[9fans] color compositing

[andrey mirtchovski <mirtchov@cpsc.ucalgary.ca>]

i'm in a pickle here trying to find out how to use duff/porter's alpha
channels properly.  here's the situation:

	- there is an image with background color which should be
	transparent

	- this image, sans the background color, should go onto
	another image

the problem is that i'm not able to extract the non-background color
from the image.  obviously the easiest soution would be to use a mask
for anything that non-background, so the first thing i tried was to
use the image as a mask on a 'draw everything in white on black
background'.  the problem i ran into was that if i use a low-bit
channel (GRAY1) i only see the parts of the image which were bright
enough originally, if i use a higher-bit channel i get variations of
grey causing everything drawn afterwards to appear black'n'white...

then i tried several of the duff/porter compositing operators with
absolutely no effect whatsoever.

in my desperation i even tried creating an image with the alpha for
the particular color set to zero, but it proved fruitless.

to summarize: how can i remove a certain color from an image
programmatically?

andrey