GIF format

GIF Header

Offset   Length   Contents
  0      3 bytes  "GIF"
  3      3 bytes  "87a" or "89a"
  6      2 bytes  <Logical Screen Width>
  8      2 bytes  <Logical Screen Height>
 10      1 byte   bit 0:    Global Color Table Flag (GCTF)
                  bit 1..3: Color Resolution
                  bit 4:    Sort Flag to Global Color Table
                  bit 5..7: Size of Global Color Table: 2^(1+n)
 11      1 byte   <Background Color Index>
 12      1 byte   <Pixel Aspect Ratio>
 13      ? bytes  <Global Color Table(0..255 x 3 bytes) if GCTF is one>
         ? bytes  <Blocks>
         1 bytes  <Trailer> (0x3b)

Image Block

Offset   Length   Contents
  0      1 byte   Image Separator (0x2c)
  1      2 bytes  Image Left Position
  3      2 bytes  Image Top Position
  5      2 bytes  Image Width
  7      2 bytes  Image Height
  8      1 byte   bit 0:    Local Color Table Flag (LCTF)
                  bit 1:    Interlace Flag
                  bit 2:    Sort Flag
                  bit 2..3: Reserved
                  bit 4..7: Size of Local Color Table: 2^(1+n)
         ? bytes  Local Color Table(0..255 x 3 bytes) if LCTF is one
         1 byte   LZW Minimum Code Size
[ // Blocks
         1 byte   Block Size (s)
        (s)bytes  Image Data
]*
         1 byte   Block Terminator(0x00)

Graphic Control Extension Block

Offset   Length   Contents
  0      1 byte   Extension Introducer (0x21)
  1      1 byte   Graphic Control Label (0xf9)
  2      1 byte   Block Size (0x04)
  3      1 byte   bit 0..2: Reserved
                  bit 3..5: Disposal Method
                  bit 6:    User Input Flag
                  bit 7:    Transparent Color Flag
  4      2 bytes  Delay Time (1/100ths of a second)
  6      1 byte   Transparent Color Index
  7      1 byte   Block Terminator(0x00)

Comment Extension Block

Offset   Length   Contents
  0      1 byte   Extension Introducer (0x21)
  1      1 byte   Comment Label (0xfe)
[
         1 byte   Block Size (s)
        (s)bytes  Comment Data
]*
         1 byte   Block Terminator(0x00)

Plain Text Extension Block

Offset   Length   Contents
  0      1 byte   Extension Introducer (0x21)
  1      1 byte   Plain Text Label (0x01)
  2      1 byte   Block Size (0x0c)
  3      2 bytes  Text Grid Left Position
  5      2 bytes  Text Grid Top Position
  7      2 bytes  Text Grid Width
  9      2 bytes  Text Grid Height
 10      1 byte   Character Cell Width(
 11      1 byte   Character Cell Height
 12      1 byte   Text Foreground Color Index(
 13      1 byte   Text Background Color Index(
[
         1 byte   Block Size (s)
        (s)bytes  Plain Text Data
]*
         1 byte   Block Terminator(0x00)

Application Extension Block

Offset   Length   Contents
  0      1 byte   Extension Introducer (0x21)
  1      1 byte   Application Label (0xff)
  2      1 byte   Block Size (0x0b)
  3      8 bytes  Application Identifire
[
         1 byte   Block Size (s)
        (s)bytes  Application Data
]*
         1 byte   Block Terminator(0x00)

GIF87a:

GIF Header
Image Block
Trailer

GIF89a:

GIF Header
Graphic Control Extension
Image Block
Trailer

GIF Animation

GIF Header
Application Extension
[
  Graphic Control Extension
  Image Block
]*
Trailer