Creating a bitmap mask from a pixmap

[Harry Tily <hjt882 bham ac uk>]

Hi - 

I'm trying to do something that I'm sure is _really_ easy, but I've been
stuck on it for a long time.

I want to draw a pixmap to the screen, but I want the white background
of the pixmap to be transparent; that is, whatever is already on the
screen should show through the white areas of my pixmap.

I can do this easily by setting a clip mask on the graphics context
provided I have a bitmap to use for the mask, such as the one returned
when loading from an XPM file. However, as my pixmap is drawn from
scratch using gdk_draw_* functions, I don't have one.

So, I'm trying to draw a suitable mask from my pixmap: I've tried just
drawing the pixmap to the bitmap - after all, I want white to be an
unset bit in the bitmap, and anything that isn't white to be a set bit.
However, gdk_draw_pixmap (along with all other gdk_draw_* functions)
crashes the program when called on the bitmap.

Is there a way to select one colour in the pixmap as the mask, and draw
only those pixels that aren't of that colour?

Please excuse me if this is a stupid question: the FAQ on the main page
seems to have been down for at least a couple of weeks, and I haven't
had any luck trawling the archives of this list.

Harry
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