Colour Tables Worked Example

This document presents two pictures by Van Gogh: The Starry Night and Olive Trees with Yellow Sky and Sun. Both can be seen in whole at Webmuseum, Paris.

The GIF file format is normally based around an 8-bit colour lookup table (CLUT) giving a choice from 256 colours. This page looks closely at colour table effects with 8-bit images. It looks at each image closely and at how they look when converted to CLUTs and when they are combined on an 8-bit (256 colour) screen.

To view the page at best increase the window width so the first three images appear on one line. Some browsers may have trouble with the number of images on this page - you may need to load them separately or use the slide at a time version of the page. Ideally you should be viewing this page on a 24-bit screen (although its still fairly effective down to 8-bits). All images are stored as 24-bit JPEGs for ease of loading on all browsers - they are, however, constructed from suitable images and then screendumped into JPEG format.

Single images

For viewing an image on screen the colour table usually doesn't make that big a difference - but it can make smooth colour transitions look bity no matter how good the choice of colour table. An adaptive 8-bit usually manages a very good job, while a system 8-bit a fairly poor one. The example below works really well in both 8-bit formats though.

 

The Starry Night extract in original 24bit, adaptive 8-bit and system 8-bit.

When viewed close up the dithering becomes more obvious, especially in the system CLUT version.

 

A close up of the church, in 24bit, adaptive 8 and system 8 bit CLUTs.

 

The second image has a very different colour palette and can be converted nearly as well into 8-bit formats.

 

Extract from Olive Trees in 24-bit.

 

Close-up in 24-bit, adaptive 8-bit CLUT and system 8-bit CLUT.

 

Mixing the images

When the two images are mixed together on an 8-bit screen the big problems start - either you have to use the two in lowish quality system 8-bit or you get horrible colour transitions when viewing one image with the colour table from the other.

In these examples, two windows were placed side by side (the bar in the middle is the window frame). When one image is in the foreground the other is viewed using the foreground's CLUT - which changes it pretty drastically, even though the Mac does a very good job normally to hide these effects. You can even see the colour change in the window frame.

 


The two images mixed two ways on an 8-bit screen.

 

A listing of the two CLUTs.

Of course, when both images are in the system CLUT - no such effects occur - but then both images are already lower quality.

 

The two system CLUT images mixed on an 8-bit screen.


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