SOC project

Stefan Dösinger stefandoesinger at
Tue Apr 25 12:20:29 CDT 2006

> AFAIK, there are four targets OpenGL can draw to on X: X windows, Pbuffers,
> FBOs or GLX pixmaps. EXT_framebuffer_object should be the most interesting
> approach, as it's supposed to somewhat similar to Pbuffer rendering, but
> supposed to be more portable and faster:
> "This extension defines a simple interface for drawing to rendering
> destinations other than the buffers provided to the GL by the
> window-system."
> But I guess a FBO or a GLX pixmap would still introduce a similar
> performance hit compared to Pbuffer rendering?
FBOs are very interesting for this, but the mail performance issue is to read 
back from the video card. Video cards are mainly output devices, reading from 
them is slow. AFAIK the driver even has to use PCI transfers for that, 
because AGP isn't designed for reading back data. I could be wrong with this, 
and I don't know how PCIe affects this.

> Forest Hale (and Lionel Ulmer) suggested a different, but probably more
> hacky and difficult approach (from
> "I would recommend overriding the glViewPort and glScissor calls, as well
> as gl[En/Dis]able GL_SCISSOR_TEST, and arrange the glViewPort for the
> proper projection for rendering, but use glScissor to set the region that
> can be rendered in the current DC, also there should be no slowdown from
> having scissor test permanently on."
That is the plan for Direct3D, where I consider that easier because WineD3D 
can control GL, without the chance that the app does opengl calls which 

> Sorry, I don't get this. Windowed applications should not be affected?
> There is no problem with windowed OpenGL applications, they are not
> affected by bug 2398 anyway. Only applications with embedded OpenGL
> viewports are affected, so a workaround would only need to target embedded
> viewports...?
This comes from the way DirectDraw apps handle windowed Direct3D. See my 
description for the IWineD3DDevice::SetFrontBackBuffers call:

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