Sharpen Image

Photos straight from a phone, scans and resized images often look a little soft. Sharpening raises the contrast right at edges, so outlines, eyelashes, text and textures look crisper. This tool uses the unsharp mask that photo editors use, with the same three controls, plus a quick 3 × 3 sharpen filter. It works on brightness only by default, which avoids coloured halos, and runs in your browser.

How to use this tool

1. Choose one or more images.
2. Keep "Unsharp mask" and start with amount 80%, radius 1.5 px and threshold 2.
3. Raise the amount for more bite; use a smaller radius for fine detail and a larger one for punchier local contrast.
4. Download the result, saved in the original format by default.

Formula

sharpened = original + amount × (original − blur(original, radius)), applied only where |original − blur| ≥ threshold

The blur is a Gaussian with the radius as its standard deviation in pixels: computed exactly for radii under 2 px and approximated by three box blurs above that. With "brightness only" the difference is computed on the luma Y = 0.299R + 0.587G + 0.114B and added equally to R, G and B, so colours do not shift. The simple filter adds amount × (4 × centre − the 4 neighbours), which at 100% is the classic [0 −1 0; −1 5 −1; 0 −1 0] kernel.

Example

At an edge pixel of brightness 120 whose blurred surroundings average 100, the difference is 20. At 80% the pixel becomes 120 + 0.8 × 20 = 136; the dark side of the same edge, say 90 against a blurred 100, becomes 90 − 8 = 82, so the edge contrast grows from 30 to 54 levels. In a smooth sky with a difference of 1 and threshold 2, nothing changes.