OptionalamountStrength, 0-5: how much of the difference from the blurred image is added. Default 0.5.
OptionalradiusRadius of the Gaussian blur (its standard deviation) in pixels, 0.1-50.
Default 1. Each pixel looks about ceil(3 * radius) pixels away, so time
grows with the radius.
OptionalthresholdDifferences smaller than this (sRGB-encoded, 0-1; multiply by 255 for levels) are left alone, which keeps smooth areas and noise from being sharpened. Default 0.
Sharpening (unsharp mask) parameters. Edges are found on the luminance of sRGB-encoded values and the same change is added to R, G and B, so colors do not fringe; monochrome images are sharpened on their one channel.