Unsharp Mask Radius
Determines how many pixels should be included to enhance the contrast when using the unsharp mask parameter.
Valid values are positive numbers, and the default is
2.5. This parameter will have no effect on the image if
usm is not set.
For images with general noise, we suggest using the
sharp parameter instead. Unsharp mask and radius are better for thumbnails and fine-tuned sharpening.