Block diagram
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.
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.
meta:
id: gif
title: GIF (Graphics Interchange Format) image file
file-extension: gif
xref:
forensicswiki: gif
justsolve: GIF
loc: fdd000133 # GIF 89a
mime: image/gif
pronom:
- fmt/3 # GIF 87a
- fmt/4 # GIF 89a
wikidata: Q2192
license: CC0-1.0
endian: le
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: until
repeat-until: _io.eof or _.block_type == block_type::end_of_file
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:
-webide-representation: '{block_type}'
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: application_id
- id: subblocks
type: subblock
repeat: until
repeat-until: _.len_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: _.len_bytes == 0
subblock:
seq:
- id: len_bytes
type: u1
- id: bytes
size: len_bytes
application_id:
seq:
- id: len_bytes
type: u1
valid: 11
- id: application_identifier
type: str
encoding: ASCII
size: 8
- id: application_auth_code
size: 3
enums:
block_type:
0x21: extension
0x2c: local_image_descriptor
0x3b: end_of_file
extension_label:
0xf9: graphic_control
0xfe: comment
0xff: application