is possible without slowdown.
* This comes in handy, when USM is used to enhance local contrast.
* I do this often, and this was very very slow with gimp's original USM.
*
* I use a sony dsc f717 that is known for "flat" images (low contrast) and also
for high dynamic
* range. Using USM, rather than curves, I can enhance contrast without
loosing shadows or highlights,
* its just contrary: shadows and highlights get more (local) contrast, and no
detail is lost.
* (For real strong fill-flash effect a contrast mask must be used in gimp and
afterwards local contrast
* can be enhanced using USM)
* ....................................................................................................
*
* Amount+ and Amount- are the amount factors, as known from USM,
however, "+" means amount of
* brightening and "-" means amount of darkening
* Threshold is the treshold as known from gimps USM
* ....................................................................................................
*
* Gamma changes the gamma factor for internal processing. High gamma
values can reduce sharpening halos.
* The gamma of the resulting image is not changed.
* (Try gamma of 5.0 and +/- amount upto 5.0. This sounds unreasonable, but
it works very good )
*
* Why?
*
* Most digital cameras have a gamma value from 1.5 to 2.0.
* When the image is in computer memory then the function between
intensity i in nature and intensity
* value m in computer memory is : m = pow(i,1/g). (Where g ranges from 1.5
to 2.0)
* Thats similar to a squareroot-curve.
* When we apply USM to this unmodified image, then we get more bright
halo than dark halo.
