Re: [Gnash-dev] Vblank sync and double buffering

[Bastiaan Jacques]

On Tue, 26 Aug 2008, Michael McTernan wrote:
> I've been trying to use gnash with the openGL renderer to get some smooth
> graphics.  I managed to do this building 0.8.3 from source with some changes
> to fix some problems I found on the way:
>
> 1) GtkGlExtGlue::init() may fail to get a double buffered context.
>
> This function first attempts to get a context with
> GDK_GL_MODE_RGB | GDK_GL_MODE_STENCIL | GDK_GL_MODE_DOUBLE |
> GDK_GL_MODE_ACCUM.  If this fails (which it does for my Radeon based setup)
> it goes on to get a context with just GDK_GL_MODE_RGB | GDK_GL_MODE_DEPTH.
> The problem here is that the render_handler_ogl::end_display() clears then
> redraws the screen *8* times in a row leading to really bad flickering and
> tearing.   It's trying to do antialiasing via the accumulation buffer, but
> since we don't have one and only a single buffer this just adds visual
> discomfort.

I agree, we have to adjust the end_display() issues to match the
initialization code in GtkGlExtGlue. But this is pointing to a more
complicated issue, namely that we should have a clear fallback path for
OpenGL anti-aliasing implementation differences (of which there are many).
Ideally, we should a) support all mainstream anti-aliasing methods, b)
try to detect which methods are available and c) Make the anti-aliasing
methods configurable in the GUI in cases where automatic detection does
not work correctly (very likely, depending on your hardware). I've
already taken a stab at a) when I was working on porting Gnash to OS X.

I will be resuming this work shortly, hopefully to be completed before
the release.

> 2) GtkGlExtGlue::render() doesn't vsync.
>
> I'm not sure why gdk_gl_drawable_swap_buffers() doesn't wait for a vsync,
> but it doesn't under my setup.  I forced vsync on using driconf and this
> verifiably works for GL based apps (e.g. glxgears FPS=60), but isn't doing
> it for gnash.  I suspect a GLX pixmap is being used and preventing direct
> rendering and hence sync, but not sure.  In anycase, I modified the function
> to use gdk_gl_glXWaitVideoSyncSGI():

Great, would you be willing to provide this part in a diff format to
make it easier for other people to try?

Thanks for your debugging efforts!

Bastiaan