tutorial: spatial kernel filtering with `imfilter` and convolution theorem

[johnnychen94]

Now that we have several ways to apply a spatial space kernel to an image:

• imfilter(img, kern)
• convolution theorem ifft(fft(img) .* fft(kern))

For the convolution theorem, we have utils like fftshift/ifftshift. We also have freqkern and spacekern functions.

Although these are "obvious" for people who already know them and use them in daily life, we need one or two demos to explain them.