apply filter to a *circular* image

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Michael
Michael 2012-5-23
hello
I'm trying to filter a circular image, the circle fits inside a square matrix, so for a 100x100 matrix the radius is 50 and there are some "blank" areas in the corners. When I try to filter this normally, the edges of my circular image become distorted because they attain a contribution from pixels outside the circumference.
I've had a look at "roifilt2" and the wording of the function implies it still uses values from pixels outside the circle to calculate filtered values of pixels inside the circle (the only different being it does not update the pixels outside the region of interest). So I'm not convinced this works either.
Does anyone have any ideas? My next step is to see if I can manually edit the filter2, but this will probably take a long time. Thanks for any help
Mike

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Image Analyst
Image Analyst 2012-5-23
I think you're going to have to handle those boundary effects manually. Sliding window kinds of filters have the same shape as they slide along, no matter what's underneath them. If you need to modify your sliding kernel, depending on what (x,y) coordinate it's over then you'll have to put in special code for that. You might use nlfilter for that.
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Michael
Michael 2012-5-31
Thanks. It appears nlfilter only takes as an argument the value of the image elements, not the pixel indices themselves. Perhaps I can set all the "non-circle" elements to -1 and write a function which ignores contributions with that exact value. Thanks for the help again!
Michael
Michael 2012-5-31
hmm, this is extremely slow.

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