Rendre une image plus nette

Despite the name, the method sharpens. It makes a blurred copy of the image with a gaussian blur of the chosen radius, subtracts it from the original to isolate the fine detail, and adds that detail back on top: result = original + amount × (original − blurred). Along an edge, the dark side gets a bit darker and the light side a bit lighter, which the eye reads as sharpness. Flat areas such as a clear sky have nothing to subtract and stay as they were. Values are clamped to 0–255, and transparency is left untouched.

Radius decides how wide that extra contrast is. 0.5–1 px suits fine texture and images for screens; 1.5–3 px brings out larger shapes and is a starting point for prints.

Sharpening adds contrast to detail that is already in the picture. It cannot bring back detail the camera never captured. A photo blurred by camera shake has every edge smeared in one direction, and an out-of-focus subject has lost the fine structure altogether; sharpening those mostly makes the blur look harsher. It helps most when the image is nearly right: a slightly soft lens, a scan through glass, a photo shrunk from 4000 to 1200 px.

Push the amount too far and bright halos appear around dark edges, noise turns gritty, and JPEG blocks in a heavily compressed file become visible. If you see halos in the preview, lower Amount first, then Radius.

Scanned documents gain the most: letters get firmer edges and print more cleanly at 100–150% with a 1 px radius. For product photos on a marketplace listing, a modest 40–80% keeps fabric and packaging text crisp without the over-processed look buyers notice. When you resize and sharpen the same picture, resize first and sharpen last, because shrinking an image softens it again.

Sharpened files are usually a little larger, since there is more edge detail to encode: our 224,092-byte test JPG came out at 246,531 bytes with the defaults (60%, 1 px, quality 92), about 10% more.

JPG, PNG, WebP and BMP are accepted, up to 50 MB each. Keep the original format or choose PNG, JPG or WebP; BMP comes out as PNG and JPG/WebP quality starts at 92%. JPEG photos are drawn upright according to their EXIF orientation, and the EXIF block itself, with GPS position and camera details, is not copied to the new file. The photos are never uploaded.