Image processing using Matlab

Author : Pooya Sagharchi Ha

Agenda

• Introduction to Matlab

• Basics & Examples

• Image processing with Matlab

• Basics & Examples

What is Matlab?• Matlab stand for Matrix Laboratory

• Matlab is high-level language

• perform computationally intensive tasks faster than with traditional programming languages such as c++

• The help in Matlab is very good, use it!

• Everything is treated as a matrix

The Matlab Environment• Matlab window components:

• Workspace

• Displays all the defined variables

• Command window

• To execute commands in the Matlab environment

• File editor window

• Define your functions

Matlab Help

• Matlab help is an extremely powerful assistance to learning Matlab

• Help not only contains the theoretical background, but also shows demos for implementation

• Matlab help can be opened by using the HELP pull-down menu

Basics syntax

• Comment : ctrl + / OR %

• Uncomment : ctrl + t

• Semicolon : suppress printing

• Arithmetic operators:

+ addition

- subtraction

* multiplication

^ power

' transpose

\ left division

/ right division

• Variable :

>> 2 * 25 + 6 * 22 + 100 / 2

ans =

232

>> a = 25; b = 22;

>> d = a * b

ans =

550

sqrt

returns the square root of

each element of the array X

abs absolute value for real numbers

cos cosine function

sin sin function

exp exponential function

• Who, Whos : current variables in the workspace

• Save : save workspace variables to *.mat file

• Load : load variables from *.mat file

• Clear : clear workspace variables

• Clc : clear command window

• Close : closes the current figure window

Vectors and Matrices

• How to build a matrix?

>> a = [1 5 8]

a =

1 5 8

>> a = [ 1 2 3]'

a =

1 2 3

>> a = [ 0:2:4 ; 8:1:10; 35:5:45]

a =

0 2 4

8 9 10

35 40 45

>> a = [1:3; 4:6; 7:9]

a =

1 2 3

4 5 6

7 8 9

• A particular element of a matrix can be assigned:

>> a(3,2)

ans = 40

• Place the number 5 in the first row, second column:

• >> a(3,2) = 29

• Operations ad functions that were defined scalars in the previous section can also be used on vectors and matrices, For example.

>> a = [ 1 2 3];

>> b = [ 4 5 6];

>> c = a + b

c =

5 7 9

• Special matrices:

• zeros(n,m) : n*m matrix of zeros

• ones(n,m) : n*m matrix of ones

• eye(n) : n*n identity matrix

• rand(n) : n*n random matrix

Flow Control

• Matlab has five flow control constructs:

1. if statement

2. for loop

3. while loop

4. break statement

if• If statement condition

• The general form of the IF statement is

IF expression

statements

ELSEIF expression

statements

ELSE

statements

END

• Example:

s = input(‘Please enter a scalar value =‘);

if s > 1

error(‘Error!’);

else

disp(‘ok’);

end

for• FOR repeats statements a specific number of

times

• The general form of a FOR statement is:

FOR variable = expression

statements

END

for i=1:10

x(i) = sin(i * pi / 10);

end;

for i=[ 1, 2, 3, 7]

x(i) = i + 1;

end;

while• WHILE repeats statements an indefinite number

of times

• The general form of a WHILE statement is:

• WHILE expression

• statements

• END

t = 1;

while t ~= -1

t = input(' enter ..... ');

end

Scripts and Functions

• There are two kinds of M-files:

• scripts, which do not accept input arguments or return output arguments. They operate on data in the workspace.

• Functions, which can accept input arguments and return output arguments. Internal variables are local to the function.

functionfunction [ output_args ] = functionName( input_args )

%functionName Summary of this function goes here

% Detailed explanation goes here

end

function Z=fitness(x)

Z=sum(x.^2);

end

Visualization and Graphics

• Plot ( x, y) : plot 1D function

• figure : open a new figure

• hold on, hold off : refreshing

• title(‘figure title) : add title to figure

Image Processing

Matlab and images

• An image in Matlab is treated as a matrix

• Every pixel is a matrix element

Image import and Export• Read and write images in Matlab:

image = imread(‘image fileName.format');

figure;

imshow(image);

imwrite(image, ‘new image filename.format');

Images and Matricesrow = 256;

col = 256;

img = zeros(row, col);

img(100:105, :) = 0.5;

img(:, 100:105) = 1;

figure;

imshow(img);

Column 1 to 256

Row 1 to 256

o

[0, 0]

o

[256, 256]

Histograms

• Frequency of the intensity value of the image

• Quantize frequency into intervals

• Probability density function of image intensities

img = imread(‘image Filename.image format’);

imshow(img);

figure;

imhist(rgb2gray(img));

Image Filtering

• Let’s replace each pixel with a weighted average of its neighborhood.

• The widths are called the filter kernel

• What are the weighs for the average of a 3*3 neighborhood?

111

111

111

“box filter”

Image Filtering

I = imread(‘filename.format file’);

h = fspecial(‘unsharp’);

I2 = imfilter(I,h);

imshow(I2);

Liner filter

000

010

000

Filtered (no change)Original

Liner filter

Shifted left by 1 pixel

Original

000

100

000

Liner filter

Bluer( with box filter )Original

111

111

111