Octave

MATLAB is great, I love it, but it is expensive. GNU Octave is a very viable alternative. Octave runs under Linux, Windows and Mac OS X.

In recent times the Octave project has become increasingly active and a number of toolboxes, including an image processing toolbox, are being built up. I have been impressed to find that many of my functions run under Octave with little or no modification. Most of the changes needed are due to the fact that plotting and visualization of data under Octave are not as well developed as they are under MATLAB. As far as image processing is concerned the major difficulty is the inability to overlay plots on an image. Functions with no plotting, or simple plotting commands, will often run with no change.

For computer vision work you want version 2.1.71 or greater. I suggest you check the following links

The Octave home page
GNU Octave Repository
The Octave Script Archive
HPC Mac OS X A great page of Mac OS X numerical software. Here you will find an easily installed Octave binary for the Mac. This is what I use.
Fink is another potential source of Octave for the Mac. However, when I last looked (July 2005) the fink binary was not of a sufficiently recent version to have the image processing toolbox.

Please do not mail me about any Octave installation problems, I will not know how to help you. You should refer to the appropriate FAQ pages.

Notes on code modifications to provide Octave compatibility

In some cases I have had to create slightly different execution paths for Octave and MATLAB. To detect whether the code is running under Octave I am using the following crude test.

  v = version; Octave = v(1)<'5';  % Crude Octave test

The value Octave will be 1 if the first character of the version string is less than '5', and 0 otherwise. This assumes that it will be a while before Octave gets to version 5 and it assumes most MATLAB users have version 5 or greater.

While MATLAB allows one to pass string arguments to functions without enclosing quotes and brackets, for example

  warning off
  axis equal

this is not (always) allowed under Octave. However, by writing code in the proper functional form such as

  warning('off');
  axis('equal')

compatibility under both MATLAB and Octave can be maintained.

Some additional functions to allow execution under Octave

Here are some functions I have had to write to allow execution of a number of my functions under Octave. These are mostly rather crude work arounds which will hopefully get better in time, or not be needed with future versions of Octave

(Place these in a directory that is not in your MATLAB path...)

fspecial.m replacement for MATLAB fspecial.
line.m Minimal implementation of MATLAB's LINE function using PLOT. This is needed for a number of functions.
show.m Minimal implementation of SHOW so that code that uses my MATLAB function SHOW will at least still execute under Octave.
set.m Placeholder function that does nothing except allow code written for MATLAB handle-graphics to at least execute under Octave.
get.m Placeholder
truesize.m Placeholder
shading.m Placeholder
pixval.m Placeholder
box.m Placeholder
rotate3d.m Placeholder

A few gotchas to look out for

imwrite under Octave has its arguments reversed relative to MATLAB

 > imwrite(filename, image)  % Octave
 > imwrite(image, filename)  % MATLAB

Some image processing toolbox functions test the input image against the function isgray which returns true if all values lie in the range [0 1]. If you get an error message saying your gray scale image is not a gray scale image you probably need to rescale its values from [0 255] to [0 1].
I have found on odd occasions (that I have difficulty replicating) that the SVD in 'economy' mode can sometimes be too economical and I lose a dimension in the decomposed matrices.
The use of 'private' subdirectories does not seem to be supported. You can, of course, explicitly add the private subdirectory to your LOADPATH assuming name clashes are not a problem.
Subfunctions within a file do not seem to over-ride public functions of the same name. Note this is also a problem in MATLAB when the public function happens to be a built-in function.