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One of the nice features of GIF images is that it is possible (to an extent) to specify the way in which the image loads. By saving a GIF image in a special format, one can specify whether or not some parts of the graphic are invisible, or whether some parts load and are displayed faster than others.

In some instances, the way that a graphic is saved is not the way that you want it to appear when it loads on the Internet. This is especially true if the image is not rectangular, since you are required to save GIFs and JPEGs as rectangular blocks. In order to prevent a part of the graphic from being displayed, you can make that part transparent. This can be done in many graphics editors by simply highlighting the area to be transparent with a color that is not already used by the image and specifying that color as the background color. Transparent parts of images will then display as invisible, allowing anything behind the image to show through. The following two pictures demonstrate the difference that making a GIF image transparent can make:

Non-Transparent

Transparent

Another feature of GIFs is that they can be interlaced. As a GIF image is normally saved, each line of the graphic is written in order until all the lines have been saved. When a GIF is saved in interlaced form, the lines are not saved in order, but from different areas of the image. Consequently, the images load in such a way that the lines are read into different areas of the image instead of from top to bottom. This has the effect of either causing the image to fade in as it loads or to first be very blocky but become more detailed. Note that this only occurs when the image is loading. If it loads too quickly, this effect may not be seen. The following picture demonstrates the difference between normal (non-interlaced, which loads from top to bottom) and interlaced loading patterns:

Non-Interlaced

Interlaced

Unfortunately, JPEG images do not support transparency or interlacing. There is such a thing as a progressive JPEG, which is similar to interlacing in GIFs in that the image is not loaded from top to bottom but in different areas throughout the graphic, but there is currently no JPEG equivalent to transparency supported for use on the Web.

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Last modified September 22, 1997 by Neil Wilson
nwilson@mail.millikin.edu