Unsharp Mask
A sharpening technique that subtracts a blurred copy from the original image to enhance edge contrast and perceived sharpness.
Unsharp Mask (USM) sharpens an image by subtracting a Gaussian-blurred copy from the original to isolate edge detail, then adding that detail back with amplification. The counterintuitive name derives from the use of an "unsharp" (blurred) intermediate, though the net effect is sharpening.
Three parameters control the result: Amount (strength of enhancement, typically 50-200%), Radius (blur extent, usually 0.5-3.0 pixels), and Threshold (minimum contrast difference to sharpen, 0-10 levels to avoid amplifying noise). Excessive radius produces visible halos along edges.
USM is standard in RAW development workflows and post-scan processing. For web images, applying mild USM before compression compensates for detail loss introduced by lossy encoding, maintaining visual quality at smaller file sizes.