GIF: Graphics Interchange Format

GIF89a Spec

Byte Order: Little-endian

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^(Color Resolution+1)
 11      1 byte   <Background Color Index>
 12      1 byte   <Pixel Aspect Ratio>
 13      ? bytes  <Global Color Table(2 ^ (Color Resolution+1) bytes) if GCTF is one>
         ? bytes  <Blocks (optional)>
         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^(Color Resolution+1)
         ? bytes  Local Color Table(2^(Color Resolution+1) 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
	Application Identifier set to "NETSCAPE"
	Application Authentication Code set to "2.0"
	a single sub block, 3 bytes long, with a two byte loop count (zero for infinite loop)
[
  Graphic Control Extension 
  Image Block
]*
Trailer

See also:

• http://nullsleep.tumblr.com/post/16524517190/animated-gif-minimum-frame-delay-browser-compatibility As of 2012, the actual minimum frame rate for GIFs in most browsers is 0.02 seconds. Attempting to set the rate below that, actually results in an 0.1 second rate.+
• http://www.vurdalakov.net/misc/gif/netscape-looping-application-extension how GIF animation works.
• http://giflib.sourceforge.net/whatsinagif/bits_and_bytes.html
• http://www.onicos.com/staff/iz/formats/gif.html
• Bob Cousin's Delphi page with code to read GIFs
• http://www.fileformat.info/format/gif/
• http://www.csee.umbc.edu/~squire/download/cards_gif/gif.h

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