Contents

Present a drifting Gabor patch with cogent

demonstrates how to display a bitmap image using cg library Elliot Freeman 0207

gam = 2.2;                      % gamma value from calibration procedure
contrast = .5;                  % contrast of gabor patch
windowmode = 0;                 % 0: small window; 1: full screen
horizontalResolution = 1024;  	% horizontal screen resolution in pixels: select from [640 800 1024 1152 1280 1600]
imageFile = 'plaidMovie';       % filename of image matrix
RGB = [1 0 1];                  % maximum luminance of RGB guns (before scaling by overall contrast)
contrast = 1;                   % overall contrast of images
temporalFrequency = 0;          % how many times movie is played per second
refreshRate = 75;               % refresh rate of screen in hertz
trialDuration = 1000;           % duration of each stimulus presentation (ms)
ISI = 500;                      % inter stimulus interval (ms)
fixPos = [0 0];                 % X and Y position of stimulus
stimPos = [0 100];              % X and Y position of stimulus
fixSize = 5;                    % size of fixation point
nTrial = 10;                    % number of trials
escapeKey = 52;                 % code for escape key
responseKeys = [97 98];         % codes for left and right-arrow keys
speedMod = 5;                   % amount to modulate speed

load and describe movie matrix

load(imageFile, 'im1');                                  % load movie matrix
load(imageFile, 'im2');                                  % load movie matrix
[sX sY nFrames] = size(im1);                             % get dimensions and number of frames

Error using ==> load
Unable to read file plaidMovie: No such file or directory.

computed constants

bg = .5 .^ (1/gam);                                     % calculate gamma corrected mid-gray background value

start cogent

resVal = [640 800 1024 1152 1280 1600];                 % possible screen resolutions
resCode = find(horizontalResolution == resVal);         % select one
config_display(windowmode,resCode,[0 0 0], [bg bg bg]); % configure graphics window
config_keyboard;                                        % set up keyboard
start_cogent;                                           % start cogent

keyMap = getkeymap;

make fixation sprite

fixSprite = 1000;
cgMakeSprite(fixSprite, fixSize, fixSize, bg, bg, bg);             % make a new sprite
cgSetSprite(fixSprite);
cgEllipse(fixPos(1), fixPos(2), fixSize, fixSize, [1 1 1], 'f');
cgSetSprite(0);

create each movie frame in a separate sprite

for iFrame = 1:nFrames                                  % loop through frames
    imF1 = im1(:,:,iFrame);                                  % extract frame from movie matrix
    imF2 = im2(:,:,iFrame);                                  % extract frame from movie matrix

    % To load the images into sprites, Cogent needs a three column matrix:
    % each column contains the luminance values for Red Green and Blue
    % respectively. To make this matrix,
    % (1) transpose each image frame and
    % (2) scale each column by the appropriate values of RGB to manipulate
    % the colour of the resulting image

    imR = imF1' * RGB(1);                                  % vector for red guns
    imG = imF1' * RGB(2);                                  % vector for green guns
    imB = imF2' * RGB(3);                                  % vector for blue guns

    % (3) now squash each R G or B image into a single column vector
    % - if you didn't transpose before, this operation results in 90deg rotation
    % (4) and horizontally concatenate into three column matrix
    imRGB0 = [imR(:) imG(:) imB(:)];                                  % now compile in to three-column matrix

    % finally apply overall contrast scaling, recenter values on 0.5, gamma correct
    imRGB0 = imRGB0 .* contrast;                          % apply contrast modulation to whole matrix
    imRGB1 = ( imRGB0 +1 ) /2;                            % convert values of -1->1 to 0->1 (gray is now .5)
    imRGB_gamma = imRGB1 .^ ( 1/ gam );                   % apply gamma correction to image

    % make a new sprite and put image into it
    cgMakeSprite(iFrame, sX, sY, bg, bg, bg);             % make a new sprite
    cgloadarray(iFrame, sX,sY,imRGB_gamma);               % load image frame into sprite
end

put up fixation and wait for any key

cgflip(bg, bg, bg);
cgDrawSprite(fixSprite, fixPos(1), fixPos(2));
cgflip(bg, bg, bg);
[ keyout, t, n ] = waitkeydown(inf);
if keyout == escapeKey
    error('experiment aborted');
end

trial sequence

for iTrial = 1: nTrial

    %% random speed
    randSpeed = (rand(1) * speedMod) - (speedMod /2);

    %% work out how fast to play movie
    framesPerSecond = (temporalFrequency + randSpeed) * nFrames;         % how many frames shown per second
    frameIncrement = framesPerSecond / refreshRate;          % how much to increment frame count every screen refresh

    %% display movie
    t0 = time;
    iFrame = 0;
    while time < (t0 + trialDuration)
        iFrame = iFrame + frameIncrement;
        mFrame = mod(iFrame-1, nFrames)+1;
        cgDrawSprite(mFrame, stimPos(1), stimPos(2));
        cgDrawSprite(fixSprite, fixPos(1), fixPos(2));
        cgflip(bg, bg, bg);
    end

    cgDrawSprite(fixSprite, fixPos(1), fixPos(2));
    cgflip(bg, bg, bg);
    wait(ISI);
end
stop_cogent

Published with MATLAB® 7.1