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 (2^31)-1 to accommodate languages
that have difficulty
with unsigned four-byte values. Similarly, four-byte signed integers
are limited to the range -((2^31)-1) to (2^31)-1 to
accommodate languages that have difficulty with the value -2^31.
See Rationale: Byte order.
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 (2^sampledepth)-1, not
2^sampledepth.
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.
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 and Recommendations for Decoders: Sample depth rescaling.
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.) 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). 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
(2^bitdepth)-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, Recommendations for Encoders: Alpha channel creation, and Recommendations for Decoders: Alpha channel processing.
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 and Rationale: Filtering.
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). Filtering is done on this reduced image in the usual way, and a filter-type byte is transmitted before each of its scanlines (see Filter Algorithms). Notice that the transmission order is defined so that all the scanlines transmitted in a pass will have the same number of pixels; this is necessary for proper application of some of the filters.
Caution: If the image contains fewer than five columns or fewer than five rows, some passes will be entirely empty. 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 and Recommendations for Decoders: Progressive display.
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 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 if you aren't already familiar with color representation issues.
See Rationale: Why gamma?, Recommendations for Encoders: Encoder gamma handling, and Recommendations for Decoders: Decoder gamma handling.
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 text strings [ISO/IEC-8859-1]. It is a superset of 7-bit ASCII.
Character codes not defined in Latin-1 should not be used, 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.