What is .gif file?
CompuServe Graphics Interchange Format is a raster image. It was created by CompuServe in 1987.Images are compressed.
Files always begin with ASCII string "GIF8".
File extension is .gif.
Software handling .gif files
Rest7 Free Online ConverterStructure of .gif images
The header is 13 bytes long.| Name | Type | Description |
|---|---|---|
| Magic | Char8[6] | = 'GIF87a' or 'GIF89a' |
| LogicalWidth | UInt16 | |
| LogicalHeight | UInt16 | |
| GlobalTable | UInt8 | 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 (SGCT): 2^(1+n) |
| BgColorIndex | UInt8 | |
| PixelAspectRation | UInt8 |
Global Palette
if GCTF = 1 then Global Palette is present after the header. It has 0-255 entries, each is 24 bit RGB. Number of entries is defined by SGCT.Image Block
| Name | Type | Description |
|---|---|---|
| Separator | UInt8 | = 0x2C |
| X | UInt16 | Left Position of Image |
| Y | UInt16 | Top Position of Image |
| Width | UInt16 | |
| Height | UInt16 | |
| LocalTable | UInt8 | bit #0 = Local Color Table Flag (LCTF) bit #1 = Interce Flag bit #2 = Sort Flag bit #3 = Reserved bit #4 - #7: Size of Local Color Table (SGCT): 2^(1+n) |
Local Palette
if LCTF = 1 then Local Palette is present after the previous segment. It has 0-255 entries, each is 24 bit RGB. Number of entries is defined by SGCT.Graphic Control Extension Block
This block is present only in GIF89a.| Name | Type | Description |
|---|---|---|
| Separator | UInt8 | = 0x21 |
| ControlLabel | UInt8 | = 0xF9 |
| Block Size | UInt8 | = 4 |
| Flags | UInt8 | bit #0 - #2 = Reserved bit #3 - #5 = Disposal Method bit #6 = User Input Flag bit #7 = Transparent Color Flag |
| Delay Time | UInt16 | * 1/100 second |
| TransparentColorIndex | UInt8 | |
| Block Terminator | UInt8 | = 0 |
Trailer Block
This is the last segment. It's a single byte = 0x3BMore details about GIF format
Specification GIF 87aSpecification GIF 89a
Data types:
UInt8, UInt16, UInt32, UInt64 - Unsigned Integer 8 bit, 16 bit, 32 bit, 64 bit
Int8, Int16, Int32, Int64 - Signed Integer 8 bit, 16 bit, 32 bit, 64 bit
UMint16, UMint32 - Unsigned Motorola Integer 16 bit, 32 bit (Big Endian)
Char8, Char16 - Char 8 bit, 16 bit
Real world .gif samples
Here are some real world example pictures.
