Combining masks

Hi,

Running this code

(gp:with-graphics-mask (pane (list 100 100 50 50))
  (gp:with-graphics-mask (pane (list 0 0 500 500))
    (gp:draw-rectangle pane 0 0 500 500 :filled t :foreground :red)))

results in a 500x500 rectangle being drawn. Apparently, the second mask overrides the first, while I would have expected them to be combined so that both masks restrict the drawing area.

Is this a bug in GP, or is it expected behavior? In the latter case, is there any workaround?

Regards
Erik


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Re: Combining masks


* Erik Ronström <A1A46ECF-0440-4AA6-AAA6-C38E5216C13A@doremir.com> :
Wrote on Wed, 12 Nov 2014 20:02:00 +0100:
|
| (gp:with-graphics-mask (pane (list 100 100 50 50))
|   (gp:with-graphics-mask (pane (list 0 0 500 500))
|     (gp:draw-rectangle pane 0 0 500 500 :filled t :foreground :red)))
|
| results in a 500x500 rectangle being drawn. Apparently, the second
| mask overrides the first, while I would have expected them to be
| combined so that both masks restrict the drawing area.
|
| Is this a bug in GP, or is it expected behavior? In the latter case,
| is there any workaround?

This isn't a bug in GP, WITH-GRAPHICS-MASK works as specified: it merely
`binds the mask slot of a port's graphics state across the execution of
a body of code', and here we are changing the binding twice.

I haven't really used these functions, but you there may be an ugly
workaround to get what you want: by chaining WITH-PIXMAP-GRAPHICS-PORT
within successive scopes of WITH-GRAPHICS-MASK and drawing to this port
and finally copying the pixels of the innermost pixmap port back to the
pane...---Madhu

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Re: Combining masks

* Erik Ronström <CCC87700-887D-4D76-B3E5-838B48403234@doremir.com> :
Wrote on Thu, 13 Nov 2014 21:22:59 +0100:

| I found another way, which is perhaps slightly less ugly: I wrote a
| new macro, which checks if the mask in the pane's graphics-state is
| already bound, in which case the masks are combined. Fortunately, I
| just need to deal with rectangular masks, for which the combination is
| trivial. I wouldn't go into combining different paths.

That is definitely less ugly when all the shapes all rectangles.

Perhaps there was also some way to intersect (:path ...) masks easily?
If GP was computing this, could it be exposed?  [i.e. without producing
the pixmap.  A pointer to the algorithm may also help]---Regards, Madhu

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