Graphics Interchange Format

Graphics Interchange Format

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Format type	Image
Hardware	Unknown
Colour depth	8-bit
Minimum size (pixels)	0×0
Maximum size (pixels)	65536×65536
Palette	Internal
Plane count	Unknown
Transparent pixels?	Yes
Hitmap pixels?	No
Games	Dark Legions Fantasy Empires Cyber Empires

The Graphics Interchange Format (GIF) stores images with palettes of up to 256 colors and enjoyed widespread use. It supports transparency, the possibility of multiple frames in a file, animation, and LZW Compression.

Header

The header specifies some information that applies to the image as a whole, including a maximum width and height that all frames within the file must fit inside.

Data type	Description
char[3]	"GIF"
char[3]	Version mark (e.g. "87a")
UINT16LE	How many pixels wide the entire viewport is
UINT16LE	How many pixels tall the entire viewport is
BYTE	Bits 0-2: Color count indicator for Global Color Table. The formula to calculate the number of colors is 2^(indicator+1). Bit 3: Boolean flag, is the global color table sorted by decreasing order of importance Bits 4-6: Original color depth (bits per color) minus one. E.g. binary 101 (dec. 5) = 6 bits per color (VGA) Bit 7: Boolean flag, does a global color table exist
UINT8	Palette index of transparent color; ignored if the global color table bit is unset.
UINT8	Pixel aspect ratio (0 = default)

Color tables

If the relevant bit is set in the header, then a global color table (palette) will appear immediately after the GIF header, containing the number of colors also indicated in the header.

Every color consists of three bytes, which may use their full 0-255 range of values:

Data type	Description
UINT8	Red component
UINT8	Green component
UINT8	Blue component

Local color tables (used for a single frame in a multi-frame image) are also possible.

Image descriptor

Every image in a GIF file (even single frame GIFs) must have its own image descriptor.

Data type	Description
BYTE	Image separator, always 0x2C
UINT16LE	Pixel position of frame's left edge relative to total viewport (0 = leftmost column)
UINT16LE	Pixel position of frame's top edge relative to total viewport (0 = topmost row)
UINT16LE	Width of frame in pixels
UINT16LE	Height of frame in pixels
BYTE	Bits 0-2: Indicate color count for local color table. 2^(indicator+1) = number of colors. Bits 3-4: Reserved Bit 5: Boolean flag, is the local color table sorted by color importance Bit 6: Boolean flag, is the frame interlaced Bit 7: Boolean flag, is there a local color table

If a local color table is present, it appears immediately after the image descriptor. Next comes the compressed image data for the frame.

End of file

A GIF file must end with a byte whose value is 0x3B.

Source

• GIF89a Specification

Links

• Detailed GIF article on Wikipedia
• Walkthrough of LZW and GIF de/compression that may be helpful for further understanding