[sugar] [Fwd: Re: Fast Screenshot without blocking X events]

Tomeu Vizoso tomeu
Wed Oct 17 04:58:56 EDT 2007

On Wed, 2007-10-17 at 03:58 -0400, Bernardo Innocenti wrote:
> For those of you complaining that taking screenshots
> was slow... it seems there's not much we can do even
> with Composite and the latest X server.

Well, last time I measured, it took 0.15s. to take a fullscreen
screenshot on the xo. So we are not so bad.

If with composition we can take the screenshot while the window is
hidden (and before the window is iconified/unmapped), then we'll be able
to take reliable screenshots without using much extra memory during a
long period.


> -------- Original Message --------
> Subject: Re: Fast Screenshot without blocking X events
> Date: Wed, 17 Oct 2007 09:34:53 +0200
> From: Michel D?nzer <michel at tungstengraphics.com>
> Organization: Tungsten Graphics
> To: GORIN Guillaume <guillaume.gorin at c-s.fr>
> CC: xorg at lists.freedesktop.org
> References: <6d1b4786.47866d1b at c-s.fr>
> On Tue, 2007-10-16 at 11:00 +0200, GORIN Guillaume wrote: 
> > 
> > -I don't want that all events on X are waiting in a stack, or something
> > like this. Actually when capturing (using XGetImage) i can't continue to
> > move window. It's why I suppose there is a stack of X events, that wait
> > while capturing. Is it true? (If someone can tell me where to find
> > informations about that. Xorg Wiki didn't help me).
> Yes, as the X server is mostly single threaded, event processing is
> probably delayed while a request such as GetImage is being processed.
> > I'm able to take screenshot in 3200x1200 in the framebuffer, but it
> > takes 2.8 seconds. It's too long for me. I suppose there is a stream
> > from X to the framebuffer, it's why reading is too long.
> No, CPU reads from video RAM can actually be that slow, especially with
> AGP. It's possible to accelerate this using the GPU though.
> > My questions are:
> > -How X use the framebuffer /dev/fb0 when activated?
> The nvidia driver doesn't use it at all to the best of my knowledge.
> > -Can I take screenshot without blocking X events?
> If XGetImage is inadequate, the only alternative I can think of that
> might be better is glReadPixels.
> > -Where can I get the picture of the screen before X send it to my
> > geforce? (Is it possible?)
> Probably not, at least not without modifying the driver.
> Hope this helps,
> -- 
> Earthling Michel D?nzer           |          http://tungstengraphics.com
> Libre software enthusiast         |          Debian, X and DRI developer
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