Matlab Class 5

Xiaotao Su, Ph.D.Visual Psychophysicist & IT Group Leader

Rutgers Center for Cognitive Science (RuCCS)

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Psychtoolbox Basics

• Computers are perfect for creating and displaying visual psychophysics stimuli

• Programs usually written in a low-level language (e.g. C or Pascal) to achieve full control of the hardware for precise stimulus display but are difficult to program

• Interpreted languages like Matlab are abstracted from hardware details and provide friendlier development environments

• The Psychophysics Toolbox is a software package that allows Matlab to fully control the hardware for precise stimulus display while retaining the flexibility and ease of Matlab.

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Screen drawing and psychtoolbox

Screen Geometry

X ----------- Positive ->

Y

Positive

Origin

Max X and Y

Coordinates are measured in pixels.

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How you typically work with the psychtoolbox

Back Buffer(invisible)

Front Buffer

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How you typically work with the psychtoolbox

Front Buffer

Step 1: DrawShape to the back buffer

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How you typically work with the psychtoolbox

Step 2:Flip the back bufferto the front buffer

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How you typically work with the psychtoolbox

Back Buffer is automatically cleared

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How you typically work with the psychtoolbox

Now you can continue with your next frame of animation

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How you typically work with the psychtoolbox

Flip the back bufferto the front buffer

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How you typically work with the psychtoolbox

Back Buffer is automatically cleared

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Basic Display Loop in PsychToolbox (PTB)

% Open Window

% while some condition is true do the following

% draw something

% make some changes

% repeat

% Close screen

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Set up program draw_stuff

% draw_stuff.m

%

% Draws stuff on the screen

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Skip Sync Tests

• Add to your program before opening the new screen

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

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Initialize the main window

% draw_stuff.m%% Draws stuff on the screenclear all;which_screen=0;

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Set up program draw_stuff

% draw_stuff.m%% Draws stuff on the screenclear all;which_screen=0;

[window_ptr, screen_dimensions]=Screen(0,'OpenWindow', [0, 0, 0]);

0 is main screen (allows for multiple monitors)

Command issued to the psychtoolbox Screen function

[red, green, blue]

Vector specifyingScreen color

0

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Set up program draw_stuff

% draw_stuff.m%% Draws stuff on the screenclear all;which_screen=0;[window_ptr, screen_dimensions]=Screen(0,'OpenWindow', [0, 0, 0]);

Vector [x1, y1, x2, y2]Which could be:[0, 0, 800, 600]For an 800 X 600 display

x1= 0y1 = 0

x2= 800 y2 = 600

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Draw shape in back buffer% draw_stuff.m%% Draws stuff on the screenclear all;which_screen=0;[window_ptr, screen_dimensions]=Screen(0,'OpenWindow', [0, 0, 0]);

shape_dimensions = screen_dimensions/2;Screen(window_ptr, 'FillRect', [0,0,255], shape_dimensions);

X1, Y1

X2, Y2Front buffer

Back buffer

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% draw_stuff.m%% Draws stuff on the screenclear all;which_screen=0;[window_ptr, screen_dimensions]=Screen(0,'OpenWindow', [0, 0, 0]);shape_dimensions = screen_dimensions/2;Screen(window_ptr, 'FillRect', [0,0,255], shape_dimensions);%Flip the buffersScreen(window_ptr,'Flip');

Back buffer

Flip the back buffer to the front buffer

Front buffer

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Clear screen<<<<snip>>>>clear all;which_screen=0;[window_ptr, screen_dimensions]=Screen(0,'OpenWindow', [0, 0, 0]);

shape_dimensions = screen_dimensions/2;Screen(window_pointer, 'FillRect', [0,0,255], shape_dimensions);%Copy to main windowScreen(window,'Flip');%leave the image up for 5 secondsWaitSecs(5);%Clear screen and return control to matlabclear screen;

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Task 1: Making a movie% draw_stuff.m%% Draws stuff on the screenclear all;which_screen=0;[window_ptr, screen_dimensions] …=Screen(0,'OpenWindow', [0, 0, 0]);

shape_dimensions = screen_dimensions/2;Screen(window_pointer, 'FillRect', … [0,0,255], shape_dimensions);

% then loop and increment screen_dimensions at each turn

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Task 1: Basic Display Loop in PTB (code)

% draw_stuff.m%% Draws stuff on the screenclear all;try

which_screen=0;bg_color = [0, 0, 0];[window_ptr, screen_dimensions]= …

Screen(which_screen, …

'OpenWindow',bg_color);shape_dimensions = … screen_dimensions/2;tic;

while (toc < 5) % move shape shape_dimensions = …

shape_dimensions + [0,5,0,5];

% draw shape Screen(window_ptr, 'FillRect', … [0,0,255], shape_dimensions);

% flip buffer and repeat Screen(‘Flip’, window_ptr);endclear Screen;

catch

clear Screen;

end

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Moving Shapes around the Screen

• Shapes are defined by an array of numbers e.g. sh1 = [10, 15, 40, 45] ; % [x1, y1, x2, y2]

• To move a shape in the x direction by 5 pixels we need to increment both x coordinates by 5e.g. sh1(1) = sh1(1) +5; sh1(3) = sh1(3) +5

or use another array: sh1 = sh1 + [5, 0, 5, 0];

• Same for moving in the y direction

• Increment both if you want to move diagonally

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Task 2

Make the rectangle move to the bottom of the screen at 5 pixels per frame, then move right when it hits the bottom.When it hits the right, the program ends

Hint: shape_dimensions(1) is x1, (2) is y1, (3) is x2, 4 is y2

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Task 3

When the rectangle hits the bottom make it change colors to red

Hint: color is a 3 number vector red is [255, 0, 0]