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The graphics interchange format GIF is besides JPG and
PNG
one of the most popular graphics formats for the web.
Its strength are animation (not possible with JPG and PNG, there is an emerging
MNG
although), transparency option (not possible with JPG and not consistently supported
by browsers with PNG) and excellent compression for
pictures with only a few colors and large areas of the same color.
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Viewing GIF's
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The xv viewer is an older but still one of the best image viewers in X. With
"xv file.gif" you can see the graphics and do many graphics manipulations.
In Unix, you can see with "xanim file.gif", "animate file.gif" an animated gif.
You also just can call "netscape file.gif" to see an animated or nonanimated GIF "file.gif".
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Producing GIF's
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GIF files can be exported from practially all graphics programs like the Gimp, Photoshop,
Illustrator, Fireworks etc. From
Mathematica, you get a gif "file.gif" for example with a
command
Display["file.gif",Plot[Sin[x],{x,0,3}],"GIF"].
Other programs like the ray tracing
program Povray or in C
you can export PPM files, which are then converted into GIF with
"ppmtogif file.ppm>file.gif" or "convert file.ppm file.gif". Note that the "convert" routine
produces by default badly compressed GIF's.
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Animated GIF's
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Given several gif files like "file1.gif", "file2.gif", ... ,"file10.gif", the
Unix program "gifmerge" allows to combine them to an animated gif for example with
"gifmerge -l0 -20 file*.gif>movie.gif", where the frames follow each other in a time interval
of 20 milliseconds. See the documentation
and source. A program running under Unix with
even more options is GIFsicle .
Animated gifs can also exported directly from the the Gimp,
where each layer will just be a frame of the animated GIF. The image magic
"convert" routine produces from a PS document "file.ps" an animated gif
"file.gif" with "convert file.ps file.gif". The script
flattengif (which uses
gifsicle and the now standard netppm routines) produces from an animated
gif a single-framed gif, where the frames are stacked onto each other.
This can be used in scripts like latex2all, which
turns a LaTeX file to PS,DVI,PDF,GIF files.
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Small GIF's
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Use as few colors as possible. Don't dither the picture, when making the RGB -> Indexed
transition. Clean out the picture by hand if necessary. Use conversion programs which
produce small pictures. The gimp, the conversion program ppmtogif, or the
viewer xv produce well compressed files, the
Imagemagic routines (convert) do not.
compressgif is a small shellscript which
assures that a small GIF is obtained.
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GIF Specification
The LZW patent
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Because GIF uses an adaptation of the Lempel Ziv Welch (LZW) algorithm for image compression
which Unisys unfortunatly was able to patent, developers of GIF-based software are
required to obtain a license under this patent.
Read
more information on that or
on the history of the controversy. The basic U.S. patent on LZW compression expired in
June, 2003. (See more details ).
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