IMAGE(6)                                                 IMAGE(6)

     NAME
          image - external format for images

     DESCRIPTION
          Images are described in draw-image(2), and the definition of
          pixel values is in colour(6). Fonts and images are stored in
          external files in machine-independent formats.

          Image files are read and written using Display.readimage and
          Display.writeimage (see draw-display(2)). An image is a
          rectangular array of pixels, where each pixel is organised
          as one or more channels, as determined by the image.

          An uncompressed image file starts with 5 strings: chan,
          r.min.x, r.min.y, r.max.x, and r.max.y.  Each is right-
          justified and blank padded in 11 characters, followed by a
          blank.  The chan value is a textual string describing the
          pixel format (see below for a discussion of channel descrip-
          tors), and the rectangle coordinates are decimal strings.
          The rest of the file contains the r.max.y-r.min.y rows of
          pixel data.  A row consists of the byte containing pixel
          r.min.x and all the bytes up to and including the byte con-
          taining pixel r.max.x-1.  For images with depth d less than
          eight, a pixel with x-coordinate = x will appear as d con-
          tiguous bits in a byte, with the pixel's high order bit
          starting at the byte's bit number w*(x mod (8/w)), where
          bits within a byte are numbered 0 to 7 from the high order
          to the low order bit.  Rows contain integral number of
          bytes, so there may be some unused pixels at either end of a
          row.  If d is greater than 8, the definition of images
          requires that it be a multiple of 8, so pixel values take up
          an integral number of bytes.

          The Image.readpixels and Image.writepixel operations
          described in draw-image(2) also deal with rows in this for-
          mat, stored in Limbo arrays of bytes.

          The channel format string is a sequence of two-character
          channel descriptions, each comprising a letter (r for red, g
          for green, b for blue, a for alpha, m for colour-mapped, k
          for greyscale, and x for ``don't care'') followed by a num-
          ber of bits per pixel.  The sum of the channel bits per
          pixel is the depth of the image, which must be either a
          divisor or a multiple of eight.  It is an error to have more
          than one of any channel but x.  An image must have either a
          greyscale channel; a colour-mapped channel; or red, green,
          and blue channels.  If the alpha channel is present, it must
          be at least as deep as any other channel.

          The channel string defines the format of the pixels in the

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     IMAGE(6)                                                 IMAGE(6)

          file, and should not be confused with ordering of bytes in
          the file, which is little-endian.  In particular 'r8g8b8'
          pixels have byte ordering blue, green, and red within the
          file.  See colour(6) for more details of the pixel format.

          A previous Inferno image format replaces the channel string
          with a decimal ldepth, which is the base two logarithm of
          the number of bits per pixel in the image.  In this case,
          ldepths 0, 1, 2, and 3 correspond to channel descriptors k1,
          k2, k4, and m8, respectively.  Furthermore, the pixel values
          are inverted compared to the current colour maps; in partic-
          ular, an all-zero pixel is white and all-ones is black.
          That format is still readable but cannot be written; older
          files should be converted to the newer one.  The image file
          reading operations automatically invert the pixel values to
          produce correct results.

          A compressed image file begins with the 11 bytes
          "compressed\n", immediately followed by a header as
          described above, followed by the image data.  (The pixel
          data once uncompressed has the format described above.)  The
          rest of the file is a string of compression blocks, each
          encoding a number of rows of the image's pixel data.  Com-
          pression blocks are at most 6024 bytes long, so that they
          fit comfortably in a single 9P message.  Since a compression
          block must encode a whole number of rows, there is a limit
          (about 5825 bytes) to the width of images that may be
          encoded.  Most wide images are in subfonts, which, at 1 bit
          per pixel (the usual case for fonts), can be 46600 pixels
          wide.

          A compression block begins with two decimal strings of
          twelve bytes each.  The first number is one more than the y
          coordinate of the last row in the block.  The second is the
          number of bytes of compressed data in the block, not includ-
          ing the two decimal strings.  This number must not be larger
          than 6000.

          Pixels are encoded using a version of Lempel & Ziv's sliding
          window scheme LZ77, best described in J A Storer & T G Szy-
          manski `Data Compression via Textual Substitution', JACM
          29#4, pp. 928-951.

          The compression block is a string of variable-length code
          words encoding substrings of the pixel data.  A code word
          either gives the substring directly or indicates that it is
          a copy of data occurring previously in the pixel stream.

          In a code word whose first byte has the high-order bit set,
          the rest of the byte indicates the length of a substring
          encoded directly.  Values from 0 to 127 encode lengths from
          1 to 128 bytes.  Subsequent bytes are the literal pixel

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     IMAGE(6)                                                 IMAGE(6)

          data.

          If the high-order bit is zero, the next 5 bits encode the
          length of a substring copied from previous pixels.  Values
          from 0 to 31 encode lengths from 3 to 34 bytes.  The bottom
          two bits of the first byte and the 8 bits of the next byte
          encode an offset backward from the current position in the
          pixel data at which the copy is to be found.  Values from 0
          to 1023 encode offsets from 1 to 1024.  The encoding may be
          `prescient', with the length larger than the offset, which
          works just fine: the new data is identical to the data at
          the given offset, even though the two strings overlap.

     SEE ALSO
          draw-intro(2), draw-image(2), draw(3), colour(6), font(6)

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