Cis601 02 Matlab Intro

8/4/2019 Cis601 02 Matlab Intro

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MATLAB


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MATLAB

This introduction will give

Some basic ideas

Main advantages and

drawbacks compared toother languages


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MATLAB

What Is MATLAB?

MATLAB (MATrix LABoratory) high-performance language for technical computing computation, visualization, and programming in an easy-to-

use environment

Typical uses include:

Math and computation Algorithm development

Modelling, simulation, and prototyping Data analysis, exploration, and visualization Scientific and engineering graphics Application development, including Graphical User Interface

building


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MATLAB

Matlab is an INTERPRETED languagewith all the advantages and drawbacks !

there is a Matlab Compiler, but its not

worth too much in terms of speed

Matlab takes care of the usual programmingoverhead, e.g. memory allocation, datatypeconversion, parameters by value/reference with all the advantages and drawbacks !

Matlab is NOT object oriented, although

there are some attempts to extend it


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MATLAB

Matlab is able to cooperate with otherlanguages, e.g.

C, C++

JAVA Fortran


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Why MATLAB

A good choice for vision program development

because:

Easy to do very rapid prototyping

Easy to learn, well documented A good library of image processing functions Excellent display capabilities Widely used for teaching and research in

universities and industry Another language to impress your boss with !


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Why not MATLAB

Has some drawbacks:

Slow for some kinds of processes Not geared to the web Not designed for large-scale system

development


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MATLAB Components

MATLAB consists of: The MATLAB language a high-level matrix/array language with control flow statements, functions,

data structures, input/output, and object-oriented programming features.

The MATLAB working environment the set of tools and facilities that you work with as the MATLAB user or

programmer, including tools for developing, managing, debugging, andprofiling

Handle Graphics the MATLAB graphics system. It includes high-level commands for two-

dimensional and three-dimensional data visualization, image processing,animation, and presentation graphics.

(contd)


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MATLAB Components

The MATLAB function library. a vast collection of computational algorithms ranging from elementary

functions like sum, sine, cosine, and complex arithmetic, to moresophisticated functions like matrix inverse, matrix eigenvalues, Bessel

functions, and fast Fourier transforms as well as special image processingrelated functions

The MATLAB Application Program Interface (API) a library that allows you to write C and Fortran programs that interact with

MATLAB. It include facilities for calling routines from MATLAB (dynamic

linking), calling MATLAB as a computational engine, and for reading andwriting MAT-files.


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MATLAB

Some facts for a first impression

Everything in MATLAB is a matrix !

again: MATLAB is an interpreted language, no

compilation needed (but possible)

again: MATLAB does not need any variabledeclarations, no dimension statements, has no

packaging, no storage allocation, no pointers

Programs can be run step by step, with fullaccess to all variables, functions etc.


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What does Matlab code look like?

A simple example:

a = 1while length(a) < 10a = [0 a] + [a 0]end

Whats the output ?


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A simple example:

a = 1while length(a) < 10a = [0 a] + [a 0]end which prints out Pascals triangle:

1

1 11 2 11 3 3 11 4 6 4 11 5 10 10 5 11 6 15 20 15 6 11 7 21 35 35 21 7 11 8 28 56 70 56 28 8 11 9 36 84 126 126 84 36 9 1

(with a= before each line).


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Another simple example :

t = 0:pi/100:2*pi;y = sin(t);plot(t,y)


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Another simple example:

t = 0:pi/100:2*pi;y = sin(t);plot(t,y)

Remember:

EVERYTHING IN MATLABIS A MATRIX !

creates 1 x 200 Matrix

Argument and result: 1 x 200 Matrix


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Matrices

!


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Matrices

Rows and columns are always numbered starting at 1

Matlab matrices are of various types to hold differentkinds of data (usually floats or integers)

A single number is really a 1 x 1 matrix in Matlab!

Matlab variables are not given a type, and do not needto be declared

Any matrix can be assigned to any variable


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Matrices

Building matrices with [ ]:

A = [2 7 4]

A = [2; 7; 4]

A = [2 7 4; 3 8 9]

B = [ A A ]

2 7 4

2

7

4

2 7 43 8 9

?


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Matrices

Building matrices with [ ]:

A = [2 7 4]

A = [2; 7; 4]

A = [2 7 4; 3 8 9]

B = [ A A ]

2 7 4

2

7

4

2 7 43 8 9

2 7 4

3 8 9

2 7 4

3 8 9


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Matrices


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Matrices

Some operators must be handled with care:

A = [1 2 ; 4 5]

B = A * A prints 9 1224 33

B = A .* A prints 1 4

16 25

Element by element multiplication

S b i


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Submatrices

A matrix can be indexed using another matrix, to

produce a subset of its elements:

a = [100 200 300 400 500 600 700] b = [3 5 6]

c = a(b):

300 500 600

S b t i


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Submatrices

To get a subsection of a matrix, we can produce the

index matrix with the colon operator:

a(2:5)prints

ans = 200 300 400 500

This works in 2-D as well, e.g. c(2:3, 1:2) produces a2 x 2 submatrix.

The rows and columns of the submatrix arerenumbered.

l


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loops

for loops in MATLAB iterate over matrix elements:

b = 0for i = [ 3 9 17]

b = b + i;end

Result: 29

Note:THIS IS NOT THE WAY YOU SHOULD

PROGRAM A SUM, SINCE LOOPS ARE SLOWIN MATLAB !

loops


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loops

for loops in MATLAB iterate over matrix elements:

b = 0for i = [ 3 9 17]

b = b + i;end

Result: 29

The MATLAB way to write that program would have been:b = sum([ 3 9 17]);

Avoid loops if possible !

loops


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% Example for runtime difference of loops and built in functions

a=rand(10000,1000); % Matrix with 10000000 (1e7) elements

tic

theSum=0;

for i=1:size(a,1)

for j=1:size(a,2)

theSum = theSum + a(i,j);

end

end

toc

fprintf('result of loop: %f\n',theSum);

tic

theSum=sum(a(:));

toc

fprintf('result of loop: %f\n',theSum);

-----------------------------------------------

Elapsed time is 0.203000 seconds.

result of loop: 4999093.175950

Elapsed time is 0.078000 seconds.

result of loop: 4999093.175950

loops

Here: Speed up factor 2.6

loops


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loops

The typical for loop looks like:

for i = 1:6

end

Which is the same as:

for i = [1 2 3 4 5 6]

end

loops


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loops

Or a bit more sophisticated:

for i = 1:2:7

end

Which is the same as:

for i = [1 3 5 7]

end

loops


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loops

Once again:

AVOID LOOPS

Avoiding loops


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Avoiding loops

How to avoid loops:

Task:count the elements being greater than

5 in the following matrix M:

2 7 4

3 8 9

2 7 4

3 8 9

Avoiding loops


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Thenon-MATLAB

way:

counter=0

For rows=1:2

For columns=1:6if M(rows,columns) > 5

inc counter

end

end

Avoiding loops

Avoiding loops


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TheMATLAB

way:

T = M > 5;

Count = sum (T(:)) ;

Or shorter:

Count = sum(M(:) > 5);

g p

Avoiding loops


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Another example, showing the mighty

INDEXING possibilities:

Task: eliminate the whitespaces in thefollowing vector M

g p

A B C D A B C D

Avoiding loops


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i = find (M ~= ); % result: i=[1 3 4 6]M = M(i);

Or shorter:

M=M(find(M ~= );

g p

A B C D A B C D

Avoiding loops


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survivingSubSegs = pointDensity >= minDensity;

diffSubSegs = diff([0 survivingSubSegs 0]);

startSeg = find(diffSubSegs == 1);

endSeg = find(diffSubSegs == -1) - 1;

An example how to work without loops,taken from the EMSF algorithm for line detection

Whats the purpose ?

Images


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So why MATLAB and IMAGEPROCESSING ?

Images


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Images can be treated as

matrices !

Images


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Loading an image:

a = imread(picture.jpg);imshow(a);

Images


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Image (=matrix) size:

size(a): 384 512 3

R G B

384

512

Images


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Color image:

3D Matrix of RGB planes

Images


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Show RED plane:

a(:,:,2:3) = 0;imshow(a);

Images


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Show GREEN plane:

a(:,:,[1 3]) = 0;imshow(a);

Images


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Show BLUE plane:

a(:,:,1:2) = 0;imshow(a);

Images


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Advanced: Shuffling columns

rn = rand(1,512);

[rn1,i] = sort(rn);b = a(:,i,:);

imshow(b);

Images


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By the way


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MATLAB can also handle

Movies

3D objects

Conclusion


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MATLAB is a mighty tool tomanipulate matrices

Images can be treated asmatrices

MATLAB is a mighty tool tomanipulate images

In my opinion


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MATLAB should be used to code

software prototypes

Research is mostly aboutprototypes, not runtime-optimizedsoftware

MATLAB should be used inresearch

In my opinion


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MATLAB prototypes must be re-coded (e.g. in C++) if theres need

for speed

Algorithm development time is

drastically shorter in MATLAB

Conclusion


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CONCLUSION:

Use MATLAB

How to go on


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MATLAB is exceptionally welldocumented:

Openinghelp/MATLAB help/Getting started

help/MATLAB help/Using MATLAB

Offers help to the environment,programming concepts, function

libraries etc How to go on


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The help

command shows the first comment-lines of the function given by the m-

file .

Doc does the same, but nicer

(html-style, graphics, examples)

How to go on


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The TAB key automatically extends commands

given. This is sometimes helpful if the functionname should be obvious:

Task: sort the rows of the matrix AIdea: there could be a function named sortXXXXType: sort + TABResult: sort,sortcellchar,sorted,sortiv,sortrows, sortrowsc

How to go on


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Result: sort,sortcellchar,sorted,sortiv,sortrows, sortrowsc

This looks promising !help sortrows will exclude doubt:

SORTROWS Sort rows in ascending order.Y = SORTROWS(X) sorts the rows of the matrix X in ascending

order as a group. X is a 2-D numeric or char matrix. For a charmatrix containing strings (+usage, examples etc.)

How to go on


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Another helpful command is lookfor:

lookfor looks for the string in all

first comments lines (the ones shown by thehelp command) of ALL functions, returning

functions with a hit.

e.g. lookfor sort:

How to go on


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e.g. lookfor sort:

bestof.m: %BESTOF(index,names,M,n,) returns the sorted comparison valuesof polygon(index) + filenamesCPLXPAIR Sort numbers into complex conjugate pairs.ISSORTED True for sorted vector.SORT Sort in ascending order.SORTROWS Sort rows in ascending order.

RTMDLSORTFLDS is a RTW support function.DSORT Sort complex discrete eigenvalues in descending order.ESORT Sort complex continuous eigenvalues in descending order.DSPBLKSORT DSP Blockset sort block helper function.UISORTDATA GUI for sorting matrices by row

XREGLVSORTER Function to sort items in listview

sortiv.m: % function [out,err] = sortiv(in,sortflg,nored,epp)EIGFUN Function to return sorted eigenvalues (used in GOALDEMO).V2SORT Sorts two vectors and then removes missing elements.

How to go on


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Note:a creative search using the right topics with

lookfor can save a lot of time. MATLAB comes

with mostly all basic things you can think of.

Search before you program !

a Google search MATLAB +topic is

sometimed an unbelievably satisfying source oftime-saving program collections.

How to go on


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Now: browse the documentation !

read sample codes !

simply browse the list of functions(doc) Try and type some lines, use the

graphics !

Documents

Cis601 02 Matlab Intro