I've successfully written a lot of C programs over the years using Xlib,
for displaying a (static) plot of various curves I'm studying. But for
the first time, I needed to make one of those curves (plotted wrt
position x, say) move with time t (like a wave propagating in the
direction x, with some velocity v as time progresses).
I thought I could just use a usleep function (with a delay of about 1/20
second, to get movies-like motion) between successive snapshots of the
evolution of the curve, but when I do an XFlush after doing a bunch of
incremental DrawLines to generate a given snapshot of the curve, the
display of the snapshot doesn't get displayed instantaneously ... I see
the image get drawn over a fraction of a second, enough to be very
noticeable ... slow enough per snapshot to greatly exceed the 1/20
second time-lapse between the snapshots, which will mess up the
motion-picture effect, I think. (If I do a sleep(2) after all the
DrawLines, but before the XFlush, Xlib DOES buffer those DrawLines, and
doesn't start the display of the curve until the XFlush, so the problem
ISN'T caused by each DrawLine being displayed separately ... the
buffering seems to be working as advertised).
The Xlib Programming Manual says that animation may require "double
buffering", writing the whole set of DrawLines for a snapshot into a
pixmap, and then copying that pixmap into an empty window. That sounded
like it might solve my problem (assuming the copy was sufficiently
quick), but when I tried to do it for a very trivial test case, I got
execution errors. I couldn't find any examples in that book
specifically showing how to use a pixmap in that way, and I had to do
some guessing about how to do it (and apparently I guessed wrong at
least once).
Does anyone have a very simple fragment of code that shows how it's
done? I'm looking for bare-bones sort of code fragment ... nothing
fancy or generic or portable, etc.
--
Mike Fontenot
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