XOR in GL?

[Guido van Rossum]

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 at cwi.nl>
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