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Aqua Costa

Kernel size vs number of taps

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Can someone explain the difference between kernel size and number of taps? In papers related to blurring and CG in general the two terms are sometimes mixed, and I would like to know when should I use one or another.

Thanks.

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Kernel size is how big of an area the filter covers in image space, number of taps is how many actual samples you take. I don't know if they're 'mixed' per se, but usually they're somewhat intertwined-- if you're examining a large portion of an image, it's very likely you'll want to take many actual samples in order to get an detailed/accurate view of the contents of said image area.

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Traditionally, filters are described as a convolution of the image with a matrix containing weights for each sample. So these would be an example of a box blur and Gaussian blur, respectively:

conv-simple-blur.jpg
conv-gaussian-blur.jpg

With this kind of notation the number of "taps" or samples is implicit, you take as many samples as their are entries in the matrix which means the number of taps and number of filters are the same.

Of course in realtime graphics we don't always filter with square kernels like this, or we might use bilinear filtering to approximate the result of using more taps. In these cases the size of kernel in terms of distance from the center pixel might be larger relative to the number of taps. Or you might use a seperable filter, which also results in using less taps than the filter width squared.

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