Image sharpener
Choose a picture and adjust the amount, radius and threshold of an unsharp mask. The preview shows the middle of the picture at 100 % so you judge real pixels, and you can switch back to the original to compare. The processing is done in your browser.
Runs in your browser: nothing you enter is sent anywhere.
How to use it
- Choose a picture.
- Set Amount (100 % is a good start), Radius (about 1 px for most photos) and Threshold (0 to 5 for a clean picture).
- Tick Show the original to compare, then press Download PNG.
How it works
The red, green and blue channels are blurred with a Gaussian kernel whose width is the radius. For each channel the result is the original plus amount times the difference from the blurred value, limited to 0 to 255, and only where that difference is at least the threshold. The transparency channel is not changed.
Worked example
A pixel with value 120 beside a blurred value of 110 differs by 10. At an amount of 100 % it becomes 130. At a threshold of 15 it stays at 120 because the difference is below the threshold.
Tips
- Sharpen as the last step, after resizing, because resizing changes the pixel scale.
- A small radius suits detailed pictures; a larger radius adds local contrast to soft ones.
- Sharpening cannot restore detail that was never captured.
Frequently asked questions
What is an unsharp mask?
It blurs a copy of the picture, subtracts it from the original to get the fine detail, and adds that detail back multiplied by the amount. Edges get a little lighter on the light side and darker on the dark side, which the eye reads as sharper.
What do amount, radius and threshold do?
Amount is how strongly the detail is added back. Radius is the blur size in pixels and sets how wide the edge effect is. Threshold skips differences smaller than the given level, so flat areas and noise are not boosted.
Why do I see halos or noise?
Too high an amount or radius draws bright and dark rims along edges, and a threshold of 0 sharpens noise and grain too. Lower the amount or raise the threshold a few levels.
Can I undo sharpening later?
No. Sharpening cannot be reversed in the saved file, so keep the original. The result is saved as PNG, which is lossless.