"excerpted from the PNG (Portable Network Graphics) specification, tenth draft." The PNG format (pronounced PiNG) was the replacement the Internet found, after the GIF format/CompuServe/LZW compression-patent stuff. PNG is a lossless image- compression format, which allows a large range of applications. The PNG format is in the public domain, the latest versions of the standard and related information can always be found at the PNG FTP archive site, ftp.uu.net:/graphics/png/. The maintainers of the PNG specification can be contacted by e-mail at [email protected].net. The PNG format uses Motorola byte order, scanlines always begin on byte boundaries. When pixels are less than 8 bits deep, if the scanline width is not evenly divisible by the number of pixels per byte then the low-order bits in the last byte of each scanline are wasted. The contents of the padding bits added to fill out the last byte of a scanline are unspecified. An additional "filter" byte is added to the beginning of every scanline, as described in detail below. The filter byte is not considered part of the image data, but it is included in the data stream sent to the compression step. PNG allows the image data to be filtered before it is compressed. The purpose of filtering is to improve the compressibility of the data. The filter step itself does not reduce the size of the data. All PNG filters are strictly lossless. PNG defines several different filter algorithms, including "none" which indicates no filtering. The filter algorithm is specified for each scanline by a filter type byte which precedes the filtered scanline in the precompression data stream. An intelligent encoder may switch filters from one scanline to the next. The method for choosing which filter to employ is up to the encoder. A PNG image can be stored in interlaced order to allow progressive display. The purpose of this feature is to allow images to "fade in" when they are being displayed on-the-fly. Interlacing slightly expands the file size on average, but it gives the user a meaningful display much more rapidly. Note that decoders are required to be able to read interlaced images, whether or not they actually perform progressive display. With interlace type 0, pixels are stored sequentially from left to right, and scanlines sequentially from top to bottom (no interlacing). Interlace type 1, known as Adam7 after its author, Adam M. Costello, consists of seven distinct passes over the image. Each pass transmits a subset of the pixels in the image. The pass in which each pixel is transmitted is defined by replicating the following 8-by-8 pattern over the entire image, starting at the upper left corner: 1 6 4 6 2 6 4 6 7 7 7 7 7 7 7 7 5 6 5 6 5 6 5 6 7 7 7 7 7 7 7 7 3 6 4 6 3 6 4 6 7 7 7 7 7 7 7 7 5 6 5 6 5 6 5 6 7 7 7 7 7 7 7 7 Within each pass, the selected pixels are transmitted left to right within a scanline, and selected scanlines sequentially from top to bottom. For example, pass 2 contains pixels 4, 12, 20, etc. of scanlines 0, 8, 16, etc. (numbering from 0,0 at the upper left corner). The last pass contains the entirety of scanlines 1, 3, 5, etc. The data within each pass is laid out as though it were a complete image of the appropriate dimensions. For example, if the complete image is 8x8 pixels, then pass 3 will contain a single scanline containing two pixels. When pixels are less than 8 bits deep, each such scanline is padded to fill an integral number of bytes (see Image layout). Filtering is done on this reduced image in the usual way, and a filter type byte is transmitted before each of its scanlines (see Filter Algorithms). Notice that the transmission order is defined so that all the scanlines transmitted in a pass will have the same number of pixels; this is necessary for proper application of some of the filters. Caution: If the image contains fewer than five columns or fewer than five rows, some passes will be entirely empty. Encoder and decoder authors must be careful to handle this case correctly. In particular, filter bytes are only associated with nonempty scanlines; no filter bytes are present in an empty pass. A PNG file consists of a PNG signature followed by a series of chunks. This chapter defines the signature and the basic properties of chunks. Individual chunk types are discussed in the next chapter. PNG Header OFFSET Count TYPE Description 0000h 8 char ID=89h,'PNG',13,10,26,10 Chunk layout OFFSET Count TYPE Description 0000h 1 dword Number of data bytes after this header. 0004h 4 char Chunk type. A 4-byte chunk type code. For convenience in description and in examining PNG files, type codes are restricted to consist of uppercase and lowercase ASCII letters (A-Z, a-z). However, encoders and decoders should treat the codes as fixed binary values, not character strings. For example, it would not be correct to represent the type code IDAT by the EBCDIC equivalents of those letters. ????h ? byte Data ????h 1 dword CRC calculated on the preceding bytes in that chunk, including the chunk type code and chunk data fields, but not including the length field. The CRC is always present, even for empty chunks such as IEND. The CRC algorithm is specified below. Chunk naming conventions ======================== Chunk type codes are assigned in such a way that a decoder can determine some properties of a chunk even if it does not recognize the type code. These rules are intended to allow safe, flexible extension of the PNG format, by allowing a decoder to decide what to do when it encounters an unknown chunk. The naming rules are not normally of interest when a decoder does recognize the chunk's type. Four bits of the type code, namely bit 5 (value 32) of each byte, are used to convey chunk properties. This choice means that a human can read off the assigned properties according to whether each letter of the type code is uppercase (bit 5 is 0) or lowercase (bit 5 is 1). However, decoders should test the properties of an unknown chunk by numerically testing the specified bits; testing whether a character is uppercase or lowercase is inefficient, and even incorrect if a locale-specific case definition is used. It is also worth noting that the property bits are an inherent part of the chunk name, and hence are fixed for any chunk type. Thus, TEXT and Text are completely unrelated chunk type codes. Decoders should recognize codes by simple four-byte literal comparison; it is incorrect to perform case conversion on type codes. The semantics of the property bits are: First Byte: 0 (uppercase) = critical, 1 (lowercase) = ancillary Chunks which are not strictly necessary in order to meaningfully display the contents of the file are known as "ancillary" chunks. Decoders encountering an unknown chunk in which the ancillary bit is 1 may safely ignore the chunk and proceed to display the image. The time chunk (tIME) is an example of an ancillary chunk. Chunks which are critical to the successful display of the file's contents are called "critical" chunks. Decoders encountering an unknown chunk in which the ancillary bit is 0 must indicate to the user that the image contains information they cannot safely interpret. The image header chunk (IHDR) is an example of a critical chunk. Second Byte: 0 (uppercase) = public, 1 (lowercase) = private If the chunk is public (part of this specification or a later edition of this specification), its second letter is uppercase. If your application requires proprietary chunks, and you have no interest in seeing the software of other vendors recognize them, use a lowercase second letter in the chunk name. Such names will never be assigned in the official specification. Note that there is no need for software to test this property bit; it simply ensures that private and public chunk names will not conflict. Third Byte: reserved, must be 0 (uppercase) always The significance of the case of the third letter of the chunk name is reserved for possible future expansion. At the present time all chunk names must have uppercase third letters. Fourth Byte: 0 (uppercase) = unsafe to copy, 1 (lowercase) = safe to copy This property bit is not of interest to pure decoders, but it is needed by PNG editors (programs that modify a PNG file). If a chunk's safe-to-copy bit is 1, the chunk may be copied to a modified PNG file whether or not the software recognizes the chunk type, and regardless of the extent of the file modifications. If a chunk's safe-to-copy bit is 0, it indicates that the chunk depends on the image data. If the program has made any changes to critical chunks, including addition, modification, deletion, or reordering of critical chunks, then unrecognized unsafe chunks must not be copied to the output PNG file. (Of course, if the program does recognize the chunk, it may choose to output an appropriately modified version.) A PNG editor is always allowed to copy all unrecognized chunks if it has only added, deleted, or modified ancillary chunks. This implies that it is not permissible to make ancillary chunks that depend on other ancillary chunks. PNG editors that do not recognize a critical chunk must report an error and refuse to process that PNG file at all. The safe/unsafe mechanism is intended for use with ancillary chunks. The safe-to-copy bit will always be 0 for critical chunks. For example, the hypothetical chunk type name "bLOb" has the property bits: bLOb <-- 32 bit Chunk Name represented in ASCII form |||| |||'- Safe to copy bit is 1 (lower case letter; bit 5 of byte is 1) ||'-- Reserved bit is 0 (upper case letter; bit 5 of byte is 0) |'--- Private bit is 0 (upper case letter; bit 5 of byte is 0) '---- Ancillary bit is 1 (lower case letter; bit 5 of byte is 1) Therefore, this name represents an ancillary, public, safe-to-copy chunk. See Rationale: Chunk naming conventions. CRC algorithm ============= Chunk CRCs are calculated using standard CRC methods with pre and post conditioning. The CRC polynomial employed is as follows: x^32+x^26+x^23+x^22+x^16+x^12+x^11+x^10+x^8+x^7+x^5+x^4+x^2+x+1 The 32-bit CRC register is initialized to all 1's, and then the data from each byte is processed from the least significant bit (1) to the most significant bit (128). After all the data bytes are processed, the CRC register is inverted (its ones complement is taken). This value is transmitted (stored in the file) MSB first. For the purpose of separating into bytes and ordering, the least significant bit of the 32-bit CRC is defined to be the coefficient of the x^31 term. Practical calculation of the CRC always employs a precalculated table to greatly accelerate the computation. See Appendix: Sample CRC Code. 4. Chunk Specifications ======================= This chapter defines the standard types of PNG chunks. Critical Chunks =============== All implementations must understand and successfully render the standard critical chunks. A valid PNG image must contain an IHDR chunk, one or more IDAT chunks, and an IEND chunk. IHDR Image Header This chunk must appear FIRST. Its contents are: Width: 4 bytes Height: 4 bytes Bit depth: 1 byte Color type: 1 byte Compression type: 1 byte Filter type: 1 byte Interlace type: 1 byte Width and height give the image dimensions in pixels. They are 4-byte integers. Zero is an invalid value. The maximum for each is (2^31)-1 in order to accommodate languages which have difficulty with unsigned 4-byte values. Bit depth is a single-byte integer giving the number of bits per pixel (for palette images) or per sample (for grayscale and truecolor images). Valid values are 1, 2, 4, 8, and 16, although not all values are allowed for all color types. Color type is a single-byte integer that describes the interpretation of the image data. Color type values represent sums of the following values: 1 (palette used), 2 (color used), and 4 (full alpha used). Valid values are 0, 2, 3, 4, and 6. Bit depth restrictions for each color type are imposed both to simplify implementations and to prohibit certain combinations that do not compress well in practice. Decoders must support all legal combinations of bit depth and color type. (Note that bit depths of 16 are easily supported on 8-bit display hardware by dropping the least significant byte.) The allowed combinations are: Color Allowed Interpretation Type Bit Depths 0 1,2,4,8,16 Each pixel value is a grayscale level. 2 8,16 Each pixel value is an R,G,B series. 3 1,2,4,8 Each pixel value is a palette index; a PLTE chunk must appear. 4 8,16 Each pixel value is a grayscale level, followed by an alpha channel level. 6 8,16 Each pixel value is an R,G,B series, followed by an alpha channel level. Compression type is a single-byte integer that indicates the method used to compress the image data. At present, only compression type 0 (deflate/inflate compression with a 32K sliding window) is defined. All standard PNG images must be compressed with this scheme. The compression type code is provided for possible future expansion or proprietary variants. Decoders must check this byte and report an error if it holds an unrecognized code. See Deflate/Inflate Compression for details. Filter type is a single-byte integer that indicates the preprocessing method applied to the image data before compression. At present, only filter type 0 (adaptive filtering with five basic filter types) is defined. As with the compression type code, decoders must check this byte and report an error if it holds an unrecognized code. See Filter Algorithms for details. Interlace type is a single-byte integer that indicates the transmission order of the pixel data. Two values are currently defined: 0 (no interlace) or 1 (Adam7 interlace). See Interlaced data order for details. PLTE Palette This chunk's contents are from 1 to 256 palette entries, each a three-byte series of the form: red: 1 byte (0 = black, 255 = red) green: 1 byte (0 = black, 255 = green) blue: 1 byte (0 = black, 255 = blue) The number of entries is determined from the chunk length. A chunk length not divisible by 3 is an error. This chunk must appear for color type 3, and may appear for color types 2 and 6. If this chunk does appear, it must precede the first IDAT chunk. There cannot be more than one PLTE chunk. For color type 3 (palette data), the PLTE chunk is required. The first entry in PLTE is referenced by pixel value 0, the second by pixel value 1, etc. The number of palette entries must not exceed the range that can be represented by the bit depth (for example, 2^4 = 16 for a bit depth of 4). It is permissible to have fewer entries than the bit depth would allow. In that case, any out-of-range pixel value found in the image data is an error. For color types 2 and 6 (truecolor), the PLTE chunk is optional. If present, it provides a recommended set of from 1 to 256 colors to which the truecolor image may be quantized if the viewer cannot display truecolor directly. If PLTE is not present, such a viewer must select colors on its own, but it is often preferable for this to be done once by the encoder. Note that the palette uses 8 bits (1 byte) per value regardless of the image bit depth specification. In particular, the palette is 8 bits deep even when it is a suggested quantization of a 16-bit truecolor image. IDAT Image Data This chunk contains the actual image data. To create this data, begin with image scanlines represented as described under Image layout; the layout and total size of this raw data are determinable from the IHDR fields. Then filter the image data according to the filtering method specified by the IHDR chunk. (Note that with filter method 0, the only one currently defined, this implies prepending a filter type byte to each scanline.) Finally, compress the filtered data using the compression method specified by the IHDR chunk. The IDAT chunk contains the output datastream of the compression algorithm. To read the image data, reverse this process. There may be multiple IDAT chunks; if so, they must appear consecutively with no other intervening chunks. The compressed datastream is then the concatenation of the contents of all the IDAT chunks. The encoder may divide the compressed data stream into IDAT chunks as it wishes. (Multiple IDAT chunks are allowed so that encoders can work in a fixed amount of memory; typically the chunk size will correspond to the encoder's buffer size.) It is important to emphasize that IDAT chunk boundaries have no semantic significance and can appear at any point in the compressed datastream. A PNG file in which each IDAT chunk contains only one data byte is legal, though remarkably wasteful of space. (For that matter, zero-length IDAT chunks are legal, though even more wasteful.) See Filter Algorithms and Deflate/Inflate Compression for details. IEND Image Trailer This chunk must appear LAST. It marks the end of the PNG data stream. The chunk's data field is empty. Ancillary Chunks ================ All ancillary chunks are optional, in the sense that encoders need not write them and decoders may ignore them. However, encoders are encouraged to write the standard ancillary chunks when the information is available, and decoders are encouraged to interpret these chunks when appropriate and feasible. The standard ancillary chunks are listed in alphabetical order. This is not necessarily the order in which they would appear in a file. bKGD Background Color This chunk specifies a default background color against which the image may be presented. Note that viewers are not bound to honor this chunk; a viewer may choose to use a different background color. For color type 3 (palette), the bKGD chunk contains: palette index: 1 byte The value is the palette index of the color to be used as background. For color types 0 and 4 (grayscale, with or without alpha), bKGD contains: gray: 2 bytes, range 0 .. (2^bitdepth) - 1 (For consistency, 2 bytes are used regardless of the image bit depth.) The value is the gray level to be used as background. For color types 2 and 6 (RGB, with or without alpha), bKGD contains: red: 2 bytes, range 0 .. (2^bitdepth) - 1 green: 2 bytes, range 0 .. (2^bitdepth) - 1 blue: 2 bytes, range 0 .. (2^bitdepth) - 1 (For consistency, 2 bytes per sample are used regardless of the image bit depth.) This is the RGB color to be used as background. When present, the bKGD chunk must precede the first IDAT chunk, and must follow the PLTE chunk, if any. See Recommendations for Decoders: Background color. cHRM Primary Chromaticities and White Point Applications that need precise specification of colors in a PNG file may use this chunk to specify the chromaticities of the red, green, and blue primaries used in the image, and the referenced white point. These values are based on the 1931 CIE (International Color Committee) XYZ color space. Only the chromaticities (x and y) are specified. The chunk layout is: White Point x: 4 bytes White Point y: 4 bytes Red x: 4 bytes Red y: 4 bytes Green x: 4 bytes Green y: 4 bytes Blue x: 4 bytes Blue y: 4 bytes Each value is encoded as a 4-byte unsigned integer, representing the x or y value times 100000. If the cHRM chunk appears, it must precede the first IDAT chunk, and it must also precede the PLTE chunk if present. gAMA Gamma Correction The gamma correction chunk specifies the gamma of the camera (or simulated camera) that produced the image, and thus the gamma of the image with respect to the original scene. Note that this is not the same as the gamma of the display device that will reproduce the image correctly. The chunk's contents are: Image gamma value: 4 bytes A value of 100000 represents a gamma of 1.0, a value of 45000 a gamma of 0.45, and so on (divide by 100000.0). Values around 1.0 and around 0.45 are common in practice. If the encoder does not know the gamma value, it should not write a gamma chunk; the absence of a gamma chunk indicates the gamma is unknown. If the gAMA chunk appears, it must precede the first IDAT chunk, and it must also precede the PLTE chunk if present. See Gamma correction, Recommendations for Encoders: Encoder gamma handling, and Recommendations for Decoders: Decoder gamma handling. hIST Image Histogram The histogram chunk gives the approximate usage frequency of each color in the color palette. A histogram chunk may appear only when a palette chunk appears. If a viewer is unable to provide all the colors listed in the palette, the histogram may help it decide how to choose a subset of the colors for display. This chunk's contents are a series of 2-byte (16 bit) unsigned integers. There must be exactly one entry for each entry in the PLTE chunk. Each entry is proportional to the fraction of pixels in the image that have that palette index; the exact scale factor is chosen by the encoder. Histogram entries are approximate, with the exception that a zero entry specifies that the corresponding palette entry is not used at all in the image. It is required that a histogram entry be nonzero if there are any pixels of that color. When the palette is a suggested quantization of a truecolor image, the histogram is necessarily approximate, since a decoder may map pixels to palette entries differently than the encoder did. In this situation, zero entries should not appear. The hIST chunk, if it appears, must follow the PLTE chunk, and must precede the first IDAT chunk. See Rationale: Palette histograms, and Recommendations for Decoders: Palette histogram usage. pHYs Physical Pixel Dimensions This chunk specifies the intended resolution for display of the image. The chunk's contents are: 4 bytes: pixels per unit, X axis (unsigned integer) 4 bytes: pixels per unit, Y axis (unsigned integer) 1 byte: unit specifier The following values are legal for the unit specifier: 0: unit is unknown (pHYs defines pixel aspect ratio only) 1: unit is the meter Conversion note: one inch is equal to exactly 0.0254 meters. If this ancillary chunk is not present, pixels are assumed to be square, and the physical size of each pixel is unknown. If present, this chunk must precede the first IDAT chunk. See Recommendations for Decoders: Pixel dimensions. sBIT Significant Bits To simplify decoders, PNG specifies that only certain bit depth values be used, and further specifies that pixel values must be scaled to the full range of possible values at that bit depth. However, the sBIT chunk is provided in order to store the original number of significant bits, since this information may be of use to some decoders. We recommend that an encoder emit an sBIT chunk if it has converted the data from a different bit depth. For color type 0 (grayscale), the sBIT chunk contains a single byte, indicating the number of bits which were significant in the source data. For color type 2 (RGB truecolor), the sBIT chunk contains three bytes, indicating the number of bits which were significant in the source data for the red, green, and blue channels, respectively. For color type 3 (palette color), the sBIT chunk contains three bytes, indicating the number of bits which were significant in the source data for the red, green, and blue components of the palette entries, respectively. For color type 4 (grayscale with alpha channel), the sBIT chunk contains two bytes, indicating the number of bits which were significant in the source grayscale data and the source alpha channel data, respectively. For color type 6 (RGB truecolor with alpha channel), the sBIT chunk contains four bytes, indicating the number of bits which were significant in the source data for the red, green, blue and alpha channels, respectively. Note that sBIT does not have any implications for the interpretation of the stored image: the bit depth indicated by IHDR is the correct depth. sBIT is only an indication of the history of the image. However, an sBIT chunk showing a bit depth less than the IHDR bit depth does mean that not all possible color values occur in the image; this fact may be of use to some decoders. If the sBIT chunk appears, it must precede the first IDAT chunk, and it must also precede the PLTE chunk if present. tEXt Textual Data Any textual information that the encoder wishes to record with the image is stored in tEXt chunks. Each tEXt chunk contains a keyword and a text string, in the format: Keyword: n bytes (character string) Null separator: 1 byte Text: n bytes (character string) The keyword and text string are separated by a zero byte (null character). Neither the keyword nor the text string may contain a null character. Note that the text string is not null-terminated (the length of the chunk is sufficient information to locate the ending). The keyword must be at least one character and less than 80 characters long. The text string may be of any length from zero bytes up to the maximum permissible chunk size. Any number of tEXt chunks may appear, and more than one with the same keyword is permissible. The keyword indicates the type of information represented by the text string. The following keywords are predefined and should be used where appropriate: Title Short (one line) title or caption for image Author Name of image's creator Copyright Copyright notice Description Description of image (possibly long) Software Software used to create the image Disclaimer Legal disclaimer Warning Warning of nature of content Source Device used to create the image Comment Miscellaneous comment; conversion from GIF comment Other keywords, containing any sequence of printable characters in the character set, may be invented for other purposes. Keywords of general interest may be registered with the maintainers of the PNG specification. Keywords must be spelled exactly as registered, so that decoders may use simple literal comparisons when looking for particular keywords. In particular, keywords are considered case-sensitive. Both keyword and text are interpreted according to the ISO 8859-1 (Latin-1) character set. Newlines in the text string should be represented by a single linefeed character (decimal 10); use of other ASCII control characters is discouraged. See Recommendations for Encoders: Text chunk processing and Recommendations for Decoders: Text chunk processing. tIME Image Last-Modification Time This chunk gives the time of the last image modification (not the time of initial image creation). The chunk contents are: 2 bytes: Year (complete; for example, 1995, not 95) 1 byte: Month (1-12) 1 byte: Day (1-31) 1 byte: Hour (0-23) 1 byte: Minute (0-59) 1 byte: Second (0-60) (yes, 60, for leap seconds; not 61, a common error) Universal Time (UTC, also called GMT) should be specified rather than local time. tRNS Transparency Transparency is an alternative to the full alpha channel. Although transparency is not as elegant as the full alpha channel, it requires less storage space and is sufficient for many common cases. For color type 3 (palette), this chunk's contents are a series of alpha channel bytes, corresponding to entries in the PLTE chunk: Alpha for palette index 0: 1 byte Alpha for palette index 1: 1 byte etc. Each entry indicates that pixels of that palette index should be treated as having the specified alpha value. Alpha values have the same interpretation as in an 8-bit full alpha channel: 0 is fully transparent, 255 is fully opaque, regardless of image bit depth. The tRNS chunk may contain fewer alpha channel bytes than there are palette entries. In this case, the alpha channel value for all remaining palette entries is assumed to be 255. In the common case where only palette index 0 need be made transparent, only a one-byte tRNS chunk is needed. The tRNS chunk may not contain more bytes than there are palette entries. For color type 0 (grayscale), the tRNS chunk contains a single gray level value, stored in the format gray: 2 bytes, range 0 .. (2^bitdepth) - 1 (For consistency, 2 bytes are used regardless of the image bit depth.) Pixels of the specified gray level are to be treated as transparent (equivalent to alpha value 0); all other pixels are to be treated as fully opaque (alpha value (2^bitdepth)-1). For color type 2 (RGB), the tRNS chunk contains a single RGB color value, stored in the format red: 2 bytes, range 0 .. (2^bitdepth) - 1 green: 2 bytes, range 0 .. (2^bitdepth) - 1 blue: 2 bytes, range 0 .. (2^bitdepth) - 1 (For consistency, 2 bytes per sample are used regardless of the image bit depth.) Pixels of the specified color value are to be treated as transparent (equivalent to alpha value 0); all other pixels are to be treated as fully opaque (alpha value (2^bitdepth)-1). tRNS is prohibited for color types 4 and 6, since a full alpha channel is already present in those cases. Note: when dealing with 16-bit grayscale or RGB data, it is important to compare both bytes of the sample values to determine whether a pixel is transparent. Although decoders may drop the low-order byte of the samples for display, this must not occur until after the data has been tested for transparency. For example, if the grayscale level 0x0001 is specified to be transparent, it would be incorrect to compare only the high-order byte and decide that 0x0002 is also transparent. When present, the tRNS chunk must precede the first IDAT chunk, and must follow the PLTE chunk, if any. zTXt Compressed Textual Data A zTXt chunk contains textual data, just as tEXt does; however, zTXt takes advantage of compression. A zTXt chunk begins with an uncompressed Latin-1 keyword followed by a null (0) character, just as in the tEXt chunk. The next byte after the null contains a compression type byte, for which the only presently legitimate value is zero (deflate/inflate compression). The compression-type byte is followed by a compressed data stream which makes up the remainder of the chunk. Decompression of this data stream yields Latin-1 text which is equivalent to the text stored in a tEXt chunk. Any number of zTXt and tEXt chunks may appear in the same file. See the preceding definition of the tEXt chunk for the predefined keywords and the exact format of the text. See Deflate/Inflate Compression, Recommendations for Encoders: Text chunk processing, and Recommendations for Decoders: Text chunk processing. Summary of Standard Chunks ========================== This table summarizes some properties of the standard chunk types. Critical chunks (must appear in this order, except PLTE is optional): Name Multiple Ordering constraints OK? IHDR No Must be first PLTE No Before IDAT IDAT Yes Multiple IDATs must be consecutive IEND No Must be last Ancillary chunks (need not appear in this order): Name Multiple Ordering constraints OK? cHRM No Before PLTE and IDAT gAMA No Before PLTE and IDAT sBIT No Before PLTE and IDAT bKGD No After PLTE; before IDAT hIST No After PLTE; before IDAT tRNS No After PLTE; before IDAT pHYs No Before IDAT tIME No None tEXt Yes None zTXt Yes None Standard keywords for tEXt and zTXt chunks: Title Short (one line) title or caption for image Author Name of image's creator Copyright Copyright notice Description Description of image (possibly long) Software Software used to create the image Disclaimer Legal disclaimer Warning Warning of nature of content Source Device used to create the image Comment Miscellaneous comment; conversion from GIF comment Additional Chunk Types ====================== Additional public PNG chunk types are defined in the document "PNG Special-Purpose Public Chunks", available by FTP from ftp.uu.net:/graphics/png/ or via WWW from http://sunsite.unc.edu/boutell/pngextensions.html. 5. Deflate/Inflate Compression ============================== PNG compression type 0 (the only compression method presently defined for PNG) specifies deflate/inflate compression with a 32K window. Deflate compression is an LZ77 derivative used in zip, gzip, pkzip and related programs. Extensive research has been done supporting its patent-free status. Portable C implementations are freely available. Documentation and C code for deflate are available from the Info-Zip archives at ftp.uu.net:/pub/archiving/zip/. Deflate-compressed datastreams within PNG are stored in the "zlib" format, which has the structure: Compression method/flags code: 1 byte Additional flags/check bits: 1 byte Compressed data blocks: n bytes Checksum: 4 bytes Further details on this format may be found in the zlib specification. At this writing, the zlib specification is at draft 3.1, and is available from ftp.uu.net:/pub/archiving/zip/doc/zlib-3.1.doc. For PNG compression type 0, the zlib compression method/flags code must specify method code 8 ("deflate" compression) and an LZ77 window size of not more than 32K. The checksum stored at the end of the zlib datastream is calculated on the uncompressed data represented by the datastream. Note that the algorithm used is not the same as the CRC calculation used for PNG chunk checksums. Verifying the chunk CRCs provides adequate confidence that the PNG file has been transmitted undamaged. The zlib checksum is useful mainly as a crosscheck that the deflate and inflate algorithms are implemented correctly. In a PNG file, the concatenation of the contents of all the IDAT chunks makes up a zlib datastream as specified above. This datastream decompresses to filtered image data as described elsewhere in this document. It is important to emphasize that the boundaries between IDAT chunks are arbitrary and may fall anywhere in the zlib datastream. There is not necessarily any correlation between IDAT chunk boundaries and deflate block boundaries or any other feature of the zlib data. For example, it is entirely possible for the terminating zlib checksum to be split across IDAT chunks. PNG also uses zlib datastreams in zTXt chunks. In a zTXt chunk, the remainder of the chunk following the compression type code byte is a zlib datastream as specified above. This datastream decompresses to the user-readable text described by the chunk's keyword. Unlike the image data, such datastreams are not split across chunks; each zTXt chunk contains an independent zlib datastream. 6. Filter Algorithms ==================== This chapter describes the pixel filtering algorithms which may be applied in advance of compression. The purpose of these filters is to prepare the image data for optimum compression. PNG defines five basic filtering algorithms, which are given numeric codes as follows: Code Name 0 None 1 Sub 2 Up 3 Average 4 Paeth The encoder may choose which algorithm to apply on a scanline-by-scanline basis. In the image data sent to the compression step, each scanline is preceded by a filter type byte containing the numeric code of the filter algorithm used for that scanline. Filtering algorithms are applied to bytes, not to pixels, regardless of the bit depth or color type of the image. The filtering algorithms work on the byte sequence formed by a scanline that has been represented as described under Image layout. When the image is interlaced, each pass of the interlace pattern is treated as an independent image for filtering purposes. The filters work on the byte sequences formed by the pixels actually transmitted during a pass, and the "previous scanline" is the one previously transmitted in the same pass, not the one adjacent in the complete image. Note that the subimage transmitted in any one pass is always rectangular, but is of smaller width and/or height than the complete image. Filtering is not applied when this subimage is empty. For all filters, the bytes "to the left of" the first pixel in a scanline must be treated as being zero. For filters that refer to the prior scanline, the entire prior scanline must be treated as being zeroes for the first scanline of an image (or of a pass of an interlaced image). To reverse the effect of a filter, the decoder must use the decoded values of the prior pixel on the same line, the pixel immediately above the current pixel on the prior line, and the pixel just to the left of the pixel above. This implies that at least one scanline's worth of image data must be stored by the decoder at all times. Even though some filter types do not refer to the prior scanline, the decoder must always store each scanline as it is decoded, since the next scanline might use a filter that refers to it. PNG imposes no restriction on which filter types may be applied to an image. However, the filters are not equally effective on all types of data. See Recommendations for Encoders: Filter selection. Filter type 0: None =================== With the None filter, the scanline is transmitted unmodified; it is only necessary to insert a filter type byte before the data. Filter type 1: Sub ================== The Sub filter transmits the difference between each byte and the value of the corresponding byte of the prior pixel. To compute the Sub filter, apply the following formula to each byte of each scanline: Sub(x) = Raw(x) - Raw(x-bpp) where x ranges from zero to the number of bytes representing that scanline minus one, Raw(x) refers to the raw data byte at that byte position in the scanline, and bpp is defined as the number of bytes per complete pixel, rounding up to one. For example, for color type 2 with a bit depth of 16, bpp is equal to 6 (three channels, two bytes per channel); for color type 0 with a bit depth of 2, bpp is equal to 1 (rounding up); for color type 4 with a bit depth of 16, bpp is equal to 4 (two-byte grayscale value, plus two-byte alpha channel). Note this computation is done for each byte, regardless of bit depth. In a 16-bit image, MSBs are differenced from the preceding MSB and LSBs are differenced from the preceding LSB, because of the way that bpp is defined. Unsigned arithmetic modulo 256 is used, so that both the inputs and outputs fit into bytes. The sequence of Sub values is transmitted as the filtered scanline. For all x < 0, assume Raw(x) = 0. To reverse the effect of the Sub filter after decompression, output the following value: Sub(x) + Raw(x-bpp) (computed mod 256), where Raw refers to the bytes already decoded. Filter type 2: Up ================= The Up filter is just like the Sub filter except that the pixel immediately above the current pixel, rather than just to its left, is used as the predictor. To compute the Up filter, apply the following formula to each byte of each scanline: Up(x) = Raw(x) - Prior(x) where x ranges from zero to the number of bytes representing that scanline minus one, Raw(x) refers to the raw data byte at that byte position in the scanline, and Prior(x) refers to the unfiltered bytes of the prior scanline. Note this is done for each byte, regardless of bit depth. Unsigned arithmetic modulo 256 is used, so that both the inputs and outputs fit into bytes. The sequence of Up values is transmitted as the filtered scanline. On the first scanline of an image (or of a pass of an interlaced image), assume Prior(x) = 0 for all x. To reverse the effect of the Up filter after decompression, output the following value: Up(x) + Prior(x) (computed mod 256), where Prior refers to the decoded bytes of the prior scanline. Filter type 3: Average ====================== The Average filter uses the average of the two neighboring pixels (left and above) to predict the value of a pixel. To compute the Average filter, apply the following formula to each byte of each scanline: Average(x) = Raw(x) - floor((Raw(x-bpp)+Prior(x))/2) where x ranges from zero to the number of bytes representing that scanline minus one, Raw(x) refers to the raw data byte at that byte position in the scanline, Prior(x) refers to the unfiltered bytes of the prior scanline, and bpp is defined as for the Sub filter. Note this is done for each byte, regardless of bit depth. The sequence of Average values is transmitted as the filtered scanline. The subtraction of the predicted value from the raw byte must be done modulo 256, so that both the inputs and outputs fit into bytes. However, the sum Raw(x-bpp)+Prior(x) must be formed without overflow (using at least nine-bit arithmetic). floor() indicates that the result of the division is rounded to the next lower integer if fractional; in other words, it is an integer division or right shift operation. For all x < 0, assume Raw(x) = 0. On the first scanline of an image (or of a pass of an interlaced image), assume Prior(x) = 0 for all x. To reverse the effect of the Average filter after decompression, output the following value: Average(x) + floor((Raw(x-bpp)+Prior(x))/2) where the result is computed mod 256, but the prediction is calculated in the same way as for encoding. Raw refers to the bytes already decoded, and Prior refers to the decoded bytes of the prior scanline. Filter type 4: Paeth ==================== The Paeth filter computes a simple linear function of the three neighboring pixels (left, above, upper left), then chooses as predictor the neighboring pixel closest to the computed value. This technique is taken from Alan W. Paeth's article "Image File Compression Made Easy" in Graphics Gems II, James Arvo, editor, Academic Press, 1991. To compute the Paeth filter, apply the following formula to each byte of each scanline: Paeth(x) = Raw(x) - PaethPredictor(Raw(x-bpp),Prior(x),Prior(x-bpp)) where x ranges from zero to the number of bytes representing that scanline minus one, Raw(x) refers to the raw data byte at that byte position in the scanline, Prior(x) refers to the unfiltered bytes of the prior scanline, and bpp is defined as for the Sub filter. Note this is done for each byte, regardless of bit depth. Unsigned arithmetic modulo 256 is used, so that both the inputs and outputs fit into bytes. The sequence of Paeth values is transmitted as the filtered scanline. The PaethPredictor function is defined by the following pseudocode: function PaethPredictor (a, b, c) begin ; a = left, b = above, c = upper left p := a + b - c ; initial estimate pa := abs(p - a) ; distances to a, b, c pb := abs(p - b) pc := abs(p - c) ; return nearest of a,b,c, ; breaking ties in order a,b,c. if pa <= pb AND pa <= pc begin return a end if pb <= pc begin return b end return c end The calculations within the PaethPredictor function must be performed exactly, without overflow. Arithmetic modulo 256 is to be used only for the final step of subtracting the function result from the target pixel value. Note that the order in which ties are broken is fixed and must not be altered. The tie break order is: pixel to the left, pixel above, pixel to the upper left. (This order differs from that given in Paeth's article.) For all x < 0, assume Raw(x) = 0 and Prior(x) = 0. On the first scanline of an image (or of a pass of an interlaced image), assume Prior(x) = 0 for all x. To reverse the effect of the Paeth filter after decompression, output the following value: Paeth(x) + PaethPredictor(Raw(x-bpp),Prior(x),Prior(x-bpp)) (computed mod 256), where Raw and Prior refer to bytes already decoded. Exactly the same PaethPredictor function is used by both encoder and decoder. For more information, check out the above ftp sites. EXTENSION:PNG OCCURENCES:PC,UNIX,AMIGA PROGRAMS:???? REFERENCE:The PNG Specification
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