Re: cumulative rendering

[email protected] wrote:

> A single buffered GL context seems ideal for this, but sadly this won't
> work on OS X using glut... A glutSwapBuffers() seems necessary to make
> the rendering visible on screen. On Linux and MS Windows, this works fine.

You can't (legally) use a double-buffered context because OpenGL does not
guarantee that the previous frame's content will still be there after
a buffer swap.  It might work on some cards in some situations - but you
can't rely on that.

To do this properly, you need to run single-buffered.

I believe GLUT supports that - but don't call glutSwapBuffers - use
glFinish() instead. (I'm a little hazy on this one - it's been a lot
of years since I last used GLUT with single-buffering!)

> So I have chosen a double-buffered context, and it turns out that the
> back buffer is identical to the front buffer after swapping, which is
> exactly what I need. But according to the GL spec, this is not
> guaranteed, but depends on the driver and swapping mode in use.

Yeah - you definitely can't rely on that - either in theory or in
practice.

> Is it possible to use glCopyPixels() to copy the front buffer to the
> back buffer after a swap?

No - because your window might be overlapped (in which case part of the
front buffer will have been trashed by the other window) - or the graphics
card may have done processing on the back buffer as it swapped it into the
front buffer (eg for antialiasing).

However, you could copy your back buffer into a texture map before you
swapped - then copy the texture into the new back buffer afterwards.

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The second law of Frisbee throwing states: "Never precede any maneuver
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Steve Baker                      (817)619-2657 (Vox/Vox-Mail)
L3Com/Link Simulation & Training (817)619-2466 (Fax)
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Home: [email protected]       http://www.sjbaker.org

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