How to cite
"MATLAB and Octave Functions for Computer Vision and Image Processing"

There do not seem to be any really strong conventions for citing URLs so the following is just a suggestion

Peter Kovesi.   MATLAB and Octave Functions for Computer Vision and Image Processing.
Available from:
<http://www.peterkovesi.com/matlabfns/>.

BibTeX entry:

@misc {KovesiMATLABCode,
author = "P. D. Kovesi",
title = "{MATLAB} and {Octave} Functions for Computer Vision and Image Processing",
note = "Available from: $<$http://www.peterkovesi.com/matlabfns/$>$",
}

Defining a date for a URL can be a bit problematic. The site was first created in 2000, but it contains code going back to 1996. Updates and modifications occur every month or so. The MLA Web citation style suggests that you include the date of electronic publication, of the latest update, or of posting, and the date when the researcher accessed the source. I guess you could say that the publication date for the site is the year 2000.