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From: guido@cwi.nl (Guido van Rossum)
Newsgroups: comp.sys.sgi
Subject: XOR in GL?
Summary: How do you flip bits in GL, given a monochrome image?
Message-ID: <3227@charon.cwi.nl>
Date: 27 Mar 91 09:02:19 GMT
Sender: news@cwi.nl
Lines: 20

I have a library of graphics functions that I want to port to use GL
on the 4D/45 for rendering.  My library draws monochrome images only,
and allows an XOR drawing mode.  This is used for user interface
features.

I have studied the GL manual extensively, but cannot seem to find a
way of doing this without reading a rectangle of pixels, doing the
operation myself, and writing the rectangle back; this is not only
slow but also requires a lot of work if I want to render XOR lines.

I know it can be done, because the X server supports XOR drawing.
(I have the feeling that it may be difficult though, because earlier
versions of the X server had a clipping bug for XOR drawing...)

Changing the applications is out of the question, and so is using X
(in fact I already have a version that uses X, but I'd like to have
one that uses GL as well).

--Guido van Rossum, CWI, Amsterdam <guido@cwi.nl>
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