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From: rws@EXPO.LCS.MIT.EDU (Bob Scheifler)
Newsgroups: comp.windows.x
Subject: Re: Invisible windows (Technical Limitations of X?)
Message-ID: <9003221837.AA04927@expire.lcs.mit.edu>
Date: 22 Mar 90 18:37:46 GMT
References: <9003221746.AA00894.chandran@cesium.ASC.SLB.COM>
Sender: daemon@athena.mit.edu (Mr Background)
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"Transparent" windows are a tricky subject, there are many possible
interpretations of this term.  (The core X protocol provides no
support for any of them, I suspect.)  NeWS has a special kind of
canvas (overlay) that provides one kind of transparent window, but
I think that you will find the semantics very contrained.

    In particular, consider the situation where we have to produce
    complicated graphics into a window.  The desired effect is to have
    whatever is behind the window show through (no matter where the window
    is currently positioned).

You have switched between these two sentences.  There is a difference
between doing "overlay" graphics across multiple windows (or requiring
that it be done in a separate window) and doing overlay graphics in
the same window as the underlying graphics.  Overlay graphics can be
done within a single window, given certain assumptions.  You can examine
the color chapters of the some of the X books for examples (e.g. O'Reilly
volume 1 chapter 7).

There is display hardware that has a notion of "overlay" planes, and
of "blending" multiple images on the screen.  Several companies have
private extensions which integrate this hardware into their X server.
There is work going on in the X Consortium to try and standardize such
an extension, although it is slow going so far (it is a difficult topic).

    What is the main purpose of "window_background_pixmap" to be "None"?

The main purpose is a minor performance improvement.  If the application
is simply going to cover the entire window with graphics (i.e. there is
no reasonable "background" that can be supplied), then why bother forcing
the server to spend time painting the background?  That is the primary
reason, although the mechanism can be used for other tricks.