How could I calculate the size of dots, height, and width of cylinder by pixel?

Hello.

I am a beginner in matlab.

I got the code in psychtoolbox that can make rotating cylinder.

I remember getting help here last time, so I'm asking a new question about the problem I wasn't able to solve.

I should know the width and height of cylinder and dot size by pixel unit to calculate visual angle.

But I could not know what matlab code to do this, only found the matlab code that change vector to pixel or image to pixel.

So I want to calculate the width and height of below cylinder code and dot size by pixel unit.

If anyone knows anything about this, I would appreciate it if you could let me know.

Thank you for reading this.

Below is rotating cylinder code file. It is consist of two file.

# first code file

clear;

makeCyl = 1;

%% parameters

W = 153; %width

ndot = 900;

NumFrames = 86;

stepD = 400; % ie the half perimeter of the cylinder

cylR = stepD/pi; %guarantee the cylinder aprroaches with the same distance per 180 degree as the figure walks a step

%% cylinder

if makeCyl

backCol=0.9;

dotx = sin((rand(1,ndot)-0.5)*pi)* W/2; %dots are more concentrated on the two boundaries after ttransformed to sin, more natural than sqrt

dotc = ones(1, ndot);

angs = 2*pi* rand(1,ndot);

angpf = 2*pi/NumFrames; %angle per frame

Cylin = NaN(NumFrames*3,ndot); %keep it the same format as the mov file

dotC = cell(NumFrames,1);

for zz = 1:NumFrames

Cylin(zz*3-2,:) = dotx;

Cylin(zz*3-1,:) = cylR*cos(angs);

Cylin(zz*3,:) = cylR*sin(angs); % all dots are in the same depth so that it won'd contain perspective cue when drawn by moglDrawDots3D

angs = angs-angpf;

dotc = ones(1, ndot);

dotc(:, mod(angs,2*pi)<=pi) = backCol;

dotC{zz}= [1;1;1]*dotc;

end

save('Cylin_aaa','Cylin' ,'ndot');

end

# second code file

InitializeMatlabOpenGL;

load('Cylin_aaa.mat')

nFrames = 60;

fr = 1;

InitializeMatlabOpenGL; % open GL reset, moglDrawDots3D Required for function execution

%Screen('Preference', 'SkipSyncTests', 1);

Screen('Preference', 'SkipSyncTests', 1) % Skip sync test for testing

whichScreen = max(Screen('Screens'));

[w, screenRect] = Screen('OpenWindow', whichScreen,[0 0 0],[]); % small screen output

ScreenCenter = [screenRect(3)/2 screenRect(4)/2];

commandwindow;

while fr <= 86

thefr = fr;

while thefr > nFrames; thefr =thefr-nFrames; end

disp(thefr); % Check frame progress

this_xyz = Cylin(thefr*3-2:thefr*3,:);

Screen('DrawDots',w, this_xyz(1:2,:),[],[255 255 255],ScreenCenter, 0);

% moglDrawDots3D(w, this_xyz, 1, [1 1 1]); %

Screen('Flip',w);

%WaitSecs(0.05); % To see the dot move slowly

fr = fr +1;

end

sca;

Answer 1

[Angelo Yeo]

1. By default, each dot has a size of 1 pixel. See Psychtoolbox-3 - Screen(‘DrawDots’) for details. To quote:

“size” is the diameter of each dot in pixels (default is 1)

2. In Psychtoolbox, drawing unit of dots is pixels. For example, the script below will plot 2 dots at center and 10 pixels right to the center.

InitializeMatlabOpenGL; % open GL reset, moglDrawDots3D Required for function execution

Screen('Preference', 'SkipSyncTests', 1) % Skip sync test for testing

whichScreen = max(Screen('Screens'));

[w, screenRect] = Screen('OpenWindow', whichScreen,[0 0 0],[]); % small screen output

ScreenCenter = [screenRect(3)/2 screenRect(4)/2];

commandwindow;

myDots = ...

[0, 10;

0, 0];

Screen('DrawDots',w, myDots,5,[255 255 255],ScreenCenter, 0);

Screen('Flip', w);

KbStrokeWait

sca;

3. According to #2, the width and height of the cylinder can be calculated in pixel like below.

load('Cylin_aaa.mat');
temp = Cylin(1:2,:); x = temp(1,:); y = temp(2, :);
width = max(x) - min(x) % pixels

width = 152.9998

height = max(y) - min(y) % pixels

height = 254.6442

[희영]

Your answer is exactly what I wanted to find!

Thank you for helping me.

For you help, I can make the stimuli for my experiment!

Have a good day :)

[Angelo Yeo]

Thanks for the feedback. I am glad it helped 😊.