UNSHARP MASK

Increases the apparent contrast along edges in the image.  For visualization
purposes only.  Should be done last (even after level balance.)

Sigma:
Affects the size of the edges to be enhanced or how wide the edge rims
become, so a smaller radius enhances smaller-scale detail. Higher Radius 
values can cause halos at the edges, a detectable faint light rim around 
objects. Fine detail needs a smaller Radius. Radius and Amount interact; 
reducing one allows more of the other.

Threshold:
Controls the minimum brightness change that will be sharpened or how far 
apart adjacent tonal values have to be before the filter does anything. This
lack of action is important to prevent smooth areas from becoming speckled. 
The threshold setting can be used to sharpen more-pronounced edges, while 
leaving subtler edges untouched. Low values should sharpen more because 
fewer areas are excluded. Higher threshold values exclude areas of lower 
contrast.

Amount:
Should be a value between 0.1 and 0.9.  Controls the magnitude of each 
overshoot (how much darker and how much lighter the edge borders become). 
This can also be thought of as how much contrast is added at the edges. It 
does not affect the width of the edge rims.
