Filter node in batch

Hi. Anyone could explain how the filter node works?
I know it basically make operations between each pixel and the surrounding ones, (from 1 to 5 rows) but I would like to know exactly how to set it up to get the expected result.
What the numbers you can set for each pixel of the matrix influence the result? what are the maths behind it etc… I’ve tried to have a look at the presets but it’s still quite obscure…

I have the feeling that even if it’s not GPU optimised, it’s quite under used…

It is called a kernel filter or convolution matrix. It is one of the oldest kind of image processing methods. Actually the wikipedia explains it pretty well.