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function ifi = VideoRefreshFromMeasurement(window, samples)
% ifi = VideoRefreshFromMeasurement(window, samples)
%
% This function should determine the exact duration of the displays video
% refresh interval with the highest possible precision, using whatever
% method proves to be the most accurate on your system.
%
% You must provide 'window' the handle to the onscreen window whose
% associated display you want to be measured and (optionally) 'samples',
% the number of samples to take for computation of video refresh interval.
%
% The function returns the measured interval in units of seconds.
%
% CAUTION: This is unfinished alpha quality code. While it works well on
% some system setups, it doesn't yet on others and will need more
% fine-tuning in the future. For most purpose, the values returned by
% Screen('GetFlipInterval', window); are perfectly usable and the
% recommended way of getting the video refresh duration.
% History:
% 07/16/07 Written (MK).
if nargin < 1
error('You must provide a ''window'' handle for calibration!');
end
if Screen('WindowKind', window) ~= 1
error('You must provide an onscreen window handle for calibration!');
end
if nargin < 2
samples = 600;
end
wininfo = Screen('GetWindowInfo', window);
startvblcount = wininfo.VBLCount;
if startvblcount > 0
% VBL counter supported by OS. Use it for measurement:
% Empirically this is found to be the most reliable way of determining
% refresh interavl on Macintosh OS/X:
newvblcount = startvblcount;
while newvblcount == startvblcount
wininfo = Screen('GetWindowInfo', window);
newvblcount = wininfo.VBLCount;
end
startvblcount = newvblcount;
tstart = wininfo.LastVBLTime;
while newvblcount < startvblcount + samples
wininfo = Screen('GetWindowInfo', window);
newvblcount = wininfo.VBLCount;
end
vblcount = newvblcount - startvblcount;
telapsed = wininfo.LastVBLTime - tstart;
ifi = telapsed / vblcount;
else
% Its not yet clear if beamposition method or vbl-synced flip interval
% is the best way to do it on MS-Windows...
% No VBL counter support :( - Try beamposition method:
ifi = wininfo.VideoRefreshFromBeamposition;
% Result from beamposition method?
if ifi == 0
% Nope. Ok we return the flip interval measured by Screen:
ifi = Screen('GetFlipInterval', window);
end
end
% We should have an ifi, one or the other way...
return;
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