Matlab Class 2 Xiaotao Su, Ph.D. Visual Psychophysicist

1
Matlab Class 2Xiaotao Su, Ph.D.Visual
Psychophysicist IT Group LeaderRutgers Center
for Cognitive Science (RuCCS)
2
Character Vectors And Numeric Vectors

• 'hello world' is referred to as a character
  string or a character vector
• 35, 26.3, 11, 4 is a numeric vector
• These are the two main data types in matlab

h e l l o w o r l d
(1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11)
35 26.3 11 4
(1) (2) (3) (4)
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Character Vectors (aka Strings) And Numeric
Vectors

• How to convert between these two data types? What
  if I have a string, '39.5' and I want to add to
  it?
• num2str str2num
• str2num('39.5') 5
• 'hello w', 'orld'
• a 3, 9
• b 12.6, 8
• a, b
• c '1', num2str(39.5)
• str2num(c) 200

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Taking inputs

• Create a new program called math.m
• math.m
• Carries out a series of calculations on two
• numbers
• written by Your Name July 2010

5

• clear all
• take inputs
• num1input('what is the first number?')
• num2input('what is the second number?')
• show calculations
• disp(num2str(num1), '', num2str(num2), '',
  num2str(num1num2))
• disp(num2str(num1), '-', num2str(num2), '',
  num2str(num1-num2))

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boolean expressions

• Boolean expressions either return 1 for true or
  0 for false
• Possible operators

gt gt lt lt
equals greater than greater than or equal to less than greater than or equal to and or
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• 11
• (1 1)2
• (5 5) lt 10
• (5 5) lt10
• (1 1) (23)
• (1 1) (23)
• 5 (5 lt 10)

Note Put operations inside parenthesis so they
are preformed first.
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Conditionals (if statements)

• if(Boolean Expression)
• do something
• end
• you can also do this
• if(boolean expression)
• do something
• else
• do something else
• end

or this if(boolean expression) do
something elseif(boolean expr2) do something
else end
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Back to math.m

• if ( (round(num1)num1) (round(num2)num2) )
  
• disp('num1 and num2 are integers')
• end

10

• if ( (round(num1)num1) (round(num2)num2) )
  
• disp('num1 and num2 are integers')
• elseif(round(num1)num1)
• disp('only num1 is an integer')
• elseif(round(num2)num2)
• disp('only num2 is an integer')
• else
• disp('neither number is an integer')
• end

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Task 1

• Have math.m tell me which number is larger then
  the other
• the biggest number is

gt gt lt lt
equals greater than greater than or equal to less than greater than or equal to and or
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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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Set up program draw_stuff

• draw_stuff.m
• Draws stuff on the screen

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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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Tic and toc

• tic starts a timer
• toc tells you the number of seconds since when
  you ran toc

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While loop

• while is a type of loop that runs while a
  condition is true
• while(boolean statement)
• do something many times
• end
• you would stop repeating the loop when the
• boolean statement is false OR if you use
• break

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While program

• tic
• loops 0
• while(toclt5)
• loops loops1
• disp('number of loops ', num2str(loops))
• disp('number of seconds ', num2str(toc))
• end

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While program stopping after 10 loops

• tic
• loops 0
• while(toclt5)
• loops loops1
• disp('number of loops ', num2str(loops))
• disp('number of seconds ', num2str(toc))
• if(loopsgt10)
• break
• end
• end

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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)

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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