decoding process always needs to store each scanline as it is decoded, because the next scanline may use a filter type that references it.

S203, determining the color type according to the defined value;

The color types include grayscale images, truecolor images, indexed color images, grayscale images with alpha channel data, truecolor images with alpha channel data, truecolor images with unequal bit depths, and unequal bit depth images with alpha channel data. Truecolor images of equal depth;

S204, according to the allowable bit depth of the color type read from the image header block, calculate the pixel according to the third condition for the true color image with unequal bit depth and the true color image with unequal bit depth with alpha channel data The bit depth in the pixel; for images of other color types, the bit depth in the pixel is calculated according to the second condition;

The said calculating the bit depth in the pixel according to the third condition includes judging whether the allowable bit depth of the true color image of unequal bit depth and the true color image of unequal bit depth with alpha channel data is greater than or equal to 8, if greater than or equal to 8 , then the allowable bit depth is taken as the bit depth of the pixel, otherwise the first function with an adjustable parameter is called to calculate the bit depth of the pixel;

The calculating the bit depth of the pixel according to the second condition for images of other color types includes calling a preset first function to calculate the size of the bit depth in the pixel according to its allowable depth.

For example, in the decoding process, if the definition value of the read color type is 8, then the color type can be RGB565 or RGB232, etc. In order to determine the specific color type, the present invention first determines the image header block. Whether the allowable bit depth is greater than 8, if it is greater than or equal to 8, according to the settings in Table 2, then the bit depth is 16bit, so directly according to 16bit as the bit depth of the pixel, then the allowable bit depth as the bit depth of the pixel , otherwise call the first function with adjustable parameters to calculate the bit depth of the pixel; for example, if the allowable bit depth is 5bit, then the preset first function needs to be called and expressed as f(x)=3A+α, where A represents The bit depth of the channel, α represents the bit depth occupied by the α channel; at the same time, an adjustable parameter is attached to the preset first function; the value of the adjustable parameter is ±1 or ±2.

S205, decode the image information data from the PNG image data block according to the bit depth of each pixel;

After the bit depth of each pixel is obtained, taking RGB565 as an example, the bit depth of each pixel is 16 bits. In the image data block, the image information data is obtained by taking 16 bits as a pixel.

S206: After the decoding of the information of the image data block is completed, load the decoded information of the image header block and the image end block into the information before and after the image block respectively, and complete the decoding of the PNG image.

Fig. 3 is a kind of encoding device of PNG format provided by the present invention, as shown in Fig. 3, described encoding device comprises:

a first collection unit, used for collecting target pixels in the original picture;

The first identification unit is used for identifying the color type of the target pixel and corresponding to the preset definition value;

An encoding preparation unit, for setting the allowable bit depth in the encoding process to point to the defined value; and for determining the bit depth of each pixel when encoding is required, thereby calculating the image size and determining the content of the image header block;

The encoding unit is configured to perform PNG encoding processing according to the image blocks determined by the encoding preparation unit, and encode the image header block, the image data block and the image end block.

FIG. 4 is a decoding device in PNG format provided by the present invention. As shown in FIG. 4 , the decoding device includes:

The second collection unit is used to collect PNG images;

The second identification unit is used to read out the image header block, the image data block and the image end block from the PNG image; and identify the color type and the allowable bit depth from the image header block;

The decoding preparation unit is used to determine the number of bits of pixels in the PNG data stream and the number of bits of different channels in the pixels according to the color type;

A decoding unit, configured to parse out the information of the PNG image according to the number of bits of the pixel and the number of bits of different channels in the pixel.

It can be understood that some features of the encoding and decoding methods and apparatuses in the PNG format in the present invention can be referred to each other. In order to avoid retrospect, the present invention will not exemplify them one by one.

The present invention extends the definition of true color image with unequal bit depth and true color image with alpha channel data on the traditional PNG format, so that the present invention can not only support traditional color types, but also can It supports encoding and decoding of RGB565, RGB565A and other formats; it is convenient to display on chips such as MCU; the method of the present invention is also suitable for expanding the support of PNG for other color types of image colors, and can easily realize the difference between various color types. At the same time, the method of the present invention can also be better compatible with the original PNG color type format.

In the present invention, unless otherwise expressly specified and limited, terms such as "installation", "arrangement", "connection", "fixation" and "rotation" should be understood in a broad sense, for example, it may be a fixed connection or a It can be a detachable connection, or integrated; it can be a mechanical connection or an electrical connection; it can be directly connected or indirectly connected through an intermediate medium, it can be the internal connection of two elements or the interaction relationship between the two elements, Unless otherwise clearly defined, those of ordinary skill in the art can understand the specific meanings of the above terms in the present invention according to specific situations.

Although embodiments of the present invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, and substitutions can be made in these embodiments without departing from the principle and spirit of the invention and modifications, the scope of the present invention is defined by the appended claims and their equivalents.

Claims (9)

1. an encoding method of PNG format, is characterized in that, described encoding method comprises the following steps: Read out the target pixel in the original image, and calculate the number of bits of the current target pixel and the number of bits of different channels in each target pixel; Identify the color type of the current target pixel and correspond to the preset defined value; set the allowed bit depth in the encoding process to point to the defined value; wherein the color type at least includes grayscale image, true color image, indexed color image, band Grayscale image with alpha channel data, true color image with alpha channel data, true color image with unequal bit depth, and true color image with unequal bit depth with alpha channel data; Determine the size of the defined value of the color type to be encoded, and calculate the bit depth in the pixel according to the first condition for true color images with unequal bit depths and true color images with unequal bit depths with alpha channel data; The image of the color type calculates the bit depth in the pixel according to the second condition; Determine the bit depth of each pixel that needs to be encoded, calculate the size of the image, divide the image into blocks, preset a compression algorithm in the image header block, and use scan lines to downsample the calculated image; After filtering the scan line data, it is compressed according to the preset compression algorithm, the image data block is encoded, and the image end block is encoded at the end of the image data block, thereby completing the encoding of the PNG image. 2. the encoding method of a kind of PNG format according to claim 1 is characterized in that, before judging the size of the definition value of the color type that needs to be encoded, also comprises that the target pixel in the original picture read out carries out the color type Conversion, i.e. from grayscale image, truecolor image, indexed color image, grayscale image with alpha channel data, truecolor image with alpha channel data, truecolor image with unequal bit depth, and unequal with alpha channel data Converts any color type in a bit-depth truecolor image to another color type that is different from the previous one; and redefines the definition value of the new color type. 3. the encoding method of a kind of PNG format according to claim 1 is characterized in that, the bit depth allowed in the described setting encoding process points to this defined value including grayscale image, true color image, indexed color image, band For grayscale images with alpha channel data and true color images with alpha channel data, multiple optional third bit depths are set, and the bit depths occupied by each channel are equal; for true color images with unequal bit depths and for true color images with alpha channels A true color image of unequal bit depth of data is set to a fixed bit depth, and the fixed bit depth includes a first bit depth and a second bit depth; the first bit depth is occupied by one of the color channels in the color type Bit depth, the second bit depth is the total bit depth occupied by a pixel in this color type. 4. the encoding method of a kind of PNG format according to claim 3, is characterized in that, According to the first condition, calculating the pixel bit depth size of true color images with unequal bit depths and true color images with unequal bit depths with alpha channel data includes calling the first function with an additional adjustable parameter according to their respective allowable The first bit depth calculates the bit depth of the pixel, or directly uses the allowed second bit depth as the bit depth of the pixel; Calculating the bit depth of images of other color types according to the second condition includes calling a preset first function to calculate the bit depth size in a pixel according to the allowed third bit depth. A PNG format encoding method according to claim 4, wherein the preset first function is expressed as f(x)=3A+α, A represents the bit depth of the channel, and α represents the occupancy of the α channel bit depth; the value of the adjustable parameter is ±1 or ±2. 5. a decoding method of PNG format, is characterized in that, described decoding method comprises the following steps: Read out the PNG image, read out the image header block, the image data block and the image end block; According to the compression algorithm read from the image header block, the image data block is decompressed and filtered to restore; The color type is determined according to the defined value read from the image header block, and the color type includes grayscale image, true color image, indexed color image, grayscale image with alpha channel data, and true color with alpha channel data. images, true color images of unequal bit depth, and true color images of unequal bit depth with alpha channel data; According to the allowable bit depth of the color type read from the image header block, for the true color image of unequal bit depth and the true color image of unequal bit depth with alpha channel data, the third condition is used to calculate the number of pixels in the pixel. Bit depth; for images of other color types, the bit depth in the pixel is calculated according to the second condition; The image information data is decoded from the PNG image data block according to the bit depth of each pixel; After the decoding of the information of the image data block is completed, the decoded information of the image header block and the image end block are respectively loaded into the information before and after the image block to complete the decoding of the PNG image. 6. the decoding method of a kind of PNG format according to claim 5, is characterized in that, The said calculating the bit depth in the pixel according to the third condition includes judging whether the allowable bit depth of the true color image of unequal bit depth and the true color image of unequal bit depth with alpha channel data is greater than or equal to 8, if greater than or equal to 8 , then the allowable bit depth is taken as the bit depth of the pixel, otherwise the first function with an adjustable parameter is called to calculate the bit depth of the pixel; The calculating the bit depth of the pixel according to the second condition for images of other color types includes calling a preset first function to calculate the size of the bit depth in the pixel according to its allowable depth. 7. The decoding method of a PNG format according to claim 6, wherein the preset first function is expressed as f(x)=3A+α, A represents the bit depth of the channel, and α represents α Bit depth occupied by the channel; the value of the adjustable parameter is ±1 or ±2. 8. An encoding device in PNG format, wherein the encoding device comprises: a first collection unit, used for collecting target pixels in the original picture; The first identification unit is used for identifying the color type of the target pixel and corresponding to the preset definition value; An encoding preparation unit, for setting the allowable bit depth in the encoding process to point to the defined value; and for determining the bit depth of each pixel when encoding is required, thereby calculating the image size and determining the content of the image header block; The encoding unit is configured to perform PNG encoding processing according to the image blocks determined by the encoding preparation unit, and encode the image header block, the image data block and the image end block. 9. A decoding device in PNG format, wherein the encoding device comprises: The second collection unit is used to collect PNG images; The second identification unit is used to read out the image header block, the image data block and the image end block from the PNG image; and identify the color type and the allowable bit depth from the image header block; The decoding preparation unit is used to determine the number of bits of pixels in the PNG data stream and the number of bits of different channels in the pixels according to the color type; A decoding unit, configured to parse out the information of the PNG image according to the number of bits of the pixel and the number of bits of different channels in the pixel.

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