Unsharp mask params in popular softare

There is correspondence between Unsharp Mask parameters names and scales in widely used graphics software.

Parameter	Adobe Photoshop	ImageMagick	GIMP
Radius	1x	1x	2.5x
Amount	100x	1x	1x
Threshold	1x	1/255x	1x

These numbers are theoretical, in fact for example Photoshop and ImageMagick don't give pixel-wise same results because each program uses own approxmations that has own errors.

Note that in ImageMagick radius is called sigma, and the value called radius in ImageMagick actually is not the Blur radius, but the radius of a convolution matrix (and without special neccesarity one could feel free to set it equal to 0 which means auto-selection of these value).

In Pica and GIMP, USM is applied to the relative brightness to minimize color shift. In Photoshop and ImageMagick, USM is applied to all channels independently.