Sharpen

Sharpen runs an unsharp mask in your browser: it blurs a copy of the image, takes the difference — which is every edge, every eyelash and every speck of grain — and adds a multiple of that difference back, so edges gain contrast. Amount, radius and threshold are all exposed, and the preview runs at one image pixel per screen pixel because sharpening judged on a shrunken preview hides the halos and the detail alike. A soften mode runs the same gaussian blur on its own, with no mask over it.

How it works

  1. Drop your images Drag one or more images in. Each is decoded and the first readable one fills the preview.
  2. Choose sharpen or soften Sharpen amplifies the edge difference, up to 300%. Soften runs the same gaussian and stops there, capped at 100%, where none of the original detail is left.
  3. Set radius and threshold Radius, 0.3 to 8 px, is the width of the blur and so the width of the edge being worked on. Threshold, 0 to 64, leaves pixels closer to the blur than that alone — raise it to keep flat sky, water and skin from having their own noise sharpened.
  4. Pan around the 1:1 preview Drag the preview, or use the arrow keys, to move over the image at full size. What is on screen is what gets written.
  5. Apply and download Pick a format, apply, and take one file or a ZIP of the batch. The export runs at full resolution however small the preview window is.

Questions

Does sharpening bring back detail in a blurry photo?
No. An unsharp mask redistributes contrast around edges that are already recorded in the file; nothing here adds detail the camera did not capture. Past roughly 150% amount the bright and dark fringes along an edge read as halos, and once written those do not come out again.
Why is the preview shown at full size instead of fitted to the window?
A downscaled preview hides fine detail and halos equally, so any judgement made on it is meaningless. The preview therefore shows one image pixel per screen pixel and you drag or arrow-key around the image. The exported file is processed at full resolution regardless of the preview size.
Why does sharpening a JPEG look grainy or blocky?
With the threshold at zero the mask amplifies JPEG compression blocks along with the real detail, because to the filter both are just differences from the blur. A threshold of 3 or 4 leaves them alone; the default is 3.

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