In general, radius should be roughly output dpi / 150. So
for display purposes a radius of 0.5 is suggested.
- amount: (float) Amount of the difference between original and the
blur image that gets added back to the original. Can be
thought of as the "strength" of the effect. Too high may
cause blocking of shadows and highlights. Given a decimal
value indicating percentage, e.g. 1.2 = 120%
- threshold: (float) Determines how large the brightness delta between
adjacent pixels needs to be to sharpen the edge. Larger
values == less sharpening. Useful for preventing noisy
images from being oversharpened.
- channel: (integer) Which channel to apply the sharpening to.