rfc2083.txt

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      Sample values are not necessarily linear; the gAMA chunk specifies
      the gamma characteristic of the source device, 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).

   2.3. Image layout

      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:

          * An indexed-color pixel is represented by a single sample
            that is an index into a supplied palette.  The image bit
            depth determines the maximum number of palette entries, but
            not the color precision within the palette.
          * A grayscale pixel is represented by a single sample that is
            a grayscale level, where zero is black and the largest value
            for the bit depth is white.
          * A truecolor pixel is represented by three samples: red (zero
            = black, max = red) appears first, then green (zero = black,
            max = green), then blue (zero = black, max = blue).  The bit
            depth specifies the size of each sample, not the total pixel
            size.



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      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.  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.

   2.4. Alpha channel

      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.





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      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).

   2.5. Filtering

      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).

   2.6. Interlaced data order

      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



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      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 only associated with nonempty scanlines; no filter
      type bytes are present in an empty pass.



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      See Rationale: Interlacing (Section 12.6) and Recommendations for
      Decoders: Progressive display (Section 10.9).

   2.7. Gamma correction

      PNG images can specify, via the gAMA chunk, the gamma
      characteristic of the image with respect to the original scene.
      Display programs are strongly encouraged to use this information,
      plus information about the display device they are using and room
      lighting, 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.  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).

   2.8. Text strings

      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 8859-1 (Latin-1) is the character set recommended for use in
      text strings [ISO-8859].  This character set 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 (Section 12.10).



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3. File Structure

   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.

   3.1. PNG file signature

      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 12.11).

   3.2. Chunk layout

      Each chunk consists of four parts:

      Length
         A 4-byte unsigned integer giving the number of bytes in the
         chunk's data field. The length counts only the data field, not