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Old 10 May 2019, 08:21   #23
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Location: Freiburg / Germany
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Hi Frode!

Originally Posted by FrodeSolheim View Post
I haven't seen a way to get beam position via OpenGL :-/
I have found a quote in the deep web which might help. Another possibility would be to look into the wine code how it translates the beam racing code from DX to OpenGL.

Apr '13
I could be wrong, but here’s my understanding from the OpenGL side of
things. WGL and GLX don’t have a method to poll for the refresh rate or the
vertical retrace status, instead they have an extension
(GLX_EXT_swap_control) to set the swap interval, exposed in SDL as
SDL_GL_SetSwapInterval and SDL_GL_GetSwapInterval.

A swap interval of 0 means buffers are swapped as fast as possible with no
regard for vsync, 1 means the buffer swap call will block by sleeping until
the vertical retrace finishes, 2 means the same but for every 2nd retrace,
and so forth. Therefore, in the 100Hz case a draw call made at 25ms would
sleep until it hits 30ms, then release.

Also there’s a further extension (GLX_EXT_swap_control_tear) for
"Xbox-style" vsync handling, where any errant draw call that misses the
retrace will trigger a buffer swap ASAP, then afterwards revert back to
vsync. But that doesn’t really help in this case.

The OpenGL platform APIs don’t support callbacks, so you’re probably out of
luck. If you really need vertical sync, you will most likely have to poll
your external devices from a separate thread, or perhaps change your
strategy (e.g. set the swap interval to 1, determine the refresh rate, set
the swap interval to 0, start polling and trigger buffer swaps by checking
a timer).
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