3.4 GIF. Graphics Interchange Format

Cf.

• www.dcs.ed.ac.uk/.../GIF87a.txt
• www.dcs.ed.ac.uk/.../GIF89a.txt
• www.dcs.ed.ac.uk/.../GIF-comp.txt

This format is organized by a block structure.

• Header-block:
  • 'GIF' ... 3 byte identifier
  • version ... 3 bytes (e.g. '87a', '89a')
• Logical Screen Descriptor:
  • Logical Screen Width ... 2 bytes
  • Logical Screen Height ... 2 bytes
  • Flags ... 1 byte consisting of
    • Global Color Table Flag ... 1 bit
    • Color Resolution ... 3 bits
    • Color Table Sort Flag .. 1 bit
    • Size of Color Table ... 3 bits
  • Background Color Index ... 1 byte
  • Pixel Aspect Ratio ... 1 byte
• Global Color Table (if Global Color Table Flag is 1): Colors are coded by a selectable color-table of 256 entries. $3\times 2^$(Size of Color Table) bytes of R/G/B information.
• Image Descriptor:
  • 0x2C ... 1 byte Image Separator
  • Image Left Pos ... 2 bytes
  • Image Top Pos .... 2 bytes
  • Image Width ...... 2 bytes
  • Image Height ..... 2 bytes
  • Flags ............ 1 byte consisting of Local Color Table Flag, Interlace Flag, Sort Flag, Reserved, Size Local Color Table
• Table Based Image Data (packed with LZW):
  • LZW minimum code ... 1 byte
  • Subblocks of at most 255 bytes each, these are LZW compressed with variable length output codes starting at codesize+1 (minimum 3 bits) and ending at 12 bits (Maximal code is thus 4095=0xFFF)

    Uses LZ78 (patented by Unisys), see below.

    Bits are packed from right to left. Then output is cut into subblocks of length $\leq 255$.

  • Terminated by a zero length subblock
• Graphic Control Extension (Identifier: 0x21 0xF9)
• Comment Extension (Identifier: 0x21 0xFE)
• Plain Text Extension (Identifier: 0x21 0x01)
• Application Extension (Identifier: 0x21 0xFF)
• Trailer: 0x3B ... 1 byte

This format is best used for small images (e.g. icons), images with sharp edges, and images with few colors.

For details on LZ78 used by GIF see www.dcs.ed.ac.uk/.../GIF-comp.txt