suggestions wanted for tricky shaped window problem

[Paul Barton-Davis <pbd Op Net>]

I've grown quite comfortable with shape windows created from xpm files.

I'm now working on a more difficult problem. I have a pixmap. I draw
into it (lets assume with gdk_draw_lines()). Before each drawing, I
clear the pixmap, and after it, I want to reset its shape to include
only those pixels that were set by the drawing operation.

This means, I think, computing a bitmap from the pixmap to pass into
gdk_window_shape_combine(). I tried the naive approach of using
XCopyPlane for every plane in the source pixmap, and then resetting
the shape. This is *dog* slow.

I'm looking for ideas. The overall goal is an essentially transparent
window where things can be drawn and redrawn, and whatever is
underneath is visible except where something has been drawn in the
"essentially transparent" window.

Let me know if you have any ideas. I think this is a hard thing to do
with X, but I could be wrong.

--thanks
--p