Matlab Introduction

CSC 262 - Computer Vision - Weinman

Background

This primer will give you a brief introduction to Matlab (http://mathworks.com). This is the software that we will be using to complete our laboratory assignments for this class. While it may seem difficult at first, as the course progresses, you will be satisfied at how quickly it allows you to do many complicated things with relatively little pain. In addition, it is a standard package used widely in research and industry, so becoming familiar with it may be of long term utility.

There are a plethora of tutorials for Matlab on the web, including those written by the company that produces it. Using Google, you are likely to find more than you would need. However, here are a few potentially useful leads:

The Matlab Manuals, Mathworks http://www.mathworks.com/access/helpdesk/help/techdoc/matlab.shtml
MATLAB Primer, Kermit Sigmon http://math.ucsd.edu/~driver/21d-s99/matlab-primer.html
A Practical Introduction to Matlab, Mark Gockenbach http://www.math.mtu.edu/~msgocken/intro/intro.html

Many of these are lengthy with more details than you may need just now. Thus, we will get started with a very brief primer and you'll do a laboratory to reinforce some of the basics.

Getting Started

Change to the directory containing code for our course and run Matlab:

: $ cd ~weinman/courses/CSC262/toolbox
$ matlab &

Calling matlab will automatically run the latest verion of Matlab. If you dislike the GUI you can run the command line mode using:

: $ matlab -nodesktop

The directory you are in when you run Matlab will later be important, because Matlab will automatically loads any .m files (Matlab programs) that are in your current directory. Nevertheless, if you started from the wrong directory, you can either change the directory using the GUI or you can use

: >> cd directory

at the Matlab command prompt. Other Unix-like shell command such as ls and pwd are supported as well.

To quit Matlab, type:

: >> quit

Matlab has two different kinds of programs:

Scripts

Scripts are simply a number of Matlab commands stored in a file with the extension .m.

Functions

Functions are what they are in other programming languages. The important thing about functions is that each function has to be in a separate file with the same name as the function (plus the suffix .m). For example, you would save the function my_test in a file called my_test.m. Function files also have a special syntax, but more about that later.

Getting Help

The Matlab GUI comes with a pretty extensive help system. If you don't like GUIs or you just need to quickly look something up, you can use the following commands:

: >> help function

and for searching

: >> lookfor keyword

Matlab Commands

The name Matlab comes from MATrix LABoratory, and hence it is not surprising that matrix operations are the most important types of computation in Matlab. Actually, Matlab supports arbitrary n-dimensional arrays (called tensors), of which matrices and vectors are just special cases. We will only describe the latter, however. Images are nothing but matrices, so it should be fairly obvious why matrix algebra will be important for us.

Instead of spending hours on reading long tutorials, we have provided some starter tutorial code for you. This well- commented source code should be easy to understand (though you probably won't remember it all). Study it before the lab, where you'll practice and begin to learn (and thereby remember) how to use the Matlab environment.

The most important Matlab commands are described in the script intro.m. You can run it all at once by changing Matlab's work directory and typing intro, but this way you will not be able to see and learn very much. Instead, you should run it piece by piece: If you use the Matlab GUI, you should load the file,

: >> edit intro.m

then bit by bit mark a few lines of code at a time and run the marked code by choosing Evaluate Selection from the context menu (right mouse click). The output of the operations appears in the main window. If you don't like the GUI, you can copy and paste a few lines of code at a time into the command line and after pressing enter the last pasted code block is evaluated.

If you've looked though that file and read the help pages for a few important functions you should know enough about Matlab for our purposes. If you're still curious for more, have a look at the Matlab Primer, which contains more detail and explains additional commands.

We'll reinforce this in class with a some lab exercises.

Acknowledgments

Adapted from "CS143: Introduction to Matlab," by Stefan Roth and Teodor Moldovan.