Re: [GtkGLExt] How to only update at screen refresh rate?

[Bryan Green <bgreen nas nasa gov>]

Eric Jonas writes:
> 
> On Thu, 2007-04-26 at 19:13 -0600, Timothy M. Shead wrote:
> > On Mon, 2007-04-23 at 10:38 -0400, Eric Jonas wrote:
> > > Hello! Apologies if this question is misdirected, as I'm somewhat new to
> > > gtk in general. Is there a recommended way of writing my app such that
> > > the expose event is only called once per screen redraw? Right now it
> > > seems like I'm looping and doing a lot of extra work (and getting 1000s
> > > of FPS) when I really don't need to be. I searched through the gtkglext
> > > examples as well as the gtk docs and couldn't find anything that seemed
> > > relevant. Thanks!
> > 
> > Not sure what you're trying to describe - the expose event gets called
> > by X whenever a window needs to be redrawn.  If you redraw when you
> > receive the expose event, you're doing the right thing.
> > 
> 
> Thanks for the quick reply! In a lot of the examples, the entire window
> is invalidated and updated in either an idle handler or a timeout from
> some timer. I'm trying to render some scientific data and on one hand I
> don't want to rerender every time a new point comes in (at 1kHz),
> because that seems really excessive (and pegs my CPU). 
> 
> I'm assuming that the screen is updated at some fixed rate, say 60 or 72
> Hz. It would be great if there were some way to only invalidate the
> window / trigger a redraw on this event. 
> 
> I could also set up a timer that ran at, say, ~72 Hz, but I worry that
> the resulting display would precess through actual screen redraws. 
> 			...Eric

It sounds like you want to use the GLX_SGI_video_sync extension.
It has a call that will syncronize on the next vertical refresh, thus
setting an upper limit on your frame rate.

I have used it this way:

#include <GL/glxext.h>
...
do_draw();
gdk_gl_drawable_swap_buffers(gtk_widget_get_gl_drawable(glxarea));

#ifdef GLX_SGI_video_sync
    glXWaitVideoSyncSGI(1,0,NULL);  // synchronize on next refresh
#endif

Good luck,
-bryan