LPF and HPF Confusion - Rectangular and Circular Cut-Offs Interpreting some homework, I'm a little confused on what it's asking for. I'm asked to filter in the frequency domain (easy enough, just a fft2 and ifft2 when I'm done) a 256x256 8bit grayscale tiff with the following filters:


"Rectangular low pass filter using cutoff frequency of
(uc=N/4,vc=M/4) where N and M are the row and column sizes of the
image." 
"Circular low pass filter using cutoff frequency of Circular
filter the cutoff frequency is given by Rc=N/4= sqrt[(uc)^2+(vc)^2]
where N and M are the row and column sizes of the image" 
"Rectangular high pass filter that has the same cutoff as part a." 
"Circular high pass filter that has the same cutoff as part b."


Now, I understand some of this well enough, but the "Rectangular/Circular" throw me.  We didn't really cover anything like that.  What we did cover were the ideal low/high pass filters and the butterworth low/high pass filters.  Is this perhaps a synonym for the two, the ideal with its immediate cutoff and the buttworth with its tapering? My book does not mention anything in this regard (though perhaps I have not found it...the book is verbose and I am an impatient student at times), only that the general form it gives for the Ideal filters is a circular cutoff, which is what prompts my second supposition: that the circular is the method the book uses, and the rectangluar means to filter anything inside a submatrix one quarter the size of my image, centered, in the frequency domain.

Or are we talking two different kinds of ideal LPFs and HPFs, 

Yes, this is homework for a course, but I'm not looking for easy answers.  I'm looking for understanding.
You're probably best off asking your professor for clarification. As you reasoned, it sounds like you're being asked to multiply the image in the frequency domain by either a rectangular or circular mask that eliminates all frequencies outside the mask. This is not typically how you would apply a filter, however, as "ideal" frequency-domain filter masks have unsatisfactory time-domain properties (e.g. a truly ideal rectangular filter has an infinitely long impulse response).

As you noted, filters with a tapered response (of which the Butterworth filter is an example) are more often used. However, the problem specification doesn't seem to contain the information that you would need to design such a filter, such as the type of filter and its order.