X[Shm]GetImage is slow; what can I do about this?

Michel Dänzer michel at daenzer.net
Mon Apr 4 21:09:51 PDT 2005

On Mon, 2005-04-04 at 15:51 -0400, Jonathan Lennox wrote:
> Some Googling seems to indicate that XGetImage slowness comes about because
> the X server keeps screen images in video RAM, not main memory, and video
> cards don't tend to be optimized for uploading images to the CPU.
> So my questions are:
> 1.  Is this true?  And is my experience typical?

Yes, this is true in the absence of hardware acceleration for GetImage,
which only few drivers provide.

> 2.  Is there anything that can be done about this, for instance some way to
> convince the X server to keep a mirrored copy of a window in main memory?
> (Could XComposite be of use here?)

For now maybe, but eventually the window contents will live in video RAM
as well with composite.

ShadowFB or backing store might be other ways to achieve an improvement,
but they both need to be enabled in the X server configuration.

Earthling Michel Dänzer      |     Debian (powerpc), X and DRI developer
Libre software enthusiast    |   http://svcs.affero.net/rm.php?r=daenzer

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