usm Unsharp Mask
Sharpens the image details using an unsharp mask (a blurred, inverted copy of the image).
Valid values are any floating point number. The default value is
0, which leaves the image unchanged. This parameter should be used in conjunction with unsharp mask radius.
For images with general noise, we suggest using the sharp parameter instead. Unsharp mask and radius are better for thumbnails and fine-tuned sharpening.