How does an unsharp mask work?

Question

I\'ve been playing around with image processing lately, and I\'d like to know how the unsharp mask algorithm works. I\'m looking at the source code for Gimp and it\'s implement

Answer 1

The key is the idea of spatial frequency. A Gaussian filter passes only low spatial frequencies, so if you do something like:

2*(original image) - (gaussian filtered image)

Then it's effect in the spacial frequency domain is:

(2 * all frequencies) - (low frequencies) = (2 * high frequencies) + (1 * low frequencies).

So, in effect, an 'unsharp mask', is boosting the high frequency components of the image --- the exact parameters of the gaussian filter size, and the weights when the images are subtracted determine the exact properties of the filter.