GIF (Graphics Interchange Format) image file: format specification

GIF (Graphics Interchange Format) is an image file format, developed in 1987. It became popular in 1990s as one of the main image formats used in World Wide Web.

GIF format allows encoding of palette-based images up to 256 colors (each of the colors can be chosen from a 24-bit RGB colorspace). Image data stream uses LZW (Lempel–Ziv–Welch) lossless compression.

Over the years, several version of the format were published and several extensions to it were made, namely, a popular Netscape extension that allows to store several images in one file, switching between them, which produces crude form of animation.

Structurally, format consists of several mandatory headers and then a stream of blocks follows. Blocks can carry additional metainformation or image data.

File extension

gif

KS implementation details

License: CC0-1.0

This page hosts a formal specification of GIF (Graphics Interchange Format) image file using Kaitai Struct. This specification can be automatically translated into a variety of programming languages to get a parsing library.

Block diagram

Format specification in Kaitai Struct YAML

meta:
  id: gif
  file-extension: gif
  title: GIF (Graphics Interchange Format) image file
  endian: le
  license: CC0-1.0
doc: |
  GIF (Graphics Interchange Format) is an image file format, developed
  in 1987. It became popular in 1990s as one of the main image formats
  used in World Wide Web.

  GIF format allows encoding of palette-based images up to 256 colors
  (each of the colors can be chosen from a 24-bit RGB
  colorspace). Image data stream uses LZW (Lempel–Ziv–Welch) lossless
  compression.

  Over the years, several version of the format were published and
  several extensions to it were made, namely, a popular Netscape
  extension that allows to store several images in one file, switching
  between them, which produces crude form of animation.

  Structurally, format consists of several mandatory headers and then
  a stream of blocks follows. Blocks can carry additional
  metainformation or image data.
seq:
  - id: hdr
    type: header
  - id: logical_screen_descriptor
    type: logical_screen_descriptor_struct
  - id: global_color_table
    type: color_table
    if: logical_screen_descriptor.has_color_table
    size: logical_screen_descriptor.color_table_size * 3
    doc-ref: https://www.w3.org/Graphics/GIF/spec-gif89a.txt - section 18
  - id: blocks
    type: block
    repeat: eos
types:
  header:
    doc-ref: https://www.w3.org/Graphics/GIF/spec-gif89a.txt - section 17
    seq:
      - id: magic
        contents: 'GIF'
      - id: version
        type: str
        size: 3
        encoding: ASCII
  logical_screen_descriptor_struct:
    doc-ref: https://www.w3.org/Graphics/GIF/spec-gif89a.txt - section 18
    seq:
      - id: screen_width
        type: u2
      - id: screen_height
        type: u2
      - id: flags
        type: u1
      - id: bg_color_index
        type: u1
      - id: pixel_aspect_ratio
        type: u1
    instances:
      has_color_table:
        value: '(flags & 0b10000000) != 0'
      color_table_size:
        value: '2 << (flags & 7)'
  color_table:
    doc-ref: https://www.w3.org/Graphics/GIF/spec-gif89a.txt - section 19
    seq:
      - id: entries
        type: color_table_entry
        repeat: eos
  color_table_entry:
    seq:
      - id: red
        type: u1
      - id: green
        type: u1
      - id: blue
        type: u1
  block:
    seq:
      - id: block_type
        type: u1
        enum: block_type
      - id: body
        type:
          switch-on: block_type
          cases:
            'block_type::extension': extension
            'block_type::local_image_descriptor': local_image_descriptor
  local_image_descriptor:
    seq:
      - id: left
        type: u2
      - id: top
        type: u2
      - id: width
        type: u2
      - id: height
        type: u2
      - id: flags
        type: u1
      - id: local_color_table
        type: color_table
        if: has_color_table
        size: color_table_size * 3
      - id: image_data
        type: image_data
    instances:
      has_color_table:
        value: '(flags & 0b10000000) != 0'
      has_interlace:
        value: '(flags & 0b01000000) != 0'
      has_sorted_color_table:
        value: '(flags & 0b00100000) != 0'
      color_table_size:
        value: '2 << (flags & 7)'
  image_data:
    doc-ref: https://www.w3.org/Graphics/GIF/spec-gif89a.txt - section 22
    seq:
      - id: lzw_min_code_size
        type: u1
      - id: subblocks
        type: subblocks
  extension:
    seq:
      - id: label
        type: u1
        enum: extension_label
      - id: body
        type:
          switch-on: label
          cases:
            'extension_label::application': ext_application
            'extension_label::comment': subblocks
            'extension_label::graphic_control': ext_graphic_control
            _: subblocks
  ext_application:
    seq:
      - id: application_id
        type: subblock
      - id: subblocks
        type: subblock
        repeat: until
        repeat-until: _.num_bytes == 0
  ext_graphic_control:
    doc-ref: https://www.w3.org/Graphics/GIF/spec-gif89a.txt - section 23
    seq:
      - id: block_size
        contents: [4]
      - id: flags
        type: u1
      - id: delay_time
        type: u2
      - id: transparent_idx
        type: u1
      - id: terminator
        contents: [0]
    instances:
      transparent_color_flag:
        value: '(flags & 0b00000001) != 0'
      user_input_flag:
        value: '(flags & 0b00000010) != 0'
  subblocks:
    seq:
      - id: entries
        type: subblock
        repeat: until
        repeat-until: _.num_bytes == 0
  subblock:
    seq:
      - id: num_bytes
        type: u1
      - id: bytes
        size: num_bytes
enums:
  block_type:
    0x21: extension
    0x2c: local_image_descriptor
    0x3b: end_of_file
  extension_label:
    0xf9: graphic_control
    0xfe: comment
    0xff: application