Spatial Filtering - Demystified (well, slightly...)

[RobA]

Well.....

The FFT stuff in gimp seems a bit different. Principally, when you run an FFT on an image, it uses 50% grey as the 0 point.

For example, your image:


has a spectrum of this:


Whereas the result of a FFT in gimp is this:


The same manipulation can be done, but to zero out a portion of the specrum, instead of black, I paint with 50% grey. (Actually, I create a layer that is 50% grey and use a layer masks). For the provided image, I did the same thing as you, decomposing the image to RGB, then masking each depending on the "hotspots", replacing those areas with 50% grey in each channel. Here is the result:


This seems a pain in the butt (all that manual painting) so I tried another experiment... After decomposing, I ran a sobel edge detection on each channel, applied a 10px gaussian blur, thresholded it, followed by a 5px gaussian blur, then normalized each channel. Blending down each of these masks and recompositing gave me:


I think it is a pretty impressive results for something that I could automate...(hmmm)

Also, for reference, here is the FFT the procedure resulted in:


I have been able to create gimp equivalents for the first 12 of the filters you show, but have not worked out the last three... how do you prevent image blowout?

-Rob A>