[cairo] 8 bit pseudo color missing.

[Bill Spitzak]

I'm certain the only way Cairo is going to work and produce the correct 
picture is to keep a local full-color buffer and then draw the resulting 
dirty area onto the output device. I think error diffusion is 
preferrable over patterned dither. Patterned dither is used by Windows 
and looks really bad, while error diffusion I have found looks quite 
reasonable, even for line graphics.

A technique I have used that works pretty well is the following. It 
works on Pseudo color visuals with as few as 4 bits and produce a 
visible  picture. It is also good for fixed-color displays with 8 bits 
or less such as the 3/4 bit VGA displays or even monochrome ones.

1. Invent your own color cube with < 256 colors. Ideally this should be 
a cube that is used by other programs, such as the Netscape cube. This 
maximizes the chances the cube will allocate exactly. Use less than 256 
colors so that you will not allocate all the colors on an 8-bit display.

2. Make a table the size of this cube, with space for color index and 
for an actual rgb color.

3. Allocate every one of the cube colors from the colormap. Cycle 
through it in some pattern so that widely-seperated colors are allocated 
first (I just cycle through with a relatively prime constant increment). 
Put the resulting index, and the resulting color (masked to the 
abilities of the device) into the table. Eventually the allocation will 
fail (either because the colormap is full or because this is a TrueColor 
visual).

4. For all remaining colors, search the entire colormap from the device 
(including the ones you allocated) to find the closest color, and put 
this closest color and index into the table. Notice that several table 
entries may get the same index.

5. The dithering algorithim dithers to the desired color cube, as though 
you really had it. It then draws the color index from the table. But the 
*error* for the neighboring pixels is calculated from the difference 
between the source color and the color in the table, which may be 
different than the color the cube should have.