From mboxrd@z Thu Jan  1 00:00:00 1970
Message-ID: <ee9e417a0409141819253407e@mail.gmail.com>
Date: Tue, 14 Sep 2004 21:19:08 -0400
From: Russ Cox <russcox@gmail.com>
To: Fans of the OS Plan 9 from Bell Labs <9fans@cse.psu.edu>
Subject: Re: [9fans] color compositing
In-Reply-To: <564b487b1feec76d33fa694aabe072ad@plan9.ucalgary.ca>
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> to summarize: how can i remove a certain color from an image
> programmatically?

first make the "image with background color which should
be transparent" into an "image with transparent background":

read it into a Memimage of (say) RGBA32,
then loop over the pixels looking for your particular
color and replacing them with 0,0,0,0.  (actual transparent
pixels.)

then you can 

draw(dst, dst->r, bottom, nil, ZP);
draw(dst, dst->r, image, nil, ZP);

and voila! you have your image.

russ