MATLAB-tutorial for Image Processing with Lecture 3.ppt

MATLAB for Image Processing
April 10th
, 2015

Outline
• Introduction to MATLAB
– Basics & Examples
• Image Processing with MATLAB
– Basics & Examples

What is MATLAB?
• MATLAB = Matrix Laboratory
• “MATLAB is a high-level language and
interactive environment that enables you to
perform computationally intensive tasks faster
than with traditional programming languages
such as C, C++ and Fortran.” (
www.mathworks.com)
• MATLAB is an interactive, interpreted language
that is designed for fast numerical matrix
calculations

The MATLAB Environment
• MATLAB window
components:
Workspace
> Displays all the defined
variables
Command Window
> To execute commands
in the MATLAB
environment
Command History
> Displays record of the
commands used
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

MATLAB Help (cont.)
• Any command description
can be found by typing the
command in the search field
• As shown above, the
command to take square
root (sqrt) is searched
• We can also utilize
MATLAB Help from the
command window as shown

More about the Workspace
• who, whos – current variables in the
workspace
• save – save workspace variables to *.mat
file
• load – load variables from *.mat file
• clear – clear workspace variables

Matrices in MATLAB
• Matrix is the main MATLAB data type
• How to build a matrix?
– A=[1 2 3; 4 5 6; 7 8 9];
– Creates matrix A of size 3 x 3
• Special matrices:
– zeros(n,m), ones(n,m), eye(n,m),
rand(), randn()
• Numbers are always double (64 bits)
unless you specify a different data
type

Basic Operations on Matrices
• All operators in MATLAB are defined on
matrices: +, -, *, /, ^, sqrt,
sin, cos, etc.
• Element-wise operators defined with a
preceding dot: .*, ./, .^
• size(A) – size vector
• sum(A) – columns sums vector
• sum(sum(A)) – sum of all the elements

Variable Name in Matlab
• Variable naming rules
- must be unique in the first 63 characters
- must begin with a letter
- may not contain blank spaces or other types of punctuation
- may contain any combination of letters, digits, and underscores
- are case-sensitive
- should not use Matlab keyword
• Pre-defined variable names
• pi

Logical Operators
• ==, <, >, (not equal) ~=, (not) ~
• find(‘condition’) – Returns indexes
of A’s elements that satisfy the condition

Logical Operators (cont.)
• Example:
>>A=[7 3 5; 6 2 1], Idx=find(A<4)
A=
7 3 5
6 2 1
Idx=
3
4
6

Flow Control
• MATLAB has five flow control constructs:
– if statement
– switch statement
– for loop
– while loop
– break statement

if
• IF statement condition
– The general form of the IF statement is
IF expression
statements
ELSEIF expression
statements
ELSE
statements
END
• CODE

switch
• SWITCH – Switch among several cases based
on expression
• The general form of SWITCH statement is:
SWITCH switch_expr
CASE case_expr,
statement, …, statement
CASE {case_expr1, case_expr2, case_expr3, …}
…
OTHERWISE
END

switch (cont.)
• Note:
– Only the statements between the matching
CASE and the next CASE, OTHERWISE, or END
are executed
– Unlike C, the SWITCH statement does not fall
through (so BREAKs are unnecessary)
• CODE

for
• FOR repeats statements a specific number of
times
• The general form of a FOR statement is:
FOR variable=expr
statements
END
• CODE

while
• WHILE repeats statements an indefinite
number of times
• The general form of a WHILE statement is:
WHILE expression
statements
END
• CODE

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

Functions in MATLAB (cont.)
• Example:
– A file called STAT.M:
function [mean, stdev]=stat(x)
%STAT Interesting statistics.
n=length(x);
mean=sum(x)/n;
stdev=sqrt(sum((x-mean).^2)/n);
– Defines a new function called STAT that calculates
the mean and standard deviation of a vector. Function
name and file name should be the SAME!
– CODE

Visualization and Graphics
• plot(x,y),plot(x,sin(x)) – plot 1D function
• figure, figure(k) – open a new figure
• hold on, hold off – refreshing
• axis([xmin xmax ymin ymax]) – change axes
• title(‘figure titile’) – add title to figure
• mesh(x_ax,y_ax,z_mat) – view surface
• contour(z_mat) – view z as topo map
• subplot(3,1,2) – locate several plots in figure

Saving your Work
• save mysession
% creates mysession.mat with all variables
• save mysession a b
% save only variables a and b
• clear all
% clear all variables
• clear a b
% clear variables a and b
• load mysession
% load session

What is the Image Processing Toolbox?
• The Image Processing Toolbox is a collection of
functions that extend the capabilities of the MATLAB’s
numeric computing environment. The toolbox supports a
wide range of image processing operations, including:
– Geometric operations
– Neighborhood and block operations
– Linear filtering and filter design
– Transforms
– Image analysis and enhancement
– Binary image operations
– Region of interest operations
• YOU CANNOT USE THE IMAGE PROCESSING TOOLBOX FOR
HOMEWORK OR FINAL PROJECT

Images in MATLAB
• MATLAB can import/export
several image formats:
– BMP (Microsoft Windows
Bitmap)
– GIF (Graphics Interchange Files)
– HDF (Hierarchical Data Format)
– JPEG (Joint Photographic
Experts Group)
– PCX (Paintbrush)
– PNG (Portable Network
Graphics)
– TIFF (Tagged Image File
Format)
– XWD (X Window Dump)
– raw-data and other types of
image data
• Typically switch images to double
• Data types in MATLAB
– Double (64-bit double-precision
floating point)
– Single (32-bit single-precision
floating point)
– Int32 (32-bit signed integer)
– Uint32 (32-bit unsigned integer)

Images in MATLAB
• Binary images : {0,1}
• Intensity images : [0,1] or uint8, double etc.
• RGB images : m × n × 3
• Multidimensional images: m × n × p (p is the number of layers)

Image Import and Export
• Read and write images in Matlab
img = imread('apple.jpg');
dim = size(img);
figure;
imshow(img);
imwrite(img, 'output.bmp', 'bmp');
• Alternatives to imshow
imagesc(I)
imtool(I)
image(I)

Images and Matrices
Column 1 to 256
Row
1
to
256
o
[0, 0]
o
[256, 256]
How to build a matrix
(or image)?
Intensity Image:
row = 256;
col = 256;
img = zeros(row, col);
img(100:105, :) = 0.5;
img(:, 100:105) = 1;
figure;
imshow(img);

Images and Matrices
Binary Image:
row = 256;
col = 256;
img = rand(row,
col);
img = round(img);
figure;
imshow(img);
size(im)

Image Display
• image - create and display image object
• imagesc - scale and display as image
• imshow - display image

Performance Issues
• The idea: MATLAB is
– very fast on vector and matrix operations
– Correspondingly slow with loops
• Try to avoid loops
• Try to vectorize your code
http://www.mathworks.com/support/tech-notes/
1100/1109.html

THE END
• Thank you 
• Questions?

MATLAB-tutorial for Image Processing with Lecture 3.ppt

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