Matlab Class 8 Xiaotao Su, Ph.D. Visual Psychophysicist

1
Matlab Class 8Xiaotao Su, Ph.D.Visual
Psychophysicist IT Group LeaderRutgers Center
for Cognitive Science (RuCCS)andChris Kourtev
2
Small Pieces

• Starting with matlab version 7.0 you can execute
  small chunks of code
• This is called cells (nothing to do with cell
  arrays)
• mark off the beginning and end of cell region
• Cell regions are seen as yellow
• Pressing ctrl/cmd return causes the workspace
  to execute the command in the active cell

If you do not see a yellow region, in the
menu bar select Cell-gtEnable Cell Mode
3
Creating a vector of numbers

• This is similar to the way you create an
  iteration for a for loop
• x (020) ----gt x 0, 1, 20
• x (0.10.11) ---gt x 0.1, 0.2, , 1.0
• x (0220) ----gt x 0, 2, 4, 20
• x (0220) ---gt vertical vector like the
  previous one

4
discrete distributions and selecting segments

• you can use something like x(020) to specify a
  discrete distribution of 21 different
  possibilities
• to select the first 6 elements you could sayy
  x(16) ---gt y would be 0, 1 5
• Or to select the last 4 elements
• y x(end-3end) ---gt y would be 17, 18, 19, 20

5
y binopdf(x(16), 5, .8)

• used to generate a binomial distribution function
  the case in which one flips a weighted coin
• .8 probability of getting heads
• there are 6 possibilities (0 heads, 1 head 5
  heads
• y gives the probability of each of these outcomes

This function requires the stats toolbox
6
Plotting

• Subplot will create a figure window for plotting
• subplot (2, 1, 2) will say we are going to create
  a (2 row, 1 column, ) figure. The last number
  specifies that we are currently going to write in
  the second element of this figure.
• plot(y) will create a graph on currently chosen
  section of the figure (specified by the third
  parameter in subplot)

7
setting up the plot

• xlim(1 5) sets the limits of the x axis
• bar(y) will use a bar graph
• ylabel(Probability of n)
• xlabel(N)
• title(Discrete distributions should be plotted
  as histograms)

8
more then one figure?

• figure opens a new figure window
• All new subplots will go to the new figure
• subplot(2, 1, 1)
• bar(y)
• xlim(1 21) we want 21 possibilities

9
more then one graph on a plot

• hold on tells matlab to keep the previous graph
  and draw a new one on top
• stairs(cumsum(y), r)
• cumsum(y) lt--- each value is the sum of previous
  values of y
• r makes the graph line red
• stairs will plot it as a staircase
• hold off now any future graph additions will
  clear previous graphs on this plot
• legend(distribution, cumsum)

10
Using images in experiments

• Images are stored in matlab as
• Width x Height x 3 matrix

x
R
G
y
B




11
file -gt matrix and drawing it on the screen

• img imread(winter.jpg, jpg)
• You draw an image just like you would a
  rectangle
• Screen(window, PutImage, img, 0, 0, 200,
  200)

12
Image info and structures

• img_info imfinfo(winter.jpg)
• img_info.Width will be the width of the image
• screen(window, PutImage, img,
• 0, 0, img_info.Width, img_info.Height)

13
Making sounds

• A sound is a 2 x N matrix where N is the number
  of bits that make up the sound file and the two
  channels are for left and right

14
Making sounds

• sound, samplerate, samplesize
• wavread(feedback.wav)
• sound sound
• Snd(Play, sound, samplerate, samplesize)

Tip To make your own wav files I recommend
using an application called audacity http//audac
ity.sourceforge.net/