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From: madd@world.std.com (jim frost)
Newsgroups: comp.windows.x
Subject: Re: Transparent Pixel values
Keywords: Pixmap, Window, Pixels
Message-ID: <1989Nov6.214359.15681@world.std.com>
Date: 6 Nov 89 21:43:59 GMT
References: <809@thor.wright.EDU> <9692@zodiac.ADS.COM>
Organization: Software Tool & Die
Lines: 32

medmunds@Verity.COM (Mike Edmunds x7645) writes:
>In article <809@thor.wright.EDU> ardatta@odin.wright.edu (Arpita Datta) writes:
>>How can I copy objects drawn on the Pixmap, onto the window without
>>destroying the tiled background of the window ? 
>I've had some success with setting the clip_mask in a GC to a bitmap
>with objects drawn on it, and then painting through this "stencil"
>using XFillRectangle over the entire area.  Only pixels that were set
>in the bitmap are affected in the destination window.  If your
>original pixmap was more than one plane deep, you'll have to separate
>it into components (I keep a separate bitmap for each color) and paint
>multiple times.

What I've done in the past is to draw the object twice, once on a
pixmap in full living color and once on a monochrome pixmap.  Make a
GC with the clip_mask set to the monochrome pixmap and use XCopyArea
to copy the color pixmap through the monochrome mask onto the
destination.  If you only have a single color to draw, draw on a
monochrome pixmap, use that pixmap as clip_mask for the GC, and copy
from itself through itself to the destination.  This works well on
even very complicated "transparent" images, which is why I did it in
the first place.

Note that this technique has a side-effect; the mask can be used to
accomplish picking (ie it's easy to tell if the cursor is on a solid
part of a drawn item), something which I have needed in the past and
which is generally available on true high-end hardware.

Happy hacking,

jim frost
software tool & die     "The World" Public Access Unix for the '90s
madd@std.com            +1 617-739-WRLD  24hrs {3,12,24}00bps