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function VideoCaptureToMatlabDemo(deviceIndex)
% VideoCaptureToMatlabDemo([deviceIndex])
%
% Minimalistic demo on how to capture video data and return it in a
% Matlab/Octave matrix.
%
% The demo starts video capture on a video device, auto-detected or
% specified by optional 'deviceIndex'. Instead of requesting images as
% textures and displaying them in a ptb onscreen window, it requests images
% as raw data in a matrix rawImage. After converting rawImage into the
% standard Matlab/Octave image matrix format, it imshow()'s the image in a
% standard figure window.
%
% Press any key or wait for 600 seconds to end the demo.
%
AssertOpenGL;
% Default is to auto-detect video device to use:
if nargin < 1
deviceIndex = [];
end
try
% Open a minimalistic window (only 10 x 10 pixels). This is needed
% because we need to provide a 'win'dowhandle to the videocapture
% functions. We skip startup timing tests...
skipsync = Screen('Preference','SkipSyncTests', 2);
% Disable high precision timestamping:
timstampm = Screen('Preference', 'VBLTimestampingMode', -1);
% ... and we make the window completely invisible, as we don't need it
% for showing anything to the user. A shielding level of -1 does this:
winlevel = Screen('Preference','WindowShieldinglevel', -1);
% Open an invisible dummy window of 10 x 10 pixels size:
win = Screen('OpenWindow', 0, 0, [0 0 10 10]);
% Open videocapture device, requesting 640 x 480 pixels resolution:
grabber = Screen('OpenVideoCapture', win, deviceIndex, [0 0 640 480],[],[],[],[],[],[],8);
% Start low-latency capture with requested 30 fps:
Screen('StartVideoCapture', grabber, 30, 1);
oldpts = 0;
count = 0;
t=GetSecs;
% Capture for 600 seconds or until keypress:
while (GetSecs - t) < 600
if KbCheck
break;
end
% Retrieve next captured image in 'rawImage'. The 'waitforImage=2'
% flag disables texture creation, so 'tex' is actually an empty
% handle. The 'specialMode'=2 flag requests video data as matrix:
[tex pts nrdropped rawImage]=Screen('GetCapturedImage', win, grabber, 2, [], 2); %#ok<ASGLU>
% Display class of returned image matrix:
matrixClassIs = class(rawImage)
% Bits of accounting and stats to the Matlab window:
if count > 0
% Compute delta:
delta = (pts - oldpts) * 1000;
fprintf('%.4f\n', delta);
end
oldpts = pts;
% Convert rawImage matrix into a matrix suitable for display with
% Matlabs imshow(). imshow needs a height x width x 3 colors
% matrix, whereas rawImage is a c by width x height matrix with c=1
% for luminance data, c=3 or 4 for RGB data, where the 4th
% component - if present - is a useless alpha channel.
channels = min(size(rawImage,1), 3);
for ci=1:channels
if channels == 1
tci=1;
else
tci = 4 - ci;
end
matImage(1:size(rawImage,3), 1:size(rawImage,2), tci) = transpose(squeeze(rawImage(ci,:,:))); %#ok<AGROW>
end
% Show image in a figure window:
imshow(matImage);
% Count and print number of unique values in image - An indicator of net bitdepth:
uniquevalues = length(unique(matImage(:)))
drawnow;
count = count + 1;
end
telapsed = GetSecs - t;
% Stop capture, close engine and onscreen window:
Screen('StopVideoCapture', grabber);
Screen('CloseVideoCapture', grabber);
sca;
% Restore preference settings:
Screen('Preference','SkipSyncTests', skipsync);
Screen('Preference', 'VBLTimestampingMode', timstampm);
Screen('Preference','WindowShieldinglevel', winlevel);
avgfps = count / telapsed;
fprintf('\n\nElapsed time %f secs, average fps %f.\n', telapsed, avgfps);
catch %#ok<CTCH>
% Close windows and shutdown capture engine:
sca;
% Restore preference settings:
Screen('Preference','SkipSyncTests', skipsync);
Screen('Preference', 'VBLTimestampingMode', timstampm);
Screen('Preference','WindowShieldinglevel', winlevel);
% Throw error:
psychrethrow(psychlasterror);
end
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