Page 206 - DCAP303_MULTIMEDIA_SYSTEMS

Multimedia Systems

notes 12.3 Graphic interchange format (Gif)

The Graphics Interchange Format (GIF) is a bitmap image format that was introduced by
CompuServe in 1987 and has since come into widespread usage on the World Wide Web due to
its wide support and portability.

The format supports up to 8 bits-per-pixel thus allowing a single image to reference a palette of up
to 256 distinct colours. The colours are chosen from the 24-bit RGB colour space. It also supports
animations and allows a separate palette of 256 colours for each frame. The colour limitation
makes the GIF format unsuitable for reproducing colour photographs and other images with
continuous colour, but it is well-suited for simpler images such as graphics or logos with solid
areas of colour.

The GIF images are compressed using the Lempel Ziv Welch (LZW) lossless data compression
technique to reduce the file size without degrading the visual quality. This compression technique
was patented in 1985.
(.GIF files extension) There have been raging debates about the pronunciation. The designers of
GIF say it is correctly pronounced to sound like Jiff. But that seems counter-intuitive, and up in
my hills, we say it sounding like Gift (without the t).

The GIF show images online (in 1987 for 8 bit video boards, before JPG and 24 bit colour was in
use). The GIF uses indexed colour, which is limited to a palette of only 256 colours (next page).
The GIF was a great match for the old 8 bit 256 colour video boards, but is inappropriate for
today’s 24 bit photo images.

The GIF files do not store the image’s scaled resolution ppi number, so scaling is necessary every
time one is printed. This is of no importance for screen or Web images. Our printers did not print
images in 1987, so it was useless information, and CompuServe simply did not bother to store
the printing resolution in GIF files.

The GIF is still an excellent format for graphics, and this is its purpose today, especially on the
Web. Graphic images (like logos or dialog boxes) use few colours. Being limited to 256 colours is
not important for a 3 colour logo. A 16 colour GIF is a very small file, much smaller, and clearer
than any JPG, and ideal for graphics on the Web.

Graphics generally use solid colours instead of graduated shades, which limits their colour count
drastically, which is ideal for GIF’s indexed colour. The GIF uses lossless LZW compression
for relatively small file size, as compared to uncompressed data. The GIF files offer optimum
compression (smallest files) for solid colour graphics, because objects of one exact colour compress
very efficiently in LZW. The LZW compression is lossless, but of course the conversion to only
256 colours may be a great loss. The JPG is much better for 24 bit photographic images on the
Web. For those continuous tone images, the JPG file is also very much smaller (although lossy).
But for graphics, GIF files will be smaller, and better quality, and (assuming no dithering) pure
and clear without JPG artifacts.

If GIF is used for continuous tone photo images, the limited colour can be poor, and the 256 colour
file is quite large as compared to JPG compression, even though it is 8 bit data instead of 24 bits.
Photos might typically contain 100,000 different colour values, so the image quality of photos is
normally rather poor when limited to 256 colours. The 24 bit JPG is a much better choice today.
The GIF format may not even be offered as a save choice until you have reduced the image to
256 colours or less.

So for graphic art or screen captures or line art, GIF is the format of choice for graphic images
on the Web. Images like a company logo or screen shots of a dialog box should be reduced to 16
colours if possible and saved as a GIF for smallest size on the Web. A complex graphics image that

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