Question about grabbing X Window screen

[Martin B. <Mar...@se...>]

Hello,

I am complete newbie to this mailing list and to programming of modern
graphic HW, but have the following problem: I want to grab complete X
window screen (eg. root window) reasonably fast. When I use standard way
by XGetImage, I am able to grab about 1.5 screen (1280x1024x32) per
second having CPU usage 100%. (Athlon XP 1.8 GHz, ATI Radeon QD (AGP),
X.Org 6.8.1, on P4 1.7GHz, integrated SiS graphic chip it was about 2.5
grabs per sec.). It is too slow, so I mmap-ed /dev/mem and memcpy linear
framebuffer. To my surprise a got only slightly better results. I am
using kernel 2.6.9, on ATI DRI was enabled, on SiS it was not.

I suppose such simple operation can be done pretty fast, but don't know
how. Could somebody with better understanding of graphic HW and Linux,
point me where to look to learn more about it? Or even better, give me
some idea how to do it simply and fast? Is the DRI right place to look
at? I was googling about it for a while but didn't got much. Some
documentation is at http://dri.freedesktop.org/wiki/Documentation but
nothing like detailed description or even tutorial.

What I actualy need is not high rate of grabbing - 4 times per second
would be ok, if the CPU usage remains low. The program has not to be
portable between various graphic cards. It would be nice if it is just a
normal user-space program, not a kernel or X Server module.

Just last question - would the more up to date graphic card make a big
difference in speed of naive XGetImage/memcpy approach?

TIA

Martin