GIF (Graphics Interchange Format)
GIF images are the most widely supported image format on the Web, being supported by all the graphical web browsers.The GIF format also supports:
- animation
- transparency
- interlacing
GIF images can only be 8 bit, i.e. they can only have up to 256 colours. As discussed earlier, there are only 216 colours that are commonly supported by all web browsers. When creating a GIF you will need to save it as an indexed colour image. There are several different modes of indexed colour. Using an exact number of colours will give you the smallest file size, but adaptive usually gives the best quality. Try different Indexed Colour modes to find the best compromise between file size and quality of image.
Transparent GIF
If you've seen an image with an irregular shape on a coloured background you were probably looking at a GIF with a transparent background. A transparent GIF usually has one colour that is transparent. When you export a GIF image from most image manipulation packages you can choose which colour will be transparent. Here is an example (opens in a new window).If your image manipulation package does not have this feature you will need to download freeware from the links on the Software page of this module.
Interlaced GIF
When you save a GIF image you can choose whether it is interlaced or not. If you do choose interlacing it will display your whole image but it will seem pixilated and indistinct at first, becoming clearer and clearer in stages until it has fully downloaded. It can be a good idea to use interlacing as it gives web users an idea of what the image is before it is fully downloaded.Here is a page with the same image displayed, but one is an interlaced GIF and the other is non-interlaced.
Animation
Animated GIFs allow you to present multiple GIFs in sequence to create an animation. This is covered in depth in the optional Core Skills module Animated GIFs.
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© 2002 Ashley Preston