Kotti - where, in your code, are you trying to call this function from?
This is not a built-in MATLAB function in the base version or its toolboxes, but seems to be part of VLFeat, a computer-vision library that contains this function to do a Scale-Invariant Feature Transform.
Have you downloaded this software? If so, you will have to add the path to the folder for this file to your MATLAB search path. Once you have done that, try running the code again.
Best Answer