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Itsagif can put more than 256 colours into a single GIF file by painting the colours in passes. Another advantage of this colour layering approach is that browsers can get faster previews, if you use a limited number of colours in the first pass.

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:

Fiscal GIF - Pass #1 of 2

After the second pass, the image appears as shown:

Fiscal GIF - Pass #2 of 2

The first pass is 4.3K while the entire image (both passes) is 25.2K. Painting the entire image in a single pass gives the following image, which is 24.2K:

Fiscal GIF - Pass #1 of 1

So the two-pass image is slightly larger. However, it gives the browser a clearer preview, as the images below show:

Fiscal GIF - 4.3K into two pass Fiscal GIF - 4.3K into one pass

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.