Graphic Interchange format, a compact and popular way of storing images. [LZW]-encoded. 
Tk's [photo] type of [image] allows both reading and writing GIF files.
See http://www.datacompression.info/GIF.shtml for a series of references relating to GIF.

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The GIF format may now be freely used as the primary LZW patent (4,558,302) has expired.
See [[http://en.wikipedia.org/wiki/GIF]

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[Salvatore Sanfilippo] 14Oct2004: you can find BSD-licensed code for pure-Tcl GIF LZW encoding at [http://wiki.hping.org/125].

**How to make a single-pixel GIF file**

 % set im [image create photo -height 1 -width 1] 
 image4
 % $im put red
 % $im write im.gif -format gif
 % hexdump im.gif
 47 49 46 38 37 61 01 00 01 00 91 00 00 FF 00 00 # 0000 GIF87a..........
 FF FF FF FF FF FF FF FF FF 2C 00 00 00 00 01 00 # 0010 .........,......
 01 00 00 02 02 44 01 00 3B                      # 0020 .....D..;
Loading that file indeed shows a single red pixel.
(A "hexdump" command is described at [Dump a file in hex and ASCII].)

See also "[.gif color dot]" for a [pure-Tcl] version.

**Writing a larger GIF file**

    image create photo fooble -width 100 -height 100
    fooble blank
    for { set x 0 } { $x < 100 } { incr x } {
        for { set y 0 } { $y < 100 } { incr y } {
            if { $x >= 38 && $x <= 62 && $y >= 12 && $y <= 87
                 || $x >= 12 && $x <= 87 && $y >= 38 && $y <= 62 } {
                fooble put \#ffffff -to $x $y
            } else {
                fooble put \#ff0000 -to $x $y
            }
        }
    }
    grid [label .test -image fooble]
    fooble write swissflag.gif -format gif

**Reading GIF image dimensions**

[AMG]: This [proc] returns a two-element [list] consisting of the width and height, in that order, of the named image file.  If the image isn't a valid GIF, it raises an [error].  It only needs to look at the first ten bytes of the file to get the data it needs.  It probably won't work right for images with width or height greater than 32,767 pixels. ;^)

======
proc get_gif_dimensions {filename} {
  set img [open $filename rb]
  set data [read $img 10]
  close $img
  if {[binary scan $data a6s2 header size] != 2 || $header ni {GIF87a GIF89a}} {
    error "invalid GIF"
  } else {
    return $size
  }
}
======

**See also**
   * [gifBalls] for examples how to specify base64-encoded GIF data directly.
   * [Creating image photo data from GIF files]
   * [AniGIF] - an extension for animated GIFs, and a way to read and display them in plain Tk
   * [gifblock] - simple package to play with GIF streams

<<categories>> Acronym | Graphics | File