Psychtoolbox>Screen.{mex*} subfunction

Returns a struct with miscellaneous info about finished flips on the specified
onscreen window.

This function is currently only supported on Linux X11/GLX with the free
graphics drivers,
and on Linux/Wayland with support for the presentation_feedback extension.

The function allows you to enable logging of timestamps and other status
information about all completed bufferswaps, as triggered via Screen(‘Flip’),
Screen(‘AsyncFlipBegin’) etc. Whenever a flip completes, a little info struct is
stored in a internal queue. This function allows you to fetch these info structs
from the queue. It allows you to enable and disable logging of these info
structs. Logging is disabled by default.

“windowPtr” is the handle of the onscreen window for which info should be
“infoType” If left out or set to zero, the flip handle for the last invocation
of flip is returned. E.g., if a value of 53 is returned, then the info struct
with a info.SwapbuffersCount field of 53 will contain the info for that last
invocation of flip.
This allows to associate specific flips with the returned logged timestamps and
other info.
If set to 1, logging of flip completion info is enabled.
If set to 2, logging of flip completion info is disabled.
If set to 3, the oldest stored flip completion info is returned in a struct

The info struct contains the following fields

OnsetTime: Visual stimulus onset time of the completed Screen(‘Flip’) operation.
OnsetVBLCount: Video refresh cycle count when the flip completed.
SwapbuffersCount: Serial number of this info struct. Corresponds to the handle
returned for ‘infoType’ zero.
SwapType: How was the flip executed? Low level info about strategy chosen by
BackendFeedbackString: A string with more info, which is specific to a display
backend, e.g., GLX vs. Wayland.
Note: Currently only flips with a ‘SwapType’ field containing the (sub-)string
‘Pageflip’ are considered to have reliable timing and trustworthy timestamps. On
Wayland dislay servers a ‘SwapType’ of ‘ImpreciseCopy’ also has good timing,
although likely with less precise and accurate onset timestamps.
‘ImprecisePageflip’ is also considered having slightly less precise and accurate
onset timestamps. Other types of ‘Pageflip’s should be high quality in the time

###See also: OpenWindow, Flip, NominalFrameRate