From mboxrd@z Thu Jan  1 00:00:00 1970
Message-ID: <564b487b1feec76d33fa694aabe072ad@plan9.ucalgary.ca>
To: 9fans@cse.psu.edu
Date: Tue, 14 Sep 2004 17:52:10 -0600
From: andrey mirtchovski <mirtchov@cpsc.ucalgary.ca>
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Subject: [9fans] color compositing
Topicbox-Message-UUID: e21868f8-eacd-11e9-9e20-41e7f4b1d025

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