Matlab Class 5 Xiaotao Su, Ph.D. Visual Psychophysicist

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Matlab Class 5Xiaotao Su, Ph.D.Visual
Psychophysicist IT Group LeaderRutgers 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

Origin
Coordinates are measured in
pixels.
X ----------- Positive -gt
Y
Positive
Max X and Y
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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
Step 1 Draw Shape to the back buffer
Front Buffer
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How you typically work with the psychtoolbox
Step 2 Flip the back buffer to 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 buffer to 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
  screenScreen('Preference','SkipSyncTests', 1)

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Initialize the main window

• draw_stuff.m
• Draws stuff on the screen
• clear all
• which_screen0

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Set up program draw_stuff

• draw_stuff.m
• Draws stuff on the screen
• clear all
• which_screen0
• window_ptr, screen_dimensions
• Screen(0,'OpenWindow', 0, 0, 0)

0
Vector specifying Screen color
Command issued to the psychtoolbox Screen function
0 is main screen (allows for multiple monitors)
red, green, blue
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Set up program draw_stuff

• draw_stuff.m
• Draws stuff on the screen
• clear all
• which_screen0
• window_ptr, screen_dimensionsScreen(0,'OpenWind
  ow', 0, 0, 0)

Vector x1, y1, x2, y2 Which could be 0, 0,
800, 600 For an 800 X 600 display
x1 0 y1 0
x2 800 y2 600
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Draw shape in back buffer

• draw_stuff.m
• Draws stuff on the screen
• clear all
• which_screen0
• window_ptr, screen_dimensionsScreen(0,'OpenWind
  ow', 0, 0, 0)
• shape_dimensions screen_dimensions/2
• Screen(window_ptr, 'FillRect', 0,0,255,
  shape_dimensions)

X1, Y1
Front buffer
X2, Y2
Back buffer
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Flip the back buffer to the front buffer

• draw_stuff.m
• Draws stuff on the screen
• clear all
• which_screen0
• window_ptr, screen_dimensionsScreen(0,'OpenWind
  ow', 0, 0, 0)
• shape_dimensions screen_dimensions/2
• Screen(window_ptr, 'FillRect', 0,0,255,
  shape_dimensions)
• Flip the buffers
• Screen(window_ptr,'Flip')

Front buffer
Back buffer
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Clear screen

• ltltltltsnipgtgtgtgt
• clear all
• which_screen0
• window_ptr, screen_dimensionsScreen(0,'OpenWind
  ow', 0, 0, 0)
• shape_dimensions screen_dimensions/2
• Screen(window_pointer, 'FillRect', 0,0,255,
  shape_dimensions)
• Copy to main window
• Screen(window,'Flip')
• leave the image up for 5 seconds
• WaitSecs(5)
• Clear screen and return control to matlab
• clear screen

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Task 1 Making a movie

• draw_stuff.m
• Draws stuff on the screen
• clear all
• which_screen0
• 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 screen
• clear all
• try
• which_screen0
• bg_color 0, 0, 0
• window_ptr, screen_dimensions
• Screen(which_screen, 'OpenWindow',bg_color)
  
• shape_dimensions screen_dimensions/2
• tic

while (toc lt 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) end cl
ear 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