From mboxrd@z Thu Jan  1 00:00:00 1970
To: 9fans@cse.psu.edu
Subject: Re: [9fans] Re: moving things in a window
Date: Wed, 27 Sep 2000 09:17:01 +0900
From: okamoto@granite.cias.osakafu-u.ac.jp
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So, I wrote my comment to note this technique is not so generally
applicable to want ot avoid Russ doesn't stop his effort to implement
translucent layer.  :-)

kenji


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Subject: Re: [9fans] Re: moving things in a window
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I asked Russ Cox for an explanation of the code, and I promptly received
one:
(with hopes it's useful and he doesn't mind me sending private email)

--- begin paste ---
/* clean up previous location */
if (osize && clear) {
        draw(imgmask, imgmask->r, display->black, nil, ZP);     // transparent
        fillellipse(imgmask, subpt(oldp, p1), osize, osize, display->white, ZP);        // draw opaque old
        fillellipse(imgmask, ZP, size, size, display->black, ZP);       // cover new

        draw(screen, rectaddpt(imgmask->r, p1), display->white, imgmask, imgmask->r.min);
}

We start with a black square.
Then we draw a white circle where the old circle was.
Then we draw a black circle where the new circle will be.

The result is a black image with a sliver of white corresponding to
the area of the old circle that is outside the new circle.

Interpreted as a mask, black is transparent; white opaque.
So the mask says ``only draw where the old circle was
but the new circle is not''.
The final draw puts white down in those places.
--- end paste --

the explanation may be a bit too high level for you though :)


On Tue, 26 Sep 2000, Theo Honohan wrote:
[snip]


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