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From: mouse@LARRY.MCRCIM.MCGILL.EDU
Newsgroups: comp.windows.x
Subject: Re:  help needed: quick exchange of window contents (animation)
Message-ID: <9009032240.AA19949@Larry.McRCIM.McGill.EDU>
Date: 3 Sep 90 22:40:36 GMT
Sender: daemon@athena.mit.edu (Mr Background)
Organization: The Internet
Lines: 43

> I want to display complicated, slowly changing line drawings in an X
> window.  The problem is, that the screen starts flickering very much,
> if I

>   1) make a drawing    (1 long call to XDrawSegments)
>   2) flush the screen  (XFlush)
>   3) clear the window  (XClearWindow)
>   4) goto 1)

> Is there a possibility to build up the new drawing somewhere in the
> background while showing the old one and then switching to the new
> drawing with one (quick) command?

I know of three ways to do this.

(1) Use the Multi-Buffering extension.  This came out with R4; one
    hopes that most vendors have picked it up by now.  If available,
    this is probably the best bet.

(2) Use colormap and planemask tricks.  If you have a visual with a
    mutable colormap, and a sufficient depth (from your description, I
    would guess you need only one bit to display your picture, meaning
    a depth of two would suffice), you can set the colormap to display
    one picture and set the planemask to make that picture read-only,
    draw the next, and then display it fast by changing the colormap
    cells.  Then of course you change the planemask and draw the next
    picture, et cetera.

(3) Draw into a pixmap and copy it onto your window in one go.  You
    mention this but say

	> but I would like to avoid copying because of performance
	> reasons.

    Try it; you may find it doesn't hurt performance as much as you
    think it will.  This method is the most universally available, but
    costs server-side memory, which might be scarce (eg, an X
    terminal).

					der Mouse

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