As an example, we'll look at a chart image, rendered in two passes.
The first pass uses two colours (blue and yellow) while the
second pass uses sixty-three colours. After the first pass,
the image appears as shown:
For browsers that support it, a lowsrc tag could be
used to give a similarly small preview. Using the 4.3K two-colour
preview along with the 24.2K final image would result in a total
of 28.5K transferred in two seperate file transfers.
With a two-pass image, 25.3K is transferred in a single file
transfer. Keep in mind that each file transfer incurs a significant overhead
and reduces the number of images that can be downloaded in parallel.
Furthermore, when using a lowsrc preview, the preview
would be lost shortly after the final image starts downloading
if the final image is interlaced.
The multiple-pass GIF format is not supported by all browsers.
When limiting yourself to 256 colours by using a global palette,
it is supported by:
- Netscape Navigator 2.0 (or later) on all platforms, and
- Microsoft Internet Explorer 2.0 (or later) on the Macintosh, and
- Microsoft Internet Explorer 3.0 (or later) on Windows.
A list of programs
that support the multiple-pass format when more than 256 colours
are used is also available.