GIF (Graphics Interchange Format)

GIF stores images with a palette of at most 256 colors and compresses them losslessly with LZW. Its lasting use is animation: a GIF can hold many frames with delays and loop forever, and it plays everywhere: browsers, chat apps, email clients and social networks. Single transparent color, no partial transparency.

For still images PNG is better (more colors, smaller files, real alpha). For animations, MP4/WebM video, animated WebP or AVIF are 5-10 times smaller at better quality, which is why sites like Giphy and Twitter convert uploaded GIFs to video. GIF stays popular because it works without a video player.

When to use GIF#

• Use for short, small animations that must play anywhere without a video player: reactions, UI demos in READMEs and docs, email.
• Use for simple graphics with few colors when you need compatibility with very old software.
• Avoid for photos: 256 colors give banding and dithering noise.
• Avoid for still graphics: PNG is lossless too, smaller and supports 8-bit alpha.
• Avoid for long or large animations: use MP4/WebM video, animated WebP or AVIF, which are much smaller.

File structure#

A GIF file is a header, a logical screen descriptor and an optional global color table, followed by a sequence of blocks and a trailer byte. All multi-byte numbers are little-endian.

47 49 46 38 39 61         "GIF89a"
0A 00 0A 00               logical screen: width 10, height 10
F7                        packed: global color table, color resolution 8 bits, table size 2^(7+1) = 256
00                        background color index
00                        pixel aspect ratio (0 = none)
.. .. ..                  global color table: 256 x 3 bytes (R, G, B)
21 FF 0B                  application extension, 11 bytes follow
   4E 45 54 53 43 41 50 45 32 2E 30   "NETSCAPE2.0"
   03 01 00 00 00         sub-block: loop count 0 (forever), terminator
21 F9 04                  graphic control extension, 4 bytes
   04 0A 00 00 00         disposal 1 (keep), delay 10 (0.10 s), no transparency, terminator
2C                        image descriptor
   00 00 00 00 0A 00 0A 00   left 0, top 0, width 10, height 10
   00                     packed: no local color table, not interlaced
08                        LZW minimum code size
   16 .. ..  00           image data in sub-blocks (length byte + data), 0 terminates
...                       more graphic control extensions + images = frames
3B                        trailer

Header and logical screen descriptor#

The global color table, if present, follows with 3 bytes (RGB) per entry: 2, 4, 8 … 256 entries.

Blocks#

After the header, each block starts with an introducer byte:

Extension and image data are stored as sub-blocks: a length byte (1-255) followed by that many bytes, repeated, and ended by a 00 byte. A decoder can skip unknown extensions by reading sub-blocks.

The image descriptor has left, top, width and height (2 bytes each) and a packed byte: bit 7 local color table, bit 6 interlace, bits 2-0 local table size. An image can be smaller than the canvas and placed anywhere on it, which animated GIFs use to store only the changed rectangle.

Animation#

An animated GIF is several image descriptors in one file, each usually preceded by a Graphic Control Extension (GCE):

Looping is not in the GIF89a spec. The Netscape application extension (NETSCAPE2.0) carries a 2-byte loop count: 0 means loop forever, N means play N more times (browsers differ on whether N counts the first play). Without it, browsers play the animation once.

To show frame N correctly, a decoder must composite frames 0..N-1 on the canvas following each frame’s disposal method. Libraries that return raw frames leave this to you; others (Pillow, ImageMagick -coalesce, image crate) return full composited frames.

Compression#

Pixels are palette indices compressed with variable-length-code LZW (Lempel-Ziv-Welch). The minimum code size byte (2-8) is the number of bits per index; codes start one bit wider and grow up to 12 bits. LZW works well on runs and repeated patterns, poorly on noise and dithering. Interlaced images store rows in 4 passes (every 8th row, then the rows between, and so on) so a slow download shows a coarse preview first.

More than 256 colors#

Each frame can have its own local color table of up to 256 colors, so a file can show more colors by splitting a still image into tiles with different palettes. This works in browsers but makes large files; use PNG instead.

Making GIF files smaller#

• Reduce colors: 64 or 128 colors are often enough for UI recordings. Avoid heavy dithering, which hurts LZW compression.
• Crop each frame to the changed rectangle and use transparency for unchanged pixels. gifsicle -O3 does both.
• Lower the frame rate (10-15 fps is enough for screen recordings) and the canvas size.
• Lossy LZW (gifsicle --lossy=80, gifski for high-quality palettes) cuts size by 30-50% with little visible change.
• If the target supports it, use MP4/WebM video, animated WebP or AVIF instead.

Specifications and documentation#

• GIF89a specification (CompuServe, 1990): the current and final version
• GIF87a specification: the original version, without extensions
• What’s in a GIF: byte-by-byte walkthrough with LZW and animation examples
• GIF bits and bytes: every block type, including the Netscape loop extension
• GIFLIB documentation: reference C library and notes on the format
• GIF on Wikipedia

Software that opens GIF files#

Libraries#

Open-source libraries that read and write GIF, including animated GIF. See all GIF libraries for a comparison.

Related formats#

• PNG: lossless still images with more colors and real transparency; APNG adds animation
• WebP: animated WebP is much smaller than GIF and supports alpha
• AVIF: animated AVIF, smaller still, newer browser support
• JPEG: for photos
• JPEG XL: also supports animation, not yet in all browsers