Originally Posted by davelaser
Opengl sync only ever works on one of the monitors if both are enabled. If only one is enabled, it works on that one. It would be nice to have an opengl monitor sync option like the xvideo sync option which decides which monitor to sync to.
This was mentioned in the README:
11C. VBLANK SYNCING
Setting the environment variable __GL_SYNC_TO_VBLANK to a non-zero value will
force glXSwapBuffers to sync to your monitor's vertical refresh (perform a
swap only during the vertical blanking period).
When using __GL_SYNC_TO_VBLANK with TwinView, OpenGL can only sync to one of
the display devices; this may cause tearing corruption on the display device
to which OpenGL is not syncing. You can use the environment variable
__GL_SYNC_DISPLAY_DEVICE to specify to which display device OpenGL should
sync. You should set this environment variable to the name of a display
device; for example "CRT-1". Look for the line "Connected display device(s):"
in your X log file for a list of the display devices present and their names.
You may also find it useful to review Chapter 13 "Configuring Twinview" and
the section on Ensuring Identical Mode Timings in Chapter 19.
Does it not work? I have not tried.