3x3 Average filter in matlab

Question

I\'ve written code to smooth an image using a 3x3 averaging filter, however the output is strange, it is almost all black. Here\'s my code.

function [filtere         


        

Answer 1

Tangentially to the question:

Especially for 5x5 or larger window you can consider averaging first in one direction and then in the other and you save some operations. So, point at 3 would be (P1+P2+P3+P4+P5). Point at 4 would be (P2+P3+P4+P5+P6). Divided by 5 in the end. So, point at 4 could be calculated as P3new + P6 - P2. Etc for point 5 and so on. Repeat the same procedure in other direction. Make sure to divide first, then sum.

I would need to time this, but I believe it could work a bit faster for larger windows. It is sequential per line which might not seem the best, but you have many lines where you can work in parallel, so it shouldn't be a problem.

This first divide, then sum also prevents saturation if you have integers, so you might use the approach even in 3x3 case, as it is less wrong (though slower) to divide twice by 3 than once by 9. But note that you will always underestimate final value with that, so you might as well add a bit of bias (say all values +1 between the steps).