Mac Forum / Programming / Mac Programming / October 2006

Hello,

I co-maintain a library which uses OpenGL to synchronize screen updates
to the Vertical Blanking Loop (VBL).

I currently have two problems with OpenGL on Mac OS X for Intel.

1) On both Mac OS X (for PPC) and Linux, we observe the following
behavior:
A call to GL_SwapBuffers blocks if and only if a call to GL_SwapBuffers
has occurred within 1/(frame-rate) seconds ago.  On a 60 Hz screen,
this means that given a call to GL_SwapBuffers at time t followed by
one at time t+15 milliseconds, the second will definitely block for
some (nonzero) time interval.  Given a delay of t+19 milliseconds,
however, the second call will NOT block.

On Mac OS X for Intel, however, the second call does not block even
when it occurs within 1/(frame-rate) seconds ago.  Instead, I need to
accumulate THREE prior calls to GL_SwapBuffers within a screen cycle in
order for one to block.

Is difference this a bug or feature?  Is it possible to set how many
calls are accumulated before one blocks?

2)  On the MacMini and MacBook (but not the MacBook Pro), a call to
glCopyPixels takes 34 milliseconds, on average.  Considering on the
PowerPC, MacBook Pro and Linux this call generally takes well under 1
millisecond, this is an unacceptable degradation in performance.  Is
this a bug?  Can anything be done about this?

Thanks,
Aaron