On Fri, 4 Apr 2003, David N. wrote:
> Billy,
>
> Thanks -- that works and solves one of my problems.
>
> The other part of my problem is that I'm trying to
> grab complete images off of the screen using
> XShmGetImage(). Despite synchronizing on with
> vertical refresh like you suggested, I'm finding that
> the buffers that I capture with XShmGetImage() have
> tearing in them - i.e. it looks like the next frame is
> getting drawn into X's buffer before XShmGetImage is
> done retrieving them.
OpenGL and X rendering are allowed to be completely asynchronous,
and in the case of NVIDIA's binary drivers, they are. If the
same app is doing the grabbing and OpenGL rendering you should
either use OpenGL for everything (glReadPixels instead of XShmGetImage)
or if you use XShmGetImage you have to use GLX's synchronization
primitives: glXWaitGL and glXWaitX. Call glXWaitGL before
XShmGetImage to ensure that all GL rendering has finished.
If the grabber and GL renderer are separate applications,
there's not much you can do. You can grab the server before
doing the XShmGetImage to ensure that nobody is going to be
rendering while you are doing the XShmGetImage, but that isn't
going to give you OpenGL flip synchronization on the GetImage.
>
> In these cases I notice that XShmGetImage is taking
> longer than 13.3ms to complete. Do you have any ideas
> on a quicker way to get a copy of X's output buffer?
It only takes that long because it's sharing the hardware
with the simultanous OpenGL rendering. The above two mechanisms
will serialize X and GL rendering.
Mark.
>
> Kind regards,
> David
>
> >
> >
> > This is currently video card specific. With
> > nVidia cards and the
> > binary drivers, you can use poll() on
> > /dev/nvidia[0-x] to have your
> > process block until the next refresh. With mga,
> > radeon, r128, i830,
> > (what else?) drivers, using the new DRM for X4.3,
> > there is a DRM ioctl
> > you can use to get information about the refresh
> > rate. Look for
> > drmWaitVBlank and associated code in libGL for how
> > to use the ioctl.
> >
> > Other then that, there are a bunch of
> > card-specific codes around
> > providing refresh interrupts as kernel modules. In
> > new versions of
> > svgalib, they include a kernel module to provide
> > this functionality
> > through their API, although it's unclear if it is
> > 'safe' for use along
> > with X. As well, the mplayer program includes some
> > kernel module
> > drivers for overlay surfaces which includes some
> > refresh sync code, but
> > it's not really used by them currently to my
> > knowledge.
> >
> > I'm working on some sort of library to abstract
> > the nvidia binary
> > drivers and the new DRM method, and maybe other
> > methods if they are
> > clean. If anyone else is interested in helping out
> > with this, let me
> > know.
> >
> > Hope that helps,
> > -Billy
> >
> > --
> > Billy Biggs
> > [EMAIL PROTECTED]
> > _______________________________________________
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>
>
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