For list of authors, see Credits (Chapter 19).
This is a revision of the PNG 1.0 specification, which has been published as RFC-2083 and as a W3C Recommendation. The revision has been released by the PNG Development Group but has not been approved by any standards body.
The PNG specification is on a standards track under the purview of ISO/IEC JTC 1 SC 24 and is expected to be released eventually as ISO/IEC International Standard 15948. It is the intent of the standards bodies to maintain backward compatibility with this specification. Implementors should periodically check the PNG online resources (see Online Resources, Chapter 16) for the current status of PNG documentation.
This document describes PNG (Portable Network Graphics), an extensible file format for the lossless, portable, well-compressed storage of raster images. PNG provides a patent-free replacement for GIF and can also replace many common uses of TIFF. Indexed-color, grayscale, and truecolor images are supported, plus an optional alpha channel. Sample depths range from 1 to 16 bits.
PNG is designed to work well in online viewing applications, such as the World Wide Web, so it is fully streamable with a progressive display option. PNG is robust, providing both full file integrity checking and simple detection of common transmission errors. Also, PNG can store gamma and chromaticity data for improved color matching on heterogeneous platforms.
This specification defines the Internet Media Type "image/png".
If "231" looks like
the number "231"
instead of 2 raised to the power
31, your viewer is not
recognizing the HTML <SUP> tag that was
introduced in HTML version 3.2; you need to look at
the HTML 2.0, ASCII text, or PostScript version
of this document instead, or use another browser.
The Portable Network Graphics (PNG) format provides a portable, legally unencumbered, well-compressed, well-specified standard for lossless bitmapped image files.
Although the initial motivation for developing PNG was to replace GIF (CompuServe's Graphics Interchange Format), the design provides some useful new features not available in GIF, with minimal cost to developers.
GIF features retained in PNG include:
Important new features of PNG, not available in GIF, include:
PNG is designed to be:
The main part of this specification gives the definition of the file format and recommendations for encoder and decoder behavior. An appendix gives the rationale for many design decisions. Although the rationale is not part of the formal specification, reading it can help implementors understand the design. Cross-references in the main text point to relevant parts of the rationale. Additional appendixes, also not part of the formal specification, provide tutorials on gamma and color theory as well as other supporting material.
The words "must", "required", "should", "recommended", "may", and "optional" in this document are to be interpreted as described in [RFC-2119], which is consistent with their plain English meanings. The word "can" carries the same force as "may".
See Rationale: Why a new file format? (Section 12.1), Why these features? (Section 12.2), Why not these features? (Section 12.3), Why not use format X? (Section 12.4).
PNG is pronounced "ping".
This chapter discusses basic data representations used in PNG files, as well as the expected representation of the image data.
All integers that require more than one byte must be in network byte order: the most significant byte comes first, then the less significant bytes in descending order of significance (MSB LSB for two-byte integers, B3 B2 B1 B0 for four-byte integers). The highest bit (value 128) of a byte is numbered bit 7; the lowest bit (value 1) is numbered bit 0. Values are unsigned unless otherwise noted. Values explicitly noted as signed are represented in two's complement notation.
Unless otherwise stated, four-byte unsigned integers are limited to
the range 0 to 231-1 to accommodate languages
that have difficulty
with unsigned four-byte values. Similarly, four-byte signed integers
are limited to the range -(231-1) to 231-1 to
accommodate languages that have difficulty with the value -231.
See Rationale: Byte order (Section 12.5).
Colors can be represented by either grayscale or RGB (red, green,
blue) sample data. Grayscale data represents luminance; RGB data
represents calibrated color information (if the cHRM chunk
is present) or uncalibrated device-dependent color (if cHRM
is absent). All color values range from zero (representing black) to
most intense at the maximum value for the sample depth. Note that
the maximum value at a given sample depth is 2sampledepth-1, not
2sampledepth.
Sample values are not necessarily proportional to light intensity; the gAMA chunk specifies the relationship between sample values and display output intensity, and viewers are strongly encouraged to compensate properly. See Gamma correction (Section 2.7).
Source data with a precision not directly supported in PNG (for example, 5 bit/sample truecolor) must be scaled up to the next higher supported bit depth. This scaling is reversible with no loss of data, and it reduces the number of cases that decoders have to cope with. See Recommendations for Encoders: Sample depth scaling (Section 9.1) and Recommendations for Decoders: Sample depth rescaling (Section 10.4).
Conceptually, a PNG image is a rectangular pixel array, with pixels appearing left-to-right within each scanline, and scanlines appearing top-to-bottom. (For progressive display purposes, the data may actually be transmitted in a different order; see Interlaced data order, Section 2.6.) The size of each pixel is determined by the bit depth, which is the number of bits per sample in the image data.
Three types of pixel are supported:
Optionally, grayscale and truecolor pixels can also include an alpha sample, as described in the next section.
Pixels are always packed into scanlines with no wasted bits between pixels. Pixels smaller than a byte never cross byte boundaries; they are packed into bytes with the leftmost pixel in the high-order bits of a byte, the rightmost in the low-order bits. Permitted bit depths and pixel types are restricted so that in all cases the packing is simple and efficient.
PNG permits multi-sample pixels only with 8- and 16-bit samples, so multiple samples of a single pixel are never packed into one byte. All 16-bit samples are stored in network byte order (MSB first).
Scanlines always begin on byte boundaries. When pixels have fewer than 8 bits and the scanline width is not evenly divisible by the number of pixels per byte, the low-order bits in the last byte of each scanline are wasted. The contents of these wasted bits are unspecified.
An additional "filter-type" byte is added to the beginning of every scanline (see Filtering, Section 2.5). The filter-type byte is not considered part of the image data, but it is included in the datastream sent to the compression step.
An alpha channel, representing transparency information on a per-pixel basis, can be included in grayscale and truecolor PNG images.
An alpha value of zero represents full transparency, and a value of
2bitdepth-1 represents a fully opaque pixel.
Intermediate values
indicate partially transparent pixels that can be combined with a
background image to yield a composite image. (Thus, alpha is really
the degree of opacity of the pixel. But most people refer to alpha as
providing transparency information, not opacity information, and we
continue that custom here.)
Alpha channels can be included with images that have either 8 or 16 bits per sample, but not with images that have fewer than 8 bits per sample. Alpha samples are represented with the same bit depth used for the image samples. The alpha sample for each pixel is stored immediately following the grayscale or RGB samples of the pixel.
The color values stored for a pixel are not affected by the alpha value assigned to the pixel. This rule is sometimes called "unassociated" or "non-premultiplied" alpha. (Another common technique is to store sample values premultiplied by the alpha fraction; in effect, such an image is already composited against a black background. PNG does not use premultiplied alpha.)
Transparency control is also possible without the storage cost of a full alpha channel. In an indexed-color image, an alpha value can be defined for each palette entry. In grayscale and truecolor images, a single pixel value can be identified as being "transparent". These techniques are controlled by the tRNS ancillary chunk type.
If no alpha channel nor tRNS chunk is present, all pixels in the image are to be treated as fully opaque.
Viewers can support transparency control partially, or not at all.
See Rationale: Non-premultiplied alpha (Section 12.8), Recommendations for Encoders: Alpha channel creation (Section 9.4), and Recommendations for Decoders: Alpha channel processing (Section 10.8).
PNG allows the image data to be filtered before it is compressed. Filtering can 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 that precedes the filtered scanline in the precompression datastream. An intelligent encoder can switch filters from one scanline to the next. The method for choosing which filter to employ is up to the encoder.
See Filter Algorithms (Chapter 6) and Rationale: Filtering (Section 12.9).
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 method 0, pixels are stored sequentially from left to right, and scanlines sequentially from top to bottom (no interlacing).
Interlace method 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 16 by 16 pixels, then pass 3 will contain two scanlines, each containing four pixels. When pixels have fewer than 8 bits, each such scanline is padded as needed to fill an integral number of bytes (see Image layout, Section 2.3). 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, Chapter 6). 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. Encoders and decoders must handle this case correctly. In particular, filter-type bytes are associated only with nonempty scanlines; no filter-type bytes are present in an empty pass.
See Rationale: Interlacing (Section 12.6) and Recommendations for Decoders: Progressive display (Section 10.9).
PNG images can specify, via the gAMA chunk, the power function relating the desired display output with the image samples. Display programs are strongly encouraged to use this information, plus information about the display system they are using, to present the image to the viewer in a way that reproduces what the image's original author saw as closely as possible. See Gamma Tutorial (Chapter 13) if you aren't already familiar with gamma issues.
Gamma correction is not applied to the alpha channel, if any. Alpha samples always represent a linear fraction of full opacity.
For high-precision applications, the exact chromaticity of the RGB data in a PNG image can be specified via the cHRM chunk, allowing more accurate color matching than gamma correction alone will provide. If the RGB data conforms to the sRGB specification [sRGB], this can be indicated with the sRGB chunk, enabling even more accurate reproduction. Alternatively, the iCCP chunk can be used to embed an ICC profile [ICC] containing detailed color space information. See Color Tutorial (Chapter 14) if you aren't already familiar with color representation issues.
See Rationale: Why gamma? (Section 12.7), Recommendations for Encoders: Encoder gamma handling (Section 9.2), and Recommendations for Decoders: Decoder gamma handling (Section 10.5).
A PNG file can store text associated with the image, such as an image description or copyright notice. Keywords are used to indicate what each text string represents.
ISO/IEC 8859-1 (Latin-1) is the character set recommended for use in the text strings appearing in tEXt and zTXt chunks [ISO/IEC-8859-1]. It is a superset of 7-bit ASCII. If it is necessary to convey characters outside of the Latin-1 set, the iTXt chunk should be used instead.
Character codes not defined in Latin-1 should not be used in tEXt and zTXt chunks, because they have no platform-independent meaning. If a non-Latin-1 code does appear in a PNG text string, its interpretation will vary across platforms and decoders. Some systems might not even be able to display all the characters in Latin-1, but most modern systems can.
Provision is also made for the storage of compressed text.
See Rationale: Text strings (Section 12.10).
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.
The first eight bytes of a PNG file always contain the following (decimal) values:
137 80 78 71 13 10 26 10
This signature indicates that the remainder of the file contains a single PNG image, consisting of a series of chunks beginning with an IHDR chunk and ending with an IEND chunk.
See Rationale: PNG file signature (Section 12.12).
Each chunk consists of four parts:
231-1 bytes.
The chunk data length can be any number of bytes up to the maximum; therefore, implementors cannot assume that chunks are aligned on any boundaries larger than bytes.
Chunks can appear in any order, subject to the restrictions placed on each chunk type. (One notable restriction is that IHDR must appear first and IEND must appear last; thus the IEND chunk serves as an end-of-file marker.) Multiple chunks of the same type can appear, but only if specifically permitted for that type.
See Rationale: Chunk layout (Section 12.13).
Chunk type codes are assigned so that a decoder can determine some properties of a chunk even when 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 the 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 worth noting that the property bits are an inherent part of the chunk name, and hence are fixed for any chunk type. Thus, BLOB and bLOb would be unrelated chunk type codes, not the same chunk with different properties. Decoders must recognize type codes by a simple four-byte literal comparison; it is incorrect to perform case conversion on type codes.
The semantics of the property bits are:
Chunks that are not strictly necessary in order to meaningfully display the contents of the file are known as "ancillary" chunks. A decoder encountering an unknown chunk in which the ancillary bit is 1 can safely ignore the chunk and proceed to display the image. The time chunk (tIME) is an example of an ancillary chunk.
Chunks that are necessary for successful display of the file's contents are called "critical" chunks. A decoder encountering an unknown chunk in which the ancillary bit is 0 must indicate to the user that the image contains information it cannot safely interpret. The image header chunk (IHDR) is an example of a critical chunk.
A public chunk is one that is part of the PNG specification or is registered in the list of PNG special-purpose public chunk types. Applications can also define private (unregistered) chunks for their own purposes. The names of private chunks must have a lowercase second letter, while public chunks will always be assigned names with uppercase second letters. Note that decoders do not need to test the private-chunk property bit, since it has no functional significance; it is simply an administrative convenience to ensure that public and private chunk names will not conflict. See Additional chunk types (Section 4.4), and Recommendations for Encoders: Use of private chunks (Section 9.8).
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. (Decoders should not complain about a lowercase third letter, however, as some future version of the PNG specification could define a meaning for this bit. It is sufficient to treat a chunk with a lowercase third letter in the same way as any other unknown chunk type.)
This property bit is not of interest to pure decoders, but it is needed by PNG editors (programs that modify PNG files). This bit defines the proper handling of unrecognized chunks in a file that is being modified.
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 can choose to output an appropriately modified version.)
A PNG editor is always allowed to copy all unrecognized chunks if it has only added, deleted, modified, or reordered ancillary chunks. This implies that it is not permissible for ancillary chunks to 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.
Rules for PNG editors are discussed further in Chunk Ordering Rules (Chapter 7).
For example, the hypothetical chunk type name bLOb has the property bits:
bLOb <-- 32 bit chunk type code represented in text form |||| |||+- Safe-to-copy bit is 1 (lowercase letter; bit 5 is 1) ||+-- Reserved bit is 0 (uppercase letter; bit 5 is 0) |+--- Private bit is 0 (uppercase letter; bit 5 is 0) +---- Ancillary bit is 1 (lowercase letter; bit 5 is 1)
Therefore, this name represents an ancillary, public, safe-to-copy chunk.
See Rationale: Chunk naming conventions (Section 12.14).
Chunk CRCs are calculated using standard CRC methods with pre and post conditioning, as defined by ISO 3309 [ISO-3309] or ITU-T V.42 [ITU-T-V42]. The CRC polynomial employed is
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
x31 term.
Practical calculation of the CRC always employs a precalculated table to greatly accelerate the computation. See Sample CRC Code (Chapter 15).
This chapter defines the standard types of PNG 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.
The IHDR chunk must appear FIRST. It contains:
Width: 4 bytes Height: 4 bytes Bit depth: 1 byte Color type: 1 byte Compression method: 1 byte Filter method: 1 byte Interlace method: 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
231-1 in order to accommodate languages that have difficulty with
unsigned 4-byte values.
Bit depth is a single-byte integer giving the number of bits per sample or per palette index (not per pixel). 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 codes represent sums of the following values: 1 (palette used), 2 (color used), and 4 (alpha channel used). Valid values are 0, 2, 3, 4, and 6.
Bit depth restrictions for each color type are imposed to simplify implementations and to prohibit combinations that do not compress well. Decoders must support all valid combinations of bit depth and color type. The allowed combinations are:
Color Allowed Interpretation
Type Bit Depths
0 1,2,4,8,16 Each pixel is a grayscale sample.
2 8,16 Each pixel is an R,G,B triple.
3 1,2,4,8 Each pixel is a palette index;
a PLTE chunk must appear.
4 8,16 Each pixel is a grayscale sample,
followed by an alpha sample.
6 8,16 Each pixel is an R,G,B triple,
followed by an alpha sample.
The sample depth is the same as the bit depth except in the case of color type 3, in which the sample depth is always 8 bits.
Compression method is a single-byte integer that indicates the method used to compress the image data. At present, only compression method 0 (deflate/inflate compression with a sliding window of at most 32768 bytes) is defined. All standard PNG images must be compressed with this scheme. The compression method field 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 (Chapter 5) for details.
Filter method is a single-byte integer that indicates the preprocessing method applied to the image data before compression. At present, only filter method 0 (adaptive filtering with five basic filter types) is defined. As with the compression method field, decoders must check this byte and report an error if it holds an unrecognized code. See Filter Algorithms (Chapter 6) for details.
Interlace method is a single-byte integer that indicates the transmission order of the image data. Two values are currently defined: 0 (no interlace) or 1 (Adam7 interlace). See Interlaced data order (Section 2.6) for details.
The PLTE chunk contains 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 can appear for color types 2 and 6; it must not appear for color types 0 and 4. If this chunk does appear, it must precede the first IDAT chunk. There must not be more than one PLTE chunk.
For color type 3 (indexed color), 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 in the image bit depth
(for example, 24 = 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 and truecolor with alpha), the PLTE chunk is optional. If present, it provides a suggested set of from 1 to 256 colors to which the truecolor image can be quantized if the viewer cannot display truecolor directly. If neither PLTE nor sPLT is present, such a viewer will need to select colors on its own, but it is often preferable for this to be done once by the encoder. (See Recommendations for Encoders: Suggested palettes, Section 9.5.)
Note that the palette uses 8 bits (1 byte) per sample 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.
There is no requirement that the palette entries all be used by the image, nor that they all be different.
The IDAT chunk contains the actual image data. To create this data:
The IDAT chunk contains the output datastream of the compression algorithm.
To read the image data, reverse this process.
There can 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 can divide the compressed datastream into IDAT chunks however 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 occur at any point in the compressed datastream. A PNG file in which each IDAT chunk contains only one data byte is valid, though remarkably wasteful of space. (For that matter, zero-length IDAT chunks are valid, though even more wasteful.)
See Filter Algorithms (Chapter 6) and Deflate/Inflate Compression (Chapter 5) for details.
The IEND chunk must appear LAST. It marks the end of the PNG datastream. The chunk's data field is empty.
All ancillary chunks are optional, in the sense that encoders need not write them and decoders can 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 described in the next four sections. This is not necessarily the order in which they would appear in a PNG datastream.
This chunk conveys transparency information in datastreams that do not include a full alpha channel.
The tRNS chunk specifies that the image uses simple transparency: either alpha values associated with palette entries (for indexed-color images) or a single transparent color (for grayscale and truecolor images). Although simple 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 (indexed color), the tRNS chunk contains a series of one-byte alpha values, 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 the corresponding palette index must 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 must not contain more alpha values than there are palette entries, but tRNS can contain fewer values than there are palette entries. In this case, the alpha value for all remaining palette entries is assumed to be 255. In the common case in which only palette index 0 need be made transparent, only a one-byte tRNS chunk is needed.
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
(If the image bit depth is less than 16, the least significant bits
are used and the others are 0.) 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 2bitdepth-1).
For color type 2 (truecolor), the tRNS chunk contains a single RGB color value, stored in the format:
Red: 2 bytes, range0 .. (2^bitdepth)-1Green: 2 bytes, range0 .. (2^bitdepth)-1Blue: 2 bytes, range0 .. (2^bitdepth)-1
(If the image bit depth is less than 16, the least significant bits
are used and the others are 0.) 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 2bitdepth-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 truecolor 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.
These chunks relate the image samples to the desired display intensity.
The gAMA chunk specifies the relationship between the image samples and the desired display output intensity as a power function:
sample = light_out ^ gamma
Here sample and light_out are normalized
to the range 0.0 (minimum
intensity) to 1.0 (maximum intensity). Therefore:
sample = integer_sample / (2^bitdepth - 1)
The gAMA chunk contains:
Gamma: 4 bytes
The value is encoded as a 4-byte unsigned integer, representing gamma times 100000. For example, a gamma of 1/2.2 would be stored as 45455.
The gamma value has no effect on alpha samples, which are always a linear fraction of full opacity.
If the encoder does not know the image's gamma value, it should not write a gAMA chunk; the absence of a gAMA chunk indicates that the gamma is unknown.
Technically, "desired display output intensity" is not specific enough; one needs to specify the viewing conditions under which the output is desired. For gAMA these are the reference viewing conditions of the sRGB specification [sRGB], which are based on ISO standards [ISO-3664]. Adjusting for different viewing conditions is a complex process normally handled by a Color Management System (CMS). If this adjustment is not performed, the error is usually small. Applications desiring high color fidelity may wish to use an sRGB chunk (see the sRGB chunk specification, Paragraph 4.2.2.3) or an iCCP chunk (see the iCCP chunk specification, Paragraph 4.2.2.4).
If the gAMA chunk appears, it must precede the first IDAT chunk, and it must also precede the PLTE chunk if present. An sRGB chunk or iCCP chunk, when present and recognized, overrides the gAMA chunk.
See Gamma correction (Section 2.7), Recommendations for Encoders: Encoder gamma handling (Section 9.2), and Recommendations for Decoders: Decoder gamma handling (Section 10.5).
Applications that need device-independent specification of
colors in a PNG file can use the cHRM chunk to specify the
1931 CIE x,y chromaticities of the red, green, and blue
primaries used in the image, and the referenced white point. See Color Tutorial (Chapter 14)
for more information.
The cHRM chunk contains:
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. For example, a value of
0.3127 would be stored as the integer 31270.
cHRM is allowed in all PNG files, although it is of little value for grayscale images.
If the encoder does not know the chromaticity values, it should not write a cHRM chunk; the absence of a cHRM chunk indicates that the image's primary colors are device-dependent.
If the cHRM chunk appears, it must precede the first IDAT chunk, and it must also precede the PLTE chunk if present.
An sRGB chunk or iCCP chunk, when present and recognized, overrides the cHRM chunk.
See the sRGB chunk specification (Paragraph 4.2.2.3), the iCCP chunk specification (Paragraph 4.2.2.4), Recommendations for Encoders: Encoder color handling (Section 9.3), and Recommendations for Decoders: Decoder color handling (Section 10.6).
If the sRGB chunk is present, the image samples conform to the sRGB color space [sRGB], and should be displayed using the specified rendering intent as defined by the International Color Consortium [ICC].
The sRGB chunk contains:
Rendering intent: 1 byte
The following values are defined for the rendering intent:
0: Perceptual 1: Relative colorimetric 2: Saturation 3: Absolute colorimetric
Perceptual intent is for images preferring good adaptation to the output device gamut at the expense of colorimetric accuracy, like photographs.
Relative colorimetric intent is for images requiring color appearance matching (relative to the output device white point), like logos.
Saturation intent is for images preferring preservation of saturation at the expense of hue and lightness, like charts and graphs.
Absolute colorimetric intent is for images requiring preservation of absolute colorimetry, like proofs (previews of images destined for a different output device).
An application that writes the sRGB chunk should also write a gAMA chunk (and perhaps a cHRM chunk) for compatibility with applications that do not use the sRGB chunk. In this situation, only the following values may be used:
gAMA: Gamma: 45455 cHRM: White Point x: 31270 White Point y: 32900 Red x: 64000 Red y: 33000 Green x: 30000 Green y: 60000 Blue x: 15000 Blue y: 6000
When the sRGB chunk is present, applications that recognize it and are capable of color management [ICC] must ignore the gAMA and cHRM chunks and use the sRGB chunk instead.
Applications that recognize the sRGB chunk but are not capable of full-fledged color management must also ignore the gAMA and cHRM chunks, because the applications already know what values those chunks should contain. The applications must therefore use the values of gAMA and cHRM given above as if they had appeared in gAMA and cHRM chunks.
If the sRGB chunk appears, it must precede the first IDAT chunk, and it must also precede the PLTE chunk if present. The sRGB and iCCP chunks should not both appear.
If the iCCP chunk is present, the image samples conform to the color space represented by the embedded ICC profile as defined by the International Color Consortium [ICC]. The color space of the ICC profile must be an RGB color space for color images (PNG color types 2, 3, and 6), or a monochrome grayscale color space for grayscale images (PNG color types 0 and 4).
The iCCP chunk contains:
Profile name: 1-79 bytes (character string) Null separator: 1 byte Compression method: 1 byte Compressed profile: n bytes
The format is like the zTXt chunk. (see the zTXt chunk specification, Paragraph 4.2.3.2). The profile name can be any convenient name for referring to the profile. It is case-sensitive and subject to the same restrictions as the keyword in a text chunk: it must contain only printable Latin-1 [ISO/IEC-8859-1] characters (33-126 and 161-255) and spaces (32), but no leading, trailing, or consecutive spaces. The only value presently defined for the compression method byte is 0, meaning zlib datastream with deflate compression (see Deflate/Inflate Compression, Chapter 5). Decompression of the remainder of the chunk yields the ICC profile.
An application that writes the iCCP chunk should also write gAMA and cHRM chunks that approximate the ICC profile's transfer function, for compatibility with applications that do not use the iCCP chunk.
When the iCCP chunk is present, applications that recognize it and are capable of color management [ICC] should ignore the gAMA and cHRM chunks and use the iCCP chunk instead, but applications incapable of full-fledged color management should use the gAMA and cHRM chunks if present.
A file should contain at most one embedded profile, whether explicit like iCCP or implicit like sRGB.
If the iCCP chunk appears, it must precede the first IDAT chunk, and it must also precede the PLTE chunk if present.
The iTXt, tEXt, and zTXt chunks are used for conveying textual information associated with the image. This specification refers to them generically as "text chunks".
Each of the text chunks contains as its first field a keyword that 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
Description Description of image (possibly long)
Copyright Copyright notice
Creation Time Time of original image creation
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
For the Creation Time keyword, the date format defined in section 5.2.14 of RFC 1123 is suggested, but not required [RFC-1123]. Decoders should allow for free-format text associated with this or any other keyword.
Other keywords may be invented for other purposes. Keywords of general interest can be registered with the maintainers of the PNG specification. However, it is also permitted to use private unregistered keywords. (Private keywords should be reasonably self-explanatory, in order to minimize the chance that the same keyword will be used for incompatible purposes by different people.)
The keyword must be at least one character and less than 80 characters long. Keywords are always interpreted according to the ISO/IEC 8859-1 (Latin-1) character set [ISO/IEC-8859-1]. They must contain only printable Latin-1 characters and spaces; that is, only character codes 32-126 and 161-255 decimal are allowed. To reduce the chances for human misreading of a keyword, leading and trailing spaces are forbidden, as are consecutive spaces. Note also that the non-breaking space (code 160) is not permitted in keywords, since it is visually indistinguishable from an ordinary space.
Keywords must be spelled exactly as registered, so that decoders can use simple literal comparisons when looking for particular keywords. In particular, keywords are considered case-sensitive.
Any number of text chunks can appear, and more than one with the same keyword is permissible.
See Recommendations for Encoders: Text chunk processing (Section 9.7) and Recommendations for Decoders: Text chunk processing (Section 10.11).
Textual information that the encoder wishes to record with the image can be stored in tEXt chunks. Each tEXt chunk contains a keyword (see above) and a text string, in the format:
Keyword: 1-79 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 can 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 text string can be of any length from zero bytes up to the maximum permissible chunk size less the length of the keyword and separator.
The text is interpreted according to the ISO/IEC 8859-1 (Latin-1) character set [ISO/IEC-8859-1]. The text string can contain any Latin-1 character. Newlines in the text string should be represented by a single linefeed character (decimal 10); use of other control characters in the text is discouraged.
The zTXt chunk contains textual data, just as tEXt does; however, zTXt takes advantage of compression. The zTXt and tEXt chunks are semantically equivalent, but zTXt is recommended for storing large blocks of text.
A zTXt chunk contains:
Keyword: 1-79 bytes (character string) Null separator: 1 byte Compression method: 1 byte Compressed text: n bytes
The keyword and null separator are exactly the same as in the tEXt chunk. Note that the keyword is not compressed. The compression method byte identifies the compression method used in this zTXt chunk. The only value presently defined for it is 0 (deflate/inflate compression). The compression method byte is followed by a compressed datastream that makes up the remainder of the chunk. For compression method 0, this datastream adheres to the zlib datastream format (see Deflate/Inflate Compression, Chapter 5). Decompression of this datastream yields Latin-1 text that is identical to the text that would be stored in an equivalent tEXt chunk.
This chunk is semantically equivalent to the tEXt and zTXt chunks, but the textual data is in the UTF-8 encoding of the Unicode character set instead of Latin-1. This chunk contains:
Keyword: 1-79 bytes (character string) Null separator: 1 byte Compression flag: 1 byte Compression method: 1 byte Language tag: 0 or more bytes (character string) Null separator: 1 byte Translated keyword: 0 or more bytes Null separator: 1 byte Text: 0 or more bytes
The keyword is described above.
The compression flag is 0 for uncompressed text, 1 for compressed text. Only the text field may be compressed. The only value presently defined for the compression method byte is 0, meaning zlib datastream with deflate compression. For uncompressed text, encoders should set the compression method to 0 and decoders should ignore it.
The language tag [RFC-1766] indicates the human language used by the translated keyword and the text. Unlike the keyword, the language tag is case-insensitive. It is an ASCII [ISO-646] string consisting of hyphen-separated words of 1-8 letters each (for example: cn, en-uk, no-bok, x-klingon). If the first word is two letters long, it is an ISO language code [ISO-639]. If the language tag is empty, the language is unspecified.
The translated keyword and text both use the UTF-8 encoding of the Unicode character set [ISO/IEC-10646-1], and neither may contain a zero byte (null character). The text, unlike the other strings, is not null-terminated; its length is implied by the chunk length.
Line breaks should not appear in the translated keyword. In the text, a newline should be represented by a single line feed character (decimal 10). The remaining control characters (1-9, 11-31, and 127-159) are discouraged in both the translated keyword and the text. Note that in UTF-8 there is a difference between the characters 128-159 (which are discouraged) and the bytes 128-159 (which are often necessary).
The translated keyword, if not empty, should contain a translation of the keyword into the language indicated by the language tag, and applications displaying the keyword should display the translated keyword in addition.
These chunks are used for conveying other information associated with the image.
The bKGD chunk specifies a default background color to present the image against. Note that viewers are not bound to honor this chunk; a viewer can choose to use a different background.
For color type 3 (indexed color), 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
(If the image bit depth is less than 16, the least significant bits are used and the others are 0.) The value is the gray level to be used as background.
For color types 2 and 6 (truecolor, with or without alpha), bKGD contains:
Red: 2 bytes, range0 .. (2^bitdepth)-1Green: 2 bytes, range0 .. (2^bitdepth)-1Blue: 2 bytes, range0 .. (2^bitdepth)-1
(If the image bit depth is less than 16, the least significant bits are used and the others are 0.) 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 (Section 10.7).
The pHYs chunk specifies the intended pixel size or aspect ratio for display of the image. It contains:
Pixels per unit, X axis: 4 bytes (unsigned integer) Pixels per unit, Y axis: 4 bytes (unsigned integer) Unit specifier: 1 byte
The following values are defined for the unit specifier:
0: unit is unknown 1: unit is the meter
When the unit specifier is 0, the pHYs chunk defines pixel aspect ratio only; the actual size of the pixels remains unspecified.
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 (Section 10.2).
To simplify decoders, PNG specifies that only certain sample depths can be used, and further specifies that sample values should be scaled to the full range of possible values at the sample depth. However, the sBIT chunk is provided in order to store the original number of significant bits. This allows decoders to recover the original data losslessly even if the data had a sample depth not directly supported by PNG. We recommend that an encoder emit an sBIT chunk if it has converted the data from a lower sample depth.
For color type 0 (grayscale), the sBIT chunk contains a single byte, indicating the number of bits that were significant in the source data.
For color type 2 (truecolor), the sBIT chunk contains three bytes, indicating the number of bits that were significant in the source data for the red, green, and blue channels, respectively.
For color type 3 (indexed color), the sBIT chunk contains three bytes, indicating the number of bits that 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 that were significant in the source grayscale data and the source alpha data, respectively.
For color type 6 (truecolor with alpha channel), the sBIT chunk contains four bytes, indicating the number of bits that were significant in the source data for the red, green, blue, and alpha channels, respectively.
Each depth specified in sBIT must be greater than zero and less than or equal to the sample depth (which is 8 for indexed-color images, and the bit depth given in IHDR for other color types).
A decoder need not pay attention to sBIT: the stored image is a valid PNG file of the sample depth indicated by IHDR. However, if the decoder wishes to recover the original data at its original precision, this can be done by right-shifting the stored samples (the stored palette entries, for an indexed-color image). The encoder must scale the data in such a way that the high-order bits match the original data.
If the sBIT chunk appears, it must precede the first IDAT chunk, and it must also precede the PLTE chunk if present.
See Recommendations for Encoders: Sample depth scaling (Section 9.1) and Recommendations for Decoders: Sample depth rescaling (Section 10.4).
This chunk can be used to suggest a reduced palette to be used when the display device is not capable of displaying the full range of colors present in the image. If present, it provides a recommended set of colors, with alpha and frequency information, that can be used to construct a reduced palette to which the PNG image can be quantized.
This chunk contains a null-terminated text string that names the palette and a one-byte sample depth, followed by a series of palette entries, each a six-byte or ten-byte series containing five unsigned integers:
Palette name: 1-79 bytes (character string) Null terminator: 1 byte Sample depth: 1 byte Red: 1 or 2 bytes Green: 1 or 2 bytes Blue: 1 or 2 bytes Alpha: 1 or 2 bytes Frequency: 2 bytes ...etc...
There can be any number of entries; a decoder determines the number of entries from the remaining chunk length after the sample depth byte. It is an error if this remaining length is not divisible by 6 (if the sPLT sample depth is 8) or by 10 (if the sPLT sample depth is 16). Entries must appear in decreasing order of frequency. There is no requirement that the entries all be used by the image, nor that they all be different.
The palette name can be any convenient name for referring to the palette (for example, "256 color including Macintosh default", "256 color including Windows-3.1 default", "Optimal 512"). It may help applications or people to choose the appropriate suggested palette when more than one appears in a PNG file. The palette name is case-sensitive and subject to the same restrictions as a text keyword: it must contain only printable Latin-1 [ISO/IEC-8859-1] characters (33-126 and 161-255) and spaces (32), but no leading, trailing, or consecutive spaces.
The sPLT sample depth must be 8 or 16.
The red, green, blue, and alpha samples are either one or two bytes each, depending on the sPLT sample depth, regardless of the image bit depth. The color samples are not premultiplied by alpha, nor are they precomposited against any background. An alpha value of 0 means fully transparent, while an alpha value of 255 (when the sPLT sample depth is 8) or 65535 (when the sPLT sample depth is 16) means fully opaque. The palette samples have the same gamma and chromaticity values as those of the PNG image.
Each frequency value is proportional to the fraction of pixels in the image that are closest to that palette entry in RGBA space, before the image has been composited against any background. The exact scale factor is chosen by the encoder, but should be chosen so that the range of individual values reasonably fills the range 0 to 65535. It is acceptable to artificially inflate the frequencies for "important" colors such as those in a company logo or in the facial features of a portrait. Zero is a valid frequency meaning the color is "least important" or that it is rarely if ever used. But when all of the frequencies are zero, they are meaningless (nothing may be inferred about the actual frequencies of the colors).
The sPLT chunk can appear for any PNG color type. Note
that entries in sPLT can fall outside the color space of the
PNG image; for example, in a grayscale PNG, sPLT entries
would typically satisfy R=G=B, but this is not required. Similarly,
sPLT entries can have nonopaque alpha values even when the PNG
image does not use transparency.
If sPLT appears, it must precede the first IDAT chunk. There can be multiple sPLT chunks, but if so they must have different palette names.
See Recommendations for Encoders: Suggested palettes (Section 9.5) and Recommendations for Decoders: Suggested-palette and histogram usage (Section 10.10)
The hIST chunk gives the approximate usage frequency of each color in the color palette. A hIST chunk can appear only when a PLTE 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.
The hIST chunk contains 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 (Section 12.15) and Recommendations for Decoders: Suggested-palette and histogram usage (Section 10.10).
The tIME chunk gives the time of the last image modification (not the time of initial image creation). It contains:
Year: 2 bytes (complete; for example, 1995, not 95)
Month: 1 byte (1-12)
Day: 1 byte (1-31)
Hour: 1 byte (0-23)
Minute: 1 byte (0-59)
Second: 1 byte (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.
The tIME chunk is intended for use as an automatically-applied time stamp that is updated whenever the image data is changed. It is recommended that tIME not be changed by PNG editors that do not change the image data. The Creation Time text keyword can be used for a user-supplied time (see the text chunk specification, Paragraph 4.2.3).
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
iCCP No Before PLTE and IDAT
sBIT No Before PLTE and IDAT
sRGB 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
sPLT Yes Before IDAT
tIME No None
iTXt Yes None
tEXt Yes None
zTXt Yes None
Standard keywords for text chunks:
Title Short (one line) title or caption for image
Author Name of image's creator
Description Description of image (possibly long)
Copyright Copyright notice
Creation Time Time of original image creation
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 public PNG chunk types are defined in the document "Extensions to the PNG 1.2 Specification, Version 1.2.0" [PNG-EXTENSIONS]. Chunks described there are expected to be less widely supported than those defined in this specification. However, application authors are encouraged to use those chunk types whenever appropriate for their applications. 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.
New public chunks will be registered only if they are of use to others and do not violate the design philosophy of PNG. Chunk registration is not automatic, although it is the intent of the authors that it be straightforward when a new chunk of potentially wide application is needed. Note that the creation of new critical chunk types is discouraged unless absolutely necessary.
Applications can also use private chunk types to carry data that is not of interest to other applications. See Recommendations for Encoders: Use of private chunks (Section 9.8).
Decoders must be prepared to encounter unrecognized public or private chunk type codes. Unrecognized chunk types must be handled as described in Chunk naming conventions (Section 3.3).
PNG compression method 0 (the only compression method presently defined for PNG) specifies deflate/inflate compression with a sliding window of at most 32768 bytes. 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.
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 Check value: 4 bytes
Further details on this format are given in the zlib specification [RFC-1950].
For PNG compression method 0, the zlib compression method/flags code must specify method code 8 ("deflate" compression) and an LZ77 window size of not more than 32768 bytes. Note that the zlib compression method number is not the same as the PNG compression method number. The additional flags must not specify a preset dictionary. A PNG decoder must be able to decompress any valid zlib datastream that satisfies these additional constraints.
If the data to be compressed contains 16384 bytes or fewer, the encoder can set the window size by rounding up to a power of 2 (256 minimum). This decreases the memory required not only for encoding but also for decoding, without adversely affecting the compression ratio.
The compressed data within the zlib datastream is stored as a series of blocks, each of which can represent raw (uncompressed) data, LZ77-compressed data encoded with fixed Huffman codes, or LZ77-compressed data encoded with custom Huffman codes. A marker bit in the final block identifies it as the last block, allowing the decoder to recognize the end of the compressed datastream. Further details on the compression algorithm and the encoding are given in the deflate specification [RFC-1951].
The check value 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 check values. The zlib check value is useful mainly as a cross-check that the deflate and inflate algorithms are implemented correctly. Verifying the chunk CRCs provides adequate confidence that the PNG file has been transmitted undamaged.
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 can 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 check value to be split across IDAT chunks.
In the same vein, there is no required correlation between the structure of the image data (i.e., scanline boundaries) and deflate block boundaries or IDAT chunk boundaries. The complete image data is represented by a single zlib datastream that is stored in some number of IDAT chunks; a decoder that assumes any more than this is incorrect. (Of course, some encoder implementations may emit files in which some of these structures are indeed related. But decoders cannot rely on this.)
PNG also uses zlib datastreams in iTXt, zTXt, and iCCP chunks, where the remainder of the chunk following the compression method byte is a zlib datastream as specified above. Unlike the image data, such datastreams are not split across chunks; each iTXt, zTXt, or iCCP chunk contains an independent zlib datastream.
Additional documentation and portable C code for deflate and inflate are available from the Info-ZIP archives at ftp://ftp.info-zip.org/pub/infozip/.
This chapter describes the filter algorithms that can be applied before compression. The purpose of these filters is to prepare the image data for optimum compression.
PNG filter method 0 defines five basic filter types:
Type Name 0 None 1 Sub 2 Up 3 Average 4 Paeth
(Note that filter method 0 in IHDR specifies exactly this set of five filter types. If the set of filter types is ever extended, a different filter method number will be assigned to the extended set, so that decoders need not decompress the data to discover that it contains unsupported filter types.)
The encoder can choose which of these filter algorithms 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 that specifies 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 in Image layout (Section 2.3). If the image includes an alpha channel, the alpha data is filtered in the same way as the image data.
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 will have to be stored by the decoder at all times. Even though some filter types do not refer to the prior scanline, the decoder will always need to 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 can be applied to an image. However, the filters are not equally effective on all types of data. See Recommendations for Encoders: Filter selection (Section 9.6).
See also Rationale: Filtering (Section 12.9).
With the None() filter, the scanline is transmitted
unmodified; it is necessary only to insert a filter-type byte before the data.
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 the scanline:
Sub(x) = Raw(x) - Raw(x-bpp)
where x ranges from zero to the number of bytes
representing the scanline minus one, Raw() 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 samples, two bytes per sample); 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
sample, plus two-byte alpha sample).
Note this computation is done for each byte,
regardless of bit depth. In a 16-bit image, each MSB is predicted from
the preceding MSB and each LSB 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.
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 the scanline:
Up(x) = Raw(x) - Prior(x)
where x ranges from zero to the number of bytes
representing the scanline minus one, Raw() 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.
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 the scanline:
Average(x) = Raw(x) - floor((Raw(x-bpp)+Prior(x))/2)
where x ranges from zero to the number of bytes
representing the scanline minus one, Raw() refers
to the raw data byte at that byte position in the scanline,
Prior() 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.
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
due to Alan W. Paeth [PAETH].
To compute the Paeth() filter, apply the following formula
to each byte of the 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 the scanline minus one, Raw() refers
to the raw data byte at that byte position in the scanline,
Prior() 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 then return a
else if pb <= pc then return b
else 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 byte
value.
Note that the order in which ties are broken is critical 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.
To allow new chunk types to be added to PNG, it is necessary to establish rules about the ordering requirements for all chunk types. Otherwise a PNG editing program cannot know what to do when it encounters an unknown chunk.
We define a "PNG editor" as a program that modifies a PNG file and wishes to preserve as much as possible of the ancillary information in the file. Two examples of PNG editors are a program that adds or modifies text chunks, and a program that adds a suggested palette to a truecolor PNG file. Ordinary image editors are not PNG editors in this sense, because they usually discard all unrecognized information while reading in an image. (Note: we strongly encourage programs handling PNG files to preserve ancillary information whenever possible.)
As an example of possible problems, consider a hypothetical new ancillary chunk type that is safe-to-copy and is required to appear after PLTE if PLTE is present. If our program to add a suggested PLTE does not recognize this new chunk, it may insert PLTE in the wrong place, namely after the new chunk. We could prevent such problems by requiring PNG editors to discard all unknown chunks, but that is a very unattractive solution. Instead, PNG requires ancillary chunks not to have ordering restrictions like this.
To prevent this type of problem while allowing for future extension, we put some constraints on both the behavior of PNG editors and the allowed ordering requirements for chunks.
The rules for PNG editors are:
These rules are expressed in terms of copying chunks from an input file to an output file, but they apply in the obvious way if a PNG file is modified in place.
See also Chunk naming conventions (Section 3.3).
The ordering rules for an ancillary chunk type cannot be any stricter than this:
The actual ordering rules for any particular ancillary chunk type may be weaker. See for example the ordering rules for the standard ancillary chunk types (Summary of standard chunks, Section 4.3).
Decoders must not assume more about the positioning of any ancillary chunk than is specified by the chunk ordering rules. In particular, it is never valid to assume that a specific ancillary chunk type occurs with any particular positioning relative to other ancillary chunks. (For example, it is unsafe to assume that your private ancillary chunk occurs immediately before IEND. Even if your application always writes it there, a PNG editor might have inserted some other ancillary chunk after it. But you can safely assume that your chunk will remain somewhere between IDAT and IEND.)
Critical chunks can have arbitrary ordering requirements, because PNG editors are required to give up if they encounter unknown critical chunks. For example, IHDR has the special ordering rule that it must always appear first. A PNG editor, or indeed any PNG-writing program, must know and follow the ordering rules for any critical chunk type that it can emit.
On systems where file names customarily include an extension signifying file type, the extension ".png" is recommended for PNG files. Lowercase ".png" is preferred if file names are case-sensitive.
The Internet Assigned Numbers Authority (IANA) has registered "image/png" as the Internet Media Type for PNG [RFC-2045], [RFC-2048]. For robustness, decoders may choose to also support the interim media type "image/x-png" that was in use before registration was complete.
In the Apple Macintosh system, the following conventions are recommended:
PNG itself is strictly a single-image format. However, it may be necessary to store multiple images within one file; for example, this is needed to convert GIF animation files. The PNG Development Group has defined and approved a multiple-image format based on PNG, called "Multiple-image Network Graphics (MNG)" [MNG]. This is considered a separate file format and has a different signature. PNG-supporting applications may or may not choose to support the multiple-image format.
See Rationale: Why not these features? (Section 12.3).
A PNG file or datastream is composed of a collection of explicitly typed "chunks". Chunks whose contents are defined by the specification could actually contain anything, including malicious code. But there is no known risk that such malicious code could be executed on the recipient's computer as a result of decoding the PNG image.
The possible security risks associated with private chunk types and future chunk types cannot be specified at this time. Security issues will be considered when evaluating chunks proposed for registration as public chunks. There is no additional security risk associated with unknown or unimplemented chunk types, because such chunks will be ignored, or at most be copied into another PNG file.
The text chunks contain keywords and data that is meant to be displayed as plain text. The iCCP, sPLT, and some public "extension" chunks contain keywords meant to be displayed as plain text. It is possible that if a decoder displays such text without filtering out control characters, especially the ESC (escape) character, certain systems or terminals could behave in undesirable and insecure ways. We recommend that decoders filter out control characters to avoid this risk; see Recommendations for Decoders: Text chunk processing (Section 10.11).
Every chunk begins with a length field, making it easier to write decoders invulnerable to fraudulent chunks that attempt to overflow buffers. The CRC at the end of every chunk provides a robust defense against accidentally corrupted data. Also, the PNG signature bytes provide early detection of common file transmission errors.
A decoder that fails to check CRCs could be subject to data corruption. The only likely consequence of such corruption is incorrectly displayed pixels within the image. Worse things might happen if the CRC of the IHDR chunk is not checked and the width or height fields are corrupted. See Recommendations for Decoders: Error checking (Section 10.1).
A poorly written decoder might be subject to buffer overflow, because
chunks can be extremely large, up to 231-1 bytes long. But
properly written decoders will handle large chunks without difficulty.
This chapter gives some recommendations for encoder behavior. The only absolute requirement on a PNG encoder is that it produce files that conform to the format specified in the preceding chapters. However, best results will usually be achieved by following these recommendations.
When encoding input samples that have a sample depth that cannot be directly represented in PNG, the encoder must scale the samples up to a sample depth that is allowed by PNG. The most accurate scaling method is the linear equation
output = ROUND(input * MAXOUTSAMPLE / MAXINSAMPLE)
where the input samples range from 0 to MAXINSAMPLE
and the outputs
range from 0 to MAXOUTSAMPLE
(which is 2sampledepth-1).
A close approximation to the linear scaling method can be achieved by "left bit replication", which is shifting the valid bits to begin in the most significant bit and repeating the most significant bits into the open bits. This method is often faster to compute than linear scaling. As an example, assume that 5-bit samples are being scaled up to 8 bits. If the source sample value is 27 (in the range from 0-31), then the original bits are:
4 3 2 1 0 --------- 1 1 0 1 1
Left bit replication gives a value of 222:
7 6 5 4 3 2 1 0
----------------
1 1 0 1 1 1 1 0
|=======| |===|
| Leftmost Bits Repeated to Fill Open Bits
|
Original Bits
which matches the value computed by the linear equation. Left bit replication usually gives the same value as linear scaling and is never off by more than one.
A distinctly less accurate approximation is obtained by simply left-shifting the input value and filling the low order bits with zeroes. This scheme cannot reproduce white exactly, since it does not generate an all-ones maximum value; the net effect is to darken the image slightly. This method is not recommended in general, but it does have the effect of improving compression, particularly when dealing with greater-than-eight-bit sample depths. Since the relative error introduced by zero-fill scaling is small at high sample depths, some encoders may choose to use it. Zero-fill must not be used for alpha channel data, however, since many decoders will special-case alpha values of all zeroes and all ones. It is important to represent both those values exactly in the scaled data.
When the encoder writes an sBIT chunk, it is required to
do the scaling in such a way that the high-order bits of the stored
samples match the original data. That is, if the sBIT chunk
specifies a sample depth of S, the high-order
S bits of
the stored data must agree with the original S-bit
data values. This allows decoders to
recover the original data by shifting right. The added low-order bits
are not constrained. Note that all the above scaling methods meet this
restriction.
When scaling up source data, it is recommended that the low-order bits be filled consistently for all samples; that is, the same source value should generate the same sample value at any pixel position. This improves compression by reducing the number of distinct sample values. However, this is not a requirement, and some encoders may choose not to follow it. For example, an encoder might instead dither the low-order bits, improving displayed image quality at the price of increasing file size.
In some applications the original source data may have a range that
is not a power of 2. The linear scaling equation still works for this
case, although the shifting methods do not. It is recommended that an
sBIT chunk not be written for such images, since sBIT
suggests that the original data range was
exactly 0..2S-1.
See Gamma Tutorial (Chapter 13) if you aren't already familiar with gamma issues.
Encoders capable of full-fledged color management [ICC] will perform more sophisticated calculations than those described here, and they may choose to use the iCCP chunk. Encoders that know that their image samples conform to the sRGB specification [sRGB] should use the sRGB chunk and not perform gamma handling. Otherwise, this section applies.
The encoder has two gamma-related decisions to make. First, it must decide how to transform whatever image samples it has into the image samples that will go into the PNG file. Second, it must decide what value to write into the gAMA chunk.
The rule for the second decision is simply to write whatever value will cause a decoder to do what you want. See Recommendations for Decoders: Decoder gamma handling (Section 10.5).
The first decision depends on the nature of the image samples and their precision. If the samples represent light intensity in floating-point or high-precision integer form (perhaps from a computer image renderer), then the encoder may perform "gamma encoding" (applying a power function with exponent less than 1) before quantizing the data to integer values for output to the file. This results in fewer banding artifacts at a given sample depth, or allows smaller samples while retaining the same visual quality. An intensity level expressed as a floating-point value in the range 0 to 1 can be converted to a file image sample by
sample = intensity ^ encoding_exponentinteger_sample = ROUND(sample * (2^bitdepth - 1))
If the intensity in the equation is the desired display output intensity, then the encoding exponent is the gamma value to be written to the file, by the definition of gAMA (See the gAMA chunk specification, Paragraph 4.2.2.1). But if the intensity available to the encoder is the original scene intensity, another transformation may be needed. Sometimes the displayed image should have higher contrast than the original image; in other words, the end-to-end transfer function from original scene to display output should have an exponent greater than 1. In this case,
gamma = encoding_exponent / end_to_end_exponent
If you don't know whether the conditions under which the original image was captured (or calculated) warrant such a contrast change, you may assume that display intensities are proportional to original scene intensities; in other words, the end-to-end exponent is 1, so gamma and the encoding exponent are equal.
If the image is being written to a file only, the encoder is free to choose the encoding exponent. Choosing a value that causes the gamma value in the gAMA chunk to be 1/2.2 is often a reasonable choice because it minimizes the work for a decoder displaying on a typical video monitor.
Some image renderers may simultaneously write the image to a PNG file and display it on-screen. The displayed pixels should be appropriate for the display system, so that the user sees a proper representation of the intended scene.
If the renderer wants to write the displayed sample values to the PNG file, avoiding a separate gamma encoding step for file output, then the renderer should approximate the transfer function of the display system by a power function, and write the reciprocal of the exponent into the gAMA chunk. This will allow a PNG decoder to reproduce what the file's originator saw on screen during rendering.
However, it is equally reasonable for a renderer to compute displayed pixels appropriate for the display device, and to perform separate gamma encoding for file storage, arranging to have a value in the gAMA chunk more appropriate to the future use of the image.
Computer graphics renderers often do not perform gamma encoding, instead making sample values directly proportional to scene light intensity. If the PNG encoder receives intensity samples that have already been quantized into integers, there is no point in doing gamma encoding on them; that would just result in further loss of information. The encoder should just write the sample values to the PNG file. This does not imply that the gAMA chunk should contain a gamma value of 1.0, because the desired end-to-end transfer function from scene intensity to display output intensity is not necessarily linear. The desired gamma value is probably not far from 1.0, however. It may depend on whether the scene being rendered is a daylight scene or an indoor scene, etc.
When the sample values come directly from a piece of hardware, the correct gamma value can in principle be inferred from the transfer function of the hardware and the lighting conditions of the scene. In the case of video digitizers ("frame grabbers"), the samples are probably in the sRGB color space, because the sRGB specification was designed to be compatible with video standards. Image scanners are less predictable. Their output samples may be proportional to the input light intensity because CCD (charge coupled device) sensors themselves are linear, or the scanner hardware may have already applied a power function designed to compensate for dot gain in subsequent printing (an exponent of about 0.57), or the scanner may have corrected the samples for display on a monitor. The device documentation might describe the transformation performed, or might describe the target display or printer for the image data (which might be configurable). You can also scan a calibrated target and use calibration software to determine the behavior of the device. Remember that gamma relates file samples to desired display output, not to scanner input.
File format converters generally should not attempt to convert supplied images to a different gamma. Store the data in the PNG file without conversion, and deduce the gamma value from information in the source file if possible. Gamma alteration at file conversion time causes re-quantization of the set of intensity levels that are represented, introducing further roundoff error with little benefit. It's almost always better to just copy the sample values intact from the input to the output file.
If the source file format describes the gamma characteristic of the image, a file format converter is strongly encouraged to write a PNG gAMA chunk. Note that some file formats specify the exponent of the function mapping file samples to display output rather than the other direction. If the source file's gamma value is greater than 1.0, it is probably a display system exponent, and you should use its reciprocal for the PNG gamma. If the source file format records the relationship between image samples and something other than display output, then deducing the PNG gamma value will be more complex.
Regardless of how an image was originally created, if an encoder
or file format converter knows that the image has been displayed
satisfactorily using a display system whose transfer function can be
approximated by a power function with exponent display_exponent,
then the image can be marked as having the gamma value:
gamma = 1 / display_exponent
It's better to write a gAMA chunk with an approximately right value than to omit the chunk and force PNG decoders to guess at an appropriate gamma.
On the other hand, if the encoder has no way to infer the gamma value, then it is better to omit the gAMA chunk entirely. If the image gamma has to be guessed at, leave it to the decoder to do the guessing.
Gamma does not apply to alpha samples; alpha is always represented linearly.
See also Recommendations for Decoders: Decoder gamma handling (Section 10.5).
See Color Tutorial (Chapter 14) if you aren't already familiar with color issues.
Encoders capable of full-fledged color management [ICC] will perform more sophisticated calculations than those described here, and they may choose to use the iCCP chunk. Encoders that know that their image samples conform to the sRGB specification [sRGB] are strongly encouraged to use the sRGB chunk. Otherwise, this section applies.
If it is possible for the encoder to determine the chromaticities of the source display primaries, or to make a strong guess based on the origin of the image or the hardware running it, then the encoder is strongly encouraged to output the cHRM chunk. If it does so, the gAMA chunk should also be written; decoders can do little with cHRM if gAMA is missing.
Video created with recent video equipment probably uses the CCIR 709 primaries and D65 white point [ITU-R-BT709], which are:
R G B White
x 0.640 0.300 0.150 0.3127
y 0.330 0.600 0.060 0.3290
An older but still very popular video standard is SMPTE-C [SMPTE-170M]:
R G B White
x 0.630 0.310 0.155 0.3127
y 0.340 0.595 0.070 0.3290
The original NTSC color primaries have not been used in decades. Although you may still find the NTSC numbers listed in standards documents, you won't find any images that actually use them.
Scanners that produce PNG files as output should insert the filter chromaticities into a cHRM chunk.
In the case of hand-drawn or digitally edited images, you have to determine what monitor they were viewed on when being produced. Many image editing programs allow you to specify what type of monitor you are using. This is often because they are working in some device-independent space internally. Such programs have enough information to write valid cHRM and gAMA chunks, and should do so automatically.
If the encoder is compiled as a portion of a computer image renderer that performs full-spectral rendering, the monitor values that were used to convert from the internal device-independent color space to RGB should be written into the cHRM chunk. Any colors that are outside the gamut of the chosen RGB device should be clipped or otherwise constrained to be within the gamut; PNG does not store out-of-gamut colors.
If the computer image renderer performs calculations directly in device-dependent RGB space, a cHRM chunk should not be written unless the scene description and rendering parameters have been adjusted to look good on a particular monitor. In that case, the data for that monitor (if known) should be used to construct a cHRM chunk.
There are often cases where an image's exact origins are unknown, particularly if it began life in some other format. A few image formats store calibration information, which can be used to fill in the cHRM chunk. For example, all PhotoCD images use the CCIR 709 primaries and D65 white point, so these values can be written into the cHRM chunk when converting a PhotoCD file. PhotoCD also uses the SMPTE-170M transfer function. (PhotoCD can store colors outside the RGB gamut, so the image data will require gamut mapping before writing to PNG format.) TIFF 6.0 files can optionally store calibration information, which if present should be used to construct the cHRM chunk. GIF and most other formats do not store any calibration information.
It is not recommended that file format converters attempt to convert supplied images to a different RGB color space. Store the data in the PNG file without conversion, and record the source primary chromaticities if they are known. Color space transformation at file conversion time is a bad idea because of gamut mismatches and rounding errors. As with gamma conversions, it's better to store the data losslessly and incur at most one conversion when the image is finally displayed.
See also Recommendations for Decoders: Decoder color handling (Section 10.6).
The alpha channel can be regarded either as a mask that temporarily hides transparent parts of the image, or as a means for constructing a non-rectangular image. In the first case, the color values of fully transparent pixels should be preserved for future use. In the second case, the transparent pixels carry no useful data and are simply there to fill out the rectangular image area required by PNG. In this case, fully transparent pixels should all be assigned the same color value for best compression.
Image authors should keep in mind the possibility that a decoder will ignore transparency control. Hence, the colors assigned to transparent pix