trouble with the function 'Screen' running code with psychtoolbox

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Hello everyone,

I am new to Matlab, but would like to use the code from a paper (Thaler, L., Schutz, A. C., Goodale, M. A., & Gegenfurtner, K. R. (2013). What is the best fixation target? The effect of target shape on stability of fixational eye movements. Vision Research, 76, 31-42.) to create a fixation cross in matlab using the psychtoolbox.

However, I have trouble running the code. I get an error message following the code.

Thank you for your help.

Juliane

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ERROR MESSAGE:

Error using Screen

Usage:

Screen('DrawLine', windowPtr [,color], fromH, fromV, toH, toV [,penWidth]);

Error in fix_cross_script (line 24)

Screen('DrawLine', w, colorCross, cx-d1/2 * ppd, cy, cx+d1/2 * ppd, cy, d2 * ppd);

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clear all;
close all;
width = 39; % horizontal dimension of display (cm)
dist = 60; % viewing distance (cm)
colorOval = [0 0 0]; % color of the two circles [R G B]
colorCross = [255 255 255]; % color of the Cross [R G B]
d1 = 0.6; % diameter of outer circle (degrees)
d2 = 0.2; % diameter of inner circle (degrees)
screen = 0;
[w,rect] = Screen('OpenWindow', screen, [], []);
[cx, cy] = RectCenter(rect);
ppd = pi * (rect(3)-rect(1)) / atan(width/ dist/2) / 360; % pixel per degree
HideCursor;
WaitSecs(2);
Screen('FillOval', w, colorOval, [cx-d1/2 * ppd, cy-d1/2 * ppd, cx+d1/2 * ppd, cy+d1/2 * ppd], d1 * ppd);
Screen('DrawLine', w, colorCross, cx-d1/2 * ppd, cy, cx+d1/2 * ppd, cy, d2 * ppd);
Screen('DrawLine', w, colorCross, cx, cy-d1/2 * ppd, cx, cy+d1/2 * ppd, d2 * ppd);
Screen('FillOval', w, colorOval, [cx-d2/2 * ppd, cy-d2/2 * ppd, cx+d2/2 * ppd, cy+d2/2 * ppd], d2 * ppd);
Screen(w, 'Flip');
WaitSecs(2);
Screen('Close', w);