unsharp

Unsharp masking is a sharpening technique used in photography and image processing to enhance perceived sharpness by increasing local contrast along edges. It works by creating a perceptual mask that highlights fine details and adding a controlled amount of that mask back to the original image.

The method derives from traditional photographic printing. It uses an unsharp (blurred) version of the image as a mask to emphasize edges; when combined with the original, it boosts local contrast around high-frequency features, making edges appear crisper without changing overall brightness.

Process and parameters. In digital workflows, a copy is blurred with a Gaussian or other blur; this blurred image is subtracted from the original to form a mask. The mask is scaled by an amount parameter and added back to the original. Radius determines the extent of blur, thus the size of features affected. A threshold can restrict effect to areas with sufficient contrast to avoid boosting noise.

Implementation and variants. Most image editors call it Unsharp Mask or USM, while some offer High Pass sharpening or Smart Sharpen. Techniques differ in how they compute the mask and how they apply adjustments, but the core principle remains edge enhancement through high-frequency amplification.

Considerations. When overapplied, it can create halos, overshoot, or amplify noise; it is often used selectively on subjects or layers, sometimes with masking to protect smooth areas. It remains a standard tool in both analog and digital workflows.