based on Elboher ‘s slides , HUJI

It allows one to perform numerical calculations, and visualize
the results without the need for complicated and time
consuming programming.

Every variable in Matlab is a multidimensional matrix.

Highly modular.

No memory allocation is necessary.

Matlab enables its own garbage collection.

Simple interface for complex mathematical concepts.

Enables OO Programming.
>> help help (explain how to get help)
>> helpbrowser / doc (open the online Matlab documentation)
>> help images (list of all commands in the Image Processing Toolbox)
>> demo (supply various guide videos)
>> lookfor read (display list of functions with ‘read’ in the name or help
text)
>> type imread (display contents of file)
>> help imread (function name + % block)
>> doc imread (function documentation in help browser)

Using [tab] is useful to auto-complete function
names and variables




Digital image representation :
2D function f(x,y) -> finite discrete quantities
Coordinate Conventions:
img(r,c)
r–rows (height)
c–cols (width)
>> size(img)
The first pixel:
img(1,1)
• Intensity images
scaled to represent intensities (uint8 – [0,255], double [0,1])
• Binary images
logical array of 0s and 1s
• Indexed images
Look up table [x, map]
• RGB images
truecolor, array of (m*n*3)

Checking the image type : isind, isbw, isgray, isrgb
Converting image types: rgb2ind, rgb2gray, gray2ind, ind2gray,….
>> f = imread(‘filename’);
filename is a string including the file type (jpg, tiff,…)
; is used for suppressing output
>> [height, width] = size(f);
>> whos f display additional information about an array
Name
Size
Bytes
Class
f
512x512x3
786432 uint8 array
Grand total is 786432 elements using 786432 byte

>> imshow(f)
display the image f according to its type

>> imshow(f, [low high])

>> imshow(f, [])

>> impixelinfo

>> figure(2), imshow(g)
display as black all values less than ‘low’ and as white all values
greater or equal to ‘high’ (in grayscale images)
set low and high as the minimal and maximal values of array f (in grayscale
images)
display intensity value of individual pixel interactively
Open a new figure before displaying the image (the default – using the
same figure)

>> imwrite(f, ‘filename’)

>> k = imfinfo(‘test1.jpg’)
f is an image array
‘filename’ must include the file format (tif, jpg, bmp,..)
Filename: 'test1.jpg'
FileModDate: '22-Oct-2005 13:07:36'
FileSize: 3464
Format: 'jpg'
FormatVersion: ''
Width: 256
Height: 256
BitDepth: 24
ColorType: 'truecolor'
FormatSignature: ''
Comment: {}
The answer is a structure variable with different fields: k.Width
Converting between types : B = data_class_name(A)
for example: B = double(A)

When converting between data classes and types it is
important to keep the value range for each data class
>> img = double(img)/255;
>> img = im2double(img);



Matlab variables do not need to be declared
in advance.
‘ans’ is a defined variable containing the last
result
Memory is allocated and freed automatically.
>> A = [1 2 3; 2 3 4; 3 4 5];
>> A = 0.0005

row vector (1xN)
>> v = [1 3 5 7]; (elements separated by space or comma (,))
>> v(2) = 3;

column vector (MX1)

To Access blocks of elements we use colon notation

Vector can be used as an index into another vector
>> w = [1;3;5;7]; (elements separated semi-comma (;))
>> w = v’ (transpose operation)
w=
1
3
5
7
>> v(2:4)
ans =
357
>> v(1:end) end is the last element in the vector
>> v(:) produce a column vector
>> v(1:2:end) enables steps (jumps)
>> v(end:-2:1) steps can be negative as well
>> v([1 3 4])
ans =
1
5
7

Image – 2D array, matrix
Matrix can be represented as a sequence of row vectors

To access an element, 2 indexes are used – row index and column index

>>A = [1 2 3; 4 5 6; 7 8 9]
A=
1
2
3
4
5
6
7
8
9
>> A(2,3)  6
>> A(:,3)
3
6
9
>> A(2,:)
456
>> a(1:2,1:3)
123
456
>> B = A;
>> B(:,3) = 0
B=
1
2
4
5
7
8

0
0
0
Using vectors to index into a matrix provide a powerful tool for element selection
A([1 3], [2 3])
2
3
8
9


Image – 2D array, matrix
A matrix is also represented as a long vector
>> A = [1 2 3;
A=
1
2
4
5
7
8

3
6
9
To access sequential elements in a matrix a single index can also be used:
>> A(1:9)
A=
1 4

4 5 6; 7 8 9]
7
2
5
8
3
6
9
Sometimes changing the shape of the matrix is of use:
>> reshapse(A(1:2,1:3), 3, 2)
A=
1 5
4 3
2 6

The size of the output matrix is the size of the index matrix!
14

A very useful approach is to use logical matrix as an index
to the matrix
>> D = logical([1 0 0; 0 0 1; 0 0 0])
D=
1
0
0
0
0
1
0
0
0
>> A(D)
ans =
1
6

The use of column operation on a matrix produce a single
column vector from the matrix (on a column by column
basis). It is very useful for image operations like sum or
max
>> s = sum(f(:))
(equivalent to: sum(sum(f)))
15

Arithmetical operators have their algebraic meaning:
>> A = [1 2 3; 4 5 6; 7 8 9];
>> A + A
ans =
2 4
6
8 10 12
14 16 18
>> A * [1 1 1]
??? Error using ==> times
Matrix dimensions must agree
>> A * [1 1 1]’
ans =
6
15
24
16

Vector multiplication depends on the order of the two
vectors
>> [1 2 3] * [2 3 4]
??? Error using ==> times
Matrix dimensions must agree
>> [1 2 3] * [2 3 4]’
ans =
20
>>[1 2 3]’ *
ans =
2 3
4 6
6 9
[2 3 4]
4
8
12
17

Element by element operators -
.*
.^
./
>> [1 2 3] .* [2 3 4]
ans =
2 6 12
>> [1 2 3] .^ 2
ans =
1 4 9
>> [1 2 3] ./ (2:2:6)
ans =
0.5000 0.5000
0.5000
18


Matlab arrays can be of any dimensions
It is useful to operate on specific dimension of the
array, for example:
>> height = size(i,1);

Usually we deal with 2D arrays but there are cases
we need to address higher dimensions (such as
color images)
>> i(200:300, 200:400, 3)

To get the number of dimensions of an array
>> d = ndims(f)
19
Generating simple array enables trying out simple
ideas and test the syntax of a function during
development
>> zeros(m,n)
>> ones(m,n)
>> true(m,n)
>> false(m,n)
>> magic(m)
>> rand(n)
>> randn(n)
>> pascal(n)

20

Arithmetic operators (numeric computations)
◦ matrix arithmetic (linear algebra A*B)
◦ array arithmetic (element by element A.*B)
+, -, ./, .^,:..

Relational operators (compare)

Logical operators can operate both on logical
and numeric data (and: &, or: |, not: ~)
◦ Compare corresponding elements of arrays of
equal dimensions (<, >,<=, >=, ==, ~=) or an array to
scalar
true: logical 1 or non-zero numeric quantity
false: logical or numerical 0

logical functions : any, all
21
>> i = imread('sowrds0040.bmp');
>> i = rgb2gray(double(i)/255);
>> imshow(i)
22
>> i = imread('sowrds0040.bmp');
>> i = rgb2gray(double(i)/255);
>> imshow(i)
23
>>
>>
>>
>>
>>
i = imread('sowrds0040.bmp');
i = rgb2gray(double(i)/255);
imshow(i)
s = i(end:-1:1, :);
imshow(s);
24
>>
>>
>>
>>
>>
i = imread('sowrds0040.bmp');
i = rgb2gray(double(i)/255);
imshow(i)
s = i(end:-1:1,:);
imshow(s);
25
>>
>>
>>
>>
>>
>>
>>
i = imread('sowrds0040.bmp');
i = rgb2gray(double(i)/255);
imshow(i)
s = i(end:-1:1,:);
imshow(s);
a = i(200:300,200:400);
imshow(a);
26
>>
>>
>>
>>
>>
>>
>>
i = imread('sowrds0040.bmp');
i = rgb2gray(double(i)/255);
imshow(i)
s = i(end:-1:1,:);
imshow(s);
a = i(200:300,200:400);
imshow(a);
27
>>
>>
>>
>>
>>
>>
>>
>>
>>
i = imread('sowrds0040.bmp');
i = rgb2gray(double(i)/255);
imshow(i)
s = i(end:-1:1,:);
imshow(s);
a = i(200:300,200:400);
imshow(a);
t = i(1:2:end, 1:2:end);
imshow(t);
28
>>
>>
>>
>>
>>
>>
>>
>>
>>
i = imread('sowrds0040.bmp');
i = rgb2gray(double(i)/255);
imshow(i)
s = i(end:-1:1,:);
imshow(s);
a = i(200:300,200:400);
imshow(a);
t = i(1:2:end, 1:2:end);
imshow(t);
29
>>
>>
>>
>>
>>
>>
>>
>>
>>
>>
>>
i = imread('sowrds0040.bmp');
i = rgb2gray(double(i)/255);
imshow(i)
s = i(end:-1:1,:);
imshow(s);
a = i(200:300,200:400);
imshow(a);
t = i(1:2:end, 1:2:end);
imshow(t);
i(200:300, 200:400) = 0;
imshow(i);
30
>>
>>
>>
>>
>>
>>
>>
>>
>>
>>
>>
i = imread('sowrds0040.bmp');
i = rgb2gray(double(i)/255);
imshow(i)
s = i(end:-1:1,:);
imshow(s);
a = i(200:300,200:400);
imshow(a);
t = i(1:2:end, 1:2:end);
imshow(t);
i(200:300, 200:400) = 0;
imshow(i);
31
>>
>>
>>
>>
>>
>>
>>
>>
>>
>>
>>
>>
i = imread('sowrds0040.bmp');
i = rgb2gray(double(i)/255);
imshow(i)
s = i(end:-1:1,:);
imshow(s);
a = i(200:300,200:400);
imshow(a);
t = i(1:2:end, 1:2:end);
imshow(t);
i(200:300, 200:400) = 0;
imshow(i);
imshow(i/2);
32
>>
>>
>>
>>
>>
>>
>>
>>
>>
>>
>>
>>
i = imread('sowrds0040.bmp');
i = rgb2gray(double(i)/255);
imshow(i)
s = i(end:-1:1,:);
imshow(s);
a = i(200:300,200:400);
imshow(a);
t = i(1:2:end, 1:2:end);
imshow(t);
i(200:300, 200:400) = 0;
imshow(i);
imshow(i/2);
33
>>
>>
>>
>>
>>
>>
>>
>>
>>
>>
>>
>>
>>
i = imread('sowrds0040.bmp');
i = rgb2gray(double(i)/255);
imshow(i)
s = i(end:-1:1,:);
imshow(s);
a = i(200:300,200:400);
imshow(a);
t = i(1:2:end, 1:2:end);
imshow(t);
i(200:300, 200:400) = 0;
imshow(i);
imshow(i/2);
imshow((i>0.8).*i);
34
>>
>>
>>
>>
>>
>>
>>
>>
>>
>>
>>
>>
>>
i = imread('sowrds0040.bmp');
i = rgb2gray(double(i)/255);
imshow(i)
s = i(end:-1:1,:);
imshow(s);
a = i(200:300,200:400);
imshow(a);
t = i(1:2:end, 1:2:end);
imshow(t);
i(200:300, 200:400) = 0;
imshow(i);
imshow(i/2);
imshow((i>0.8).*i);
35
M-Files can be one of two:
 Scripts – A series of commands that are performed on the
global scope.
No input and output variables.

Functions – A set of commands performed over a given input,
with a required output.
Functions have a scope of their own.
(accessing the global scope can be done by defining the
variables to be ‘globals’).
In both types – the m-file must be in the current directory, or in
a previously added path (added with the function addpath)
36


Components of m files:
Function definition line
function [out1 out2] = name(in1, in2, in3)
H1 line - a single comment line that follows the function
definition line.
% SQUARESUM compute the sum of the square of the matrix elements
This line appears when user writes
>> help function_name
>> lookfor keyword - display all functions where the keyword
appeared in H1 line
37

Components of m files (cont.):

Help Text - text block following the H1 line without any blank
line in between the two

Function body – the Matlab code

Comments – lines starting with %
Note: – add short and clear comments to your code!
38








if, else, elseif, end
switch, case, otherwise, end
return
try...catch…end
for i=start:increment:end, end
while, end
break (used with for or while)
continue (used with for or while)
Try not to use
39

1D indexing
Convert for / while loops to equivalent vector or matrix
operations
>> for x = 1:k
ff(x) = 5*sin((x-1)/(2*pi));
end
>> x = 0:k-1
>> ff = 5*sin(x/(2*pi));
40

2D indexing
meshgrid – convert rows vectors to arrays C and R
that can be used for evaluating function with two
variables
>> for r = 1:10
>>
for c = 1:10
>>
b(r,c) = r.^2+ c.^2
>> end
>> end
>> [C, R] = meshgrid(1:c, 1:r)
>> h = R.^2 + C.^2;
Vectorzing code accelerates the computation significantly
For Example: using meshgrid runs on the order of
30 times faster the same computation based on loops
on Image of 512x512 pixels
41

Simple way to improve code execution is to
pre-allocate the size of the arrays in the
program.
>> f = zeros(1024);
42

Cell array is multidimensional array whose
elements are copies of other arrays
>> c = {‘gauss’,[1 0;0 1], 3}
>> c{1}
ans =
gauss

Structures are similar to cell arrays (allow grouping
of a collection of dissimilar data) but they
addressed by fields rather than by numbers
>> params.nimgs = 100;
>> params.jump = 2;
>> params.baseStr = ‘testImg’
43


Matlab arguments are always passed by value
Checking whether an argument exist
>> exist(a,’var’)

Checking number of arguments to the
functions
>> nargin, nargout, nargchk

Getting variable number of arguments
>>varargin, varargout
44
>> guide (Graphic User Interface Development
Environment)
Start the GUI Layout Editor. Guide create
◦ fig file: complete description of the gui elements
and their arrangements
◦ gui m-file: the code that controls the gui
operations, initializations functions, callback
functions
45
Any Questions?