Psychtoolbox>Screen.{mex*} subfunction

hz=Screen(‘NominalFrameRate’, windowPtrOrScreenNumber [, mode=0][, reqFrameRate]);

Returns or sets the nominal video frame rate in Hz, as reported by your
computer’s video driver.
‘FrameRate’ is an alias for ‘NominalFrameRate’. By default, this function
returns the nominal framerate, as reported by your operating system, rounded to
the closest integral value. If you set the optional ‘mode’ flag to 1, then the
framerate is returned without rounding to the closest integer, but at floating
point precision, on systems that support this (macOS and Linux).
GNU/Linux/X11 only: If you set the ‘mode’ flag to 2 and specify ‘reqFrameRate’,
then Screen will try to quickly switch to a framerate as close as possible to
‘reqFrameRate’ Hz. Invalid settings are rejected, returning a value of -1. On
failure, 0 is returned to signal inability to switch refresh. On systems other
than Linux, 0 is always returned to signal failure. On successfull framerate
change the new resulting nominal framerate is returned with double precision.
NOTE: This function allows fast and almost seamless video refresh rate switching
on AMD graphics cards when used to drive a FreeSync capable display device. The
‘reqFrameRate’ should be within the supported FreeSync refresh rate range of
your display device, or this function may silently fail! On other graphics cards
or on standard fixed refresh rate displays, this may or may not work, but it
will likely result in a time consuming video modesetting operation, during which
the display may go blank for up to multiple seconds.
One limitation at the moment is that in a multi-display setup, your visual
stimulation window must be fully or at least predominantly covering the display
area of the primary video output monitor, as the refresh rate of that monitor
will be assigned for Screen(‘Flip’, …) scheduling of stimulus onset time.
Otherwise only immediate flips at each video refresh cycle will work with
reliable timing. See ‘help VRRSupport’ for possibly more background and setup
info.
Due to manufacturing tolerances and other noise in your system, the real monitor
refresh interval can differ slightly from the nominal values returned by this
function. To query the real, measured framerate use Screen(‘GetFlipInterval’)
instead.

###See also: FrameRate GetFlipInterval