unsharp parameters: GIMP -> IM

[GreenKoopa]

I was testing for an approximate parameter conversion, not the method, and this thread died for two weeks so I didn't keep my results. The graph of the GIMP unsharp was not smooth. Being 8-bit didn't help, but it seemed worse than that. Large values for amount in GIMP yielded poor results.

[joew]

http://redskiesatnight.com/2005/04/06/s ... ge-magick/ suggests to use sigma=sqrt(radius) when radisu is bigger than 1.

[snibgo]

That article, which is about 6 years old, has very dodgy advice. See the comments.

As a general rule it's best to choose the sigma, and set radius=0 so IM will choose the least radius that doesn't clip the function. If you must choose a radius, perhaps for speed reasons, then radius = 3 * sigma is usually good.