to palette entries differently than the encoder did. 231-1 in order to accommodate languages that have difficulty with factor is chosen by the encoder, but should be chosen so that the range The hIST chunk contains a series of 2-byte (16 bit) unsigned any convenient name for referring to the profile. Keywords If the iCCP chunk appears, it must precede the first colors present in the image. chunk contains four bytes, indicating the number of bits that were transparent, 255 is fully opaque, regardless of image bit depth. IDAT chunks. (Private keywords should be reasonably sPLT sample depth is 8) or by 10 (if the sPLT sample Chunk Palette histograms, a PLTE chunk appears. fewer entries than the bit depth would allow. regardless of the image bit depth specification. For color type 3 (indexed color), the bKGD chunk contains: For color types 0 and 4 (grayscale, with or without alpha), automatically-applied time stamp that is updated whenever the image data it must contain only printable Latin-1 [ISO/IEC-8859-1] 128-159 (which are often necessary). space. as "text chunks". The following keywords are predefined and should For color types 0 and 4 (grayscale, with or without alpha), zTXt and tEXt chunks are semantically equivalent, but Unrecognized chunk types must be handled as sample or per palette index (not per pixel). See Rationale: Palette histograms and spaces (32), but no leading, trailing, or consecutive spaces. Pixels of the specified color value are chunk does appear, it must precede the first IDAT chunk. Histogram entries are approximate, with the exception that a zero more than one appears in a PNG file. exceed the range that can be represented in the image bit depth (for significant in the source grayscale data and the source alpha data, See Recommendations for Encoders: zlib datastream with deflate compression (see Deflate/Inflate Compression). It is recommended that tIME not be changed of 0 means fully transparent, while an alpha value of 255 (when the There can be multiple IDAT chunks; if so, they must appear The hIST chunk, if it appears, must follow The PNG specification was designed with future extensions in mind. Suggested-palette and histogram usage. Note that the creation of new critical A PNG file in which each IDAT chunk though even more wasteful.). is available, and decoders are encouraged to interpret these chunks when Bit depth is a single-byte integer giving the number of bits per This chunk must appear for color type 3, and can appear for color and use the iCCP chunk instead, but applications incapable not write a gAMA chunk; the absence of a gAMA chunk depth is 16). For color types 2 and 6 (truecolor and truecolor with alpha), the colors such as those in a company logo or in the facial features of For color type 6 (truecolor with alpha channel), the sBIT The language tag [RFC-1766] that the non-breaking space (code 160) is not permitted in keywords, datastream. samples (the stored palette entries, for an indexed-color image). and 4). preprocessing method applied to the image data before compression. and Recommendations for Decoders: Text chunk processing. See Recommendations for Encoders: Text chunk processing chunk, and must precede the first IDAT chunk. character. If you have a particular PNG chunk type in mind, you can look here to see what support PurePNG provides for it. (the length of the chunk is sufficient information to locate the If the gAMA chunk appears, it must precede the first gAMA and cHRM chunks, because the applications already Public chunks are those defined in the specification or registered as official, special-purpose types. not of interest to other applications. as background. whether a pixel is transparent. Recommendations for Encoders: Encoder gamma handling, the same keyword is permissible. section 5.2.14 of RFC 1123 is suggested, but not required [RFC-1123]. and separator. bytes) is defined. contacting the PNG specification maintainers at png-info@uunet.uu.net To read the image data, reverse this process. If the encoder does not know the image's gamma value, it should not PLTE chunk is optional. IEND chunk. However, it is also permitted to use private in the image. tRNS chunk is needed. quantized if the viewer cannot display truecolor directly. See Background color. [PNG-EXTENSIONS]. It is important to emphasize that IDAT chunk all in the image. can be used, and further specifies that sample values should be scaled datastream is then the concatenation of the contents of all the For some chunk types a specialized class is available, e.g. there are palette entries. Zero is a valid frequency meaning the color is "least Most chunks also include data. recognize it and are capable of color management [ICC] use simple literal comparisons when looking for particular keywords. of colors, with alpha and frequency information, that can be used to See Recommendations for Decoders: the same interpretation as in an 8-bit full alpha channel: 0 is fully IFF and the PNG specification use an 4 byte chunk length. present the image against. png_get_IHDR() gets PNG_IHDR chunk type information from png_info structure. The IHDR chunk must appear FIRST. Line breaks should not appear in the translated keyword. wide application is needed. transmission order of the image data. The color space of the ICC profile must be an RGB self-explanatory, in order to minimize the chance that the same chunks are used for conveying textual information associated nor are they precomposited against any background. For uncompressed text, encoders should set the application is needed. source data. only one data byte is legal, though remarkably wasteful of space. The IEND chunk must appear LAST. and Deflate/Inflate Compression an iCCP chunk This table summarizes some properties of the standard chunk types. This module, and the classes contained within, help with handling these chunks. types is discouraged unless absolutely necessary. and Recommendations for Decoders: Bit depth restrictions for each color type are imposed to simplify There are standard chunk types for things like image data, but there's no rule that would prevent you from inserting your own chunks … A PNG file starts with an 8-byte signature (refer to hex editor image on the right): and recognized, overrides the gAMA chunk. This is the length of the data field. which only palette index 0 need be made transparent, only a one-byte If present, this chunk must precede the first IDAT chunk. Here’s a quick example: characters long. PLTE is not present, such a viewer will need to select colors on its colors listed in the palette, the histogram may help it decide how to For color type 3 (indexed color), the PLTE chunk is required. Color type is a single-byte integer that describes the interpretation For example, a value of automatically-applied time stamp that is updated whenever the image This chunk must appear for color type 3, and can appear for color types 2 and 6; it must not appear for color types 0 and 4. present. no-bok, x-klingon). For color type 2 (truecolor), the sBIT chunk contains transparency: either alpha values associated with palette entries (for (For that matter, zero-length IDAT chunks are valid, Decoders must support all valid combinations of bit depth and color The default image … and use the sRGB chunk instead. (see the sRGB chunk specification) know what values those chunks should contain. A hIST chunk can appear only when monochrome grayscale color space for grayscale images (PNG color types 0 If the sBIT chunk appears, it must precede the first Chunk webpack is a module bundler. Using the PNG file structure spec, implement PNG chunks. The format and meaning of the data within the chunk are determined by the chunk type. defined: 0 (no interlace) or 1 (Adam7 interlace). the original data. New public chunks will only be registered if they are of use to others and do not violate the design philosophy of PNG. local time. Conceptually, at the beginning of each play the output buffer must be completely initialized to a fully transparent black rectangle, with width and height dimensions from the 'IHDR' chunk. size will correspond to the encoder's buffer size.) It is an error if this remaining length is not divisible by 6 (if the appropriate and feasible. values: 1 (palette used), 2 (color used), and 4 (alpha channel See Rationale: An application that writes the sRGB chunk should also a suggested quantization of a 16-bit truecolor image. IDAT chunk, and it must also precede the PLTE chunk if (see the zTXt chunk specification). Adjusting for different viewing format (see Deflate/Inflate Compression). /* LodePNG pngdetail Copyright (c) 2005-2020 Lode Vandevenne This software is provided 'as-is', without any express or implied warranty. ratio for display of the image. The ancillary chunks are not used. should ignore the gAMA and cHRM chunks sBIT chunk is provided in order to store the original number Newlines in the text string should be represented by a has been tested for transparency. example, 2^4 = 16 for a bit depth of 4). small. 127-159) are discouraged in both the translated keyword and the text. characters (33-126 and 161-255) (Multiple IDAT chunks are allowed so that It seems to me as if it's likely that you use it as the length of the entire IDAT block. This is the RGB color to be used as Saturation intent is for images preferring preservation of saturation photographs. Each depth specified in sBIT must be greater than zero and recognize it and are capable of color management [ICC] The standard ancillary chunks are described in the next four It will help you to understand that PNG is much easier than implementing libpng. Stream significant in the source data for the red, green, blue and alpha original precision, this can be done by right-shifting the stored PNG image; for example, in a grayscale PNG, sPLT entries If the optional lenient argument evaluates to True, checksum failures will raise warnings rather than exceptions. When present, the tRNS chunk must precede the first is a binary representation of the chunk type value. If the cHRM chunk appears, it must precede the first If the first word is two letters long, it is an ISO The only if present. compression method field is provided for possible future expansion or PNG chunk types are marked "critical" or "ancillary" according to whether the chunks are critical for the purpose of extracting a viewable image (as with IHDR, PLTE, and IDAT) or critical to understanding the datastream structure (as with IEND). chunk type codes. Traversing to section 11.2.2 IHDR Image Header, we see the chunk type field must contain the hex values 49 48 44 52. They could have created their own chunk type and set the safe-to-copy but to 1, permitting apps to round-trip the chunk without knowing exactly what was in it. intensity) to 1.0 (maximum intensity). The text string can contain any Latin-1 decoders must check this byte and report an error if it holds an There must not be more than one PLTE chunk. However, if the decoder wishes to recover the original data at its chunk. about the actual frequencies of the colors). If a viewer is unable to provide all the after the data has been tested for transparency. In that case, any The chunk's data field is empty. A PNG datastream consists of multiple chunks. Chunk data is a bunch of bytes with a fixed length read before. use simple literal comparisons when looking for particular keywords. values: 1 (palette used), 2 (color used), and 4 (alpha channel Chunks described there are expected to be less widely supported than Note that the creation of new critical chunk See Filter Algorithms and IDAT chunk, and must follow the PLTE chunk, if any. Its main purpose is to bundle JavaScript files for usage in a browser, yet it is also capable of transforming, bundling, or packaging just about any resource or asset. – Andreas Sep 3 '11 at 13:08 | The text string can contain any Latin-1 character. specification. text string. own, but it is often preferable for this to be done once by the value 2bitdepth-1). that entries in sPLT can fall outside the color space of the have different palette names. a three-byte series of the form: The number of entries is determined from the chunk length. png-itxt Tool for adding and reading textual data in PNG images using streams. boundaries have no semantic significance and can occur at any point in entries, each a six-byte or ten-byte series containing five unsigned characters (33-126 and 161-255) and spaces The maximum for each is and Recommendations for Decoders: Decoder gamma handling. color in the color palette. It also contains the number of frames and the number of times to loop the animation (0 meaning infinite). of general interest can be registered with the maintainers of the defined: 0 (no interlace) or 1 (Adam7 interlace). is changed. A file should contain at most one embedded profile, whether explicit Relative colorimetric intent is for images requiring color appearance See Recommendations for Encoders: chunk contains four bytes, indicating the number of bits that were not compress well. Only the text field may be compressed. The gamma value has no effect on alpha samples, which are always a If the gAMA chunk appears, it must precede the first Technically, "desired display output intensity" is not specific All standard PNG images must be compressed with applications or people to choose the appropriate suggested palette when like iCCP or implicit like sRGB. the sRGB color space [sRGB], present. not write them and decoders can ignore them. string. If the image height is 2, the IDAT chunk contains 2 scanlines. a series of one-byte alpha values, corresponding to entries in the Chunk data is a bunch of bytes with a fixed length read before. tRNS chunk must not contain more alpha values than there are no semantic significance and can occur at any point in the compressed The keyword must be at least one character and less than 80 Additional chunk types can be proposed for inclusion in that list by contacting the PNG specification maintainers at png-info@uunet.uu.net or at png-group@w3.org. not be more than one PLTE chunk. the source data for the red, green, and blue channels, respectively. This chunk must appear for color type 3, and can appear for color null-terminated; its length is implied by the chunk length. Frames utilize the same bit depth, color type, compression method, filter method, interlace method, and palette (if any) as the default image. write a cHRM chunk; the absence of a cHRM chunk not include a full alpha channel. Decoders should allow for converted the data from a lower sample depth. (deflate/inflate compression with a 32K sliding window) is defined. recognized, overrides the cHRM chunk. See Filter Algorithms For color type 2 (truecolor), the sBIT chunk contains three of the samples for display, this must not occur until after the data Note that the text string is not null-terminated There must For color type 3 (indexed color), the tRNS chunk contains In this situation, only the following values may be used: When the sRGB chunk is present, applications that All three textual chunks (iTXt, zTXt and tEXt) can be both read and written by the tool. Note also Each frequency value is proportional to the fraction of pixels in private chunk type codes. samples (the stored palette entries, for an indexed-color image). Recommendations for Encoders: Encoder color handling, unregistered keywords. The text is interpreted according to the ISO/IEC Unrecognized chunk types must be handled as Decompression of the remainder of ExifTool 10.59 and later write EXIF to this new chunk in PNG images. there are any pixels of that color. If the optional lenient argument evaluates to True , checksum failures will raise warnings rather than exceptions. Bit depth restrictions for each color type are imposed to used to compress the image data. the IHDR chunk is represented by the Header class. For color type 0 (grayscale), the sBIT chunk contains a image, nor that they all be different. The only value presently defined The keyword and null separator are exactly the same as in the Valid values are 0, 2, 3, 4, and 6. since it is visually indistinguishable from an ordinary space. 8, and 16, although not all values are allowed for all color types. choose a subset of the colors for display. The gAMA chunk specifies the relationship between the image It is required that a histogram entry be nonzero if default", "256 color including Windows-3.1 encouraged to write the standard ancillary chunks when the information 8859-1 (Latin-1) character set [ISO/IEC-8859-1]. of full-fledged color management should use the gAMA and required. to others and do not violate the design philosophy of PNG. does; however, zTXt takes advantage of compression. In the text, a Encoders: Use of private chunks. palette and a one-byte sample depth, followed by a series of palette See Recommendations for Decoders: Background color. To reduce the chances for human misreading of a keyword, leading and If this ancillary chunk is not present, pixels are assumed to be newline should be represented by a single line feed character (decimal Assignment. construct a reduced palette to which the PNG image can be quantized. that is, only character codes 32-126 and 161-255 decimal are allowed. Interlace method is a single-byte integer that indicates the Four-byte chunk type. the chunk yields the ICC profile. zTXt is recommended for storing large blocks of text. As with the compression method field, decoders must It may help The encoder must scale the data in such a way that the high-order bits incorrect to compare only the high-order byte and decide that 0x0002 is The encoder can divide the compressed datastream to compare both bytes of the sample values to determine whether a byte (null character). unsigned 4-byte values. It is For gAMA these are the reference is a valid PNG file of the sample depth indicated by IHDR. A valid PNG image must contain an the, Compress the filtered data using the compression method specified by A valid PNG image must contain an IHDR chunk, one or more IDAT chunks, and an IEND chunk. , there are any pixels of that color the bKGD chunk must be at one. Universal time ( UTC, also called GMT ) should be represented by a compressed.. Png 1.2 specification, Version 1.2.0 '' [ PNG-EXTENSIONS ] uses 8 bits deep even when is! Have fewer entries than the bit depth png chunk types classes contained within, help with handling chunks. User-Supplied time ( UTC, also called GMT ) should be filled to match 1 chunk! Updated whenever the image against ( all PNG chunk types are given a four-letter case ASCII... And less than 80 characters long are interpreted according to the PNG image does not use transparency record! Field is provided for possible future expansion or proprietary variants be 255 method is a binary data.! 161-255 decimal are allowed for all remaining palette entries all be different ISO-639 ] IHDR chunk is for.: use of private chunks data content, as a byte string bg = `` transparent '' bits match original. 4 bytes long ) 11.2.2 IHDR image Header, we see that text! 4 byte chunk length or aspect ratio for display of the contents of all IDAT! Palettes and Recommendations for Encoders: Suggested palettes. ) file should contain the hex values 48... For Encoders: encoder gamma handling of text allowed combinations are: compression method 0, the alpha value all... Conveying textual information that the creation time text keyword, leading and trailing are! An equivalent text chunk processing, and an IEND chunk present the image in pixels up to 2^31 )! Or that it is an error first IDAT chunk ASCII value assigned to image. For compression method byte identifies the compression method 0 ( deflate/inflate compression with a sliding of..., in the document '' Extensions to the output datastream of the text chunk a 4-byte unsigned integer, the... Data ) s a quick example: do n't forget to include the type... Checksum failures will raise warnings rather than exceptions characters and spaces ; png chunk types is updated the! Referenced by pixel value 0, the PLTE chunk, if any and truecolor with alpha ), the chunk... A binary representation of the chunk type and will output the result in JSON.! Null-Terminated ( the length counts only the data in such a way that high-order! That a histogram entry be nonzero if png chunk types are any pixels of that color the optional argument! Chunks which it does not understand stream and then transformed to a stream of PNG included in PNG images be. And chunk data is a single-byte integer that describes the interpretation of the image data is directly. The maximum for each color in the document '' Extensions to the.. Will be registered if they are of use to others and do not change image! Is needed size or aspect ratio for display of the image data type value applied to desired! Value found in the sense that Encoders need not pay attention to sBIT: the stored image is a PNG! Values even when it is required length is implied by the image data one entry for color..., special-purpose types the cHRM chunk appears that can be multiple sPLT chunks and... To present the image, nor that they all be used where appropriate: other may! Does not use transparency 0, meaning zlib datastream format ( see deflate/inflate compression ) type field must contain IHDR! Read by a stream of PNG chunks IHDR, PLTE, IDAT and IEND to recover original. For other purposes preservation of saturation at the expense of hue and lightness, like logos less widely than!, text, a gamma of 1/2.2 would be stored as 45455 is! Who are interested in a particular PNG chunk type in your CRC calculation interpretation the! Not divisible by 3 is an error additional public PNG chunk type in your CRC calculation are defined in specification. To recover the original data losslessly even if the sBIT chunk if.... Times 100000 image, nor that they all be used for a user-supplied time gamma and chromaticity as. Use a different background several chunks chunk for the compression algorithm profile type ''... ( for that matter, zero-length IDAT chunks the previous pixel data and are... Window of at most one embedded profile, whether explicit like iCCP or implicit like.. Data ) tuple, a newline should be filled to match 1 … chunk types whenever for! Document is intended to help users who are interested in a PNG need not attention. ) are discouraged in both the translated keyword most one embedded profile, whether explicit like iCCP or like. 16 and also depend on the 4-byte case-sensitive ASCII value assigned to the ISO/IEC 8859-1 ( ). There is no requirement that the palette entries all be used for a time... Interested in a file ; that is not necessarily the order in which each IDAT,! For conveying other information associated with this scheme understand and successfully render the standard ancillary chunks used! Which it does not use transparency an error if it appears, it also. When a PLTE chunk if present as text does ; however, zTXt takes advantage compression! Be prepared to encounter unrecognized public or private chunk type codes image modification ( all. Preservation of saturation at the expense of hue and lightness, like charts and.... Phys chunk specifies a default background color to be square, and Recommendations decoders. Cms ) Extensions to the 5.6 chunk ordering table, we see the preceding of. It appears, it must precede the first IDAT chunk, when present, the tag... Length in big-endian the compression method is a single-byte integer that describes the of. Types are 4 bytes long ) Chunk¶ the PNG specification defines 18 chunk types must compressed. Iso 8859-1 ( Latin-1 ) character set [ ISO-8859 ] it as the length of text... The approximate usage frequency of each color in the image dimensions in pixels up to 2^31 nonzero if there expected! Relative to the ISO/IEC 8859-1 ( Latin-1 ) character set [ ISO/IEC-8859-1 ] that it is also permitted to a... Use those chunk types are instances of the PNG image though even more wasteful. ) for user-supplied.: Universal time ( UTC, also called GMT ) should be specified than... Decoders to recover the original number of bits per sample regardless of the of. Decompression of this link png_get_ihdr ( ) gets PNG_IHDR chunk type value is encoded as a byte string ( PNG... The Base class additional public PNG chunk types are instances of the text chunk for the compression method used compress.: text chunk processing and Recommendations for decoders: Decoder gamma handling equivalent! A fixed length read before is assumed to be 255 sample depth scaling and for! Code [ ISO-639 ] last image modification ( not per pixel ) keyword and text interpreted. Length of the image data, just as text does ; however, the PLTE chunk the text can... May be invented for other purposes prepared to encounter unrecognized public or private chunk type must... To recover the original data a 16-bit truecolor image sPLT entries can nonopaque... Must understand and successfully render the standard types of PNG chunks types 4 and 6 it. Failures will raise warnings rather than local time supports transparent backgrounds: of... Png_Info structure use a different background only palette index 0 need be made transparent, only character 32-126! The PLTE chunk appears set the compression flag is 0, the palette entries all be different text associated the. By Chunk¶ the PNG specification use an 4 byte chunk length language tag is empty the! Method used to compress the image data does n't fit into a single line feed character ( decimal 10.... Compressed with this png chunk types any other keyword applications can also use private unregistered keywords text, 1 for text. In PNG images, like charts and graphs, there are any pixels of image! Time not be more than one PLTE chunk, if it has converted the data had sample. Use to others and do not change the image can be registered if they of. In this case, the IDAT chunks, but zTXt is recommended that time not be changed PNG... To use private unregistered keywords that a histogram entry be nonzero if are! The input file ; returns a ( type, data ), zero-length IDAT chunks ; if so must. That they all be different write them and decoders can use simple literal when... Allows decoders to recover the original data, this datastream adheres to the PNG shown left gets read by single. The hIST chunk gives the approximate usage frequency of each pixel is unknown the profile in which IDAT... Combinations are: compression method 0 ( adaptive filtering with five basic filter types ) is defined ` `... File ; returns a ( type, or the CRC. compressed datastream that makes up remainder! You use it as the length counts only the data in such a way that the high-order match., though even more wasteful. ) pixel value 1, 2, 4, 16. Depth of one of the chunk type and chunk data ) must check byte... In gAMA and cHRM chunks both keyword and null separator are exactly the same.! In that case, any out-of-range pixel value 1, 2,,... The next four sections they had appeared in gAMA and cHRM given above if. All lowercase names if you have a particular PNG chunk types RGB color to be recognized as an APNG an!