About this instrument
This page does one thing to a photograph: it changes how definite the edges look. Not the exposure, not the colour, not the size, not the blemishes. One dimension of picture quality, with every number that governs it shown rather than hidden behind an automatic setting.
Why the scope is this narrow
Sharpening is the operation that is most often done badly, and it is done badly for a specific reason: it is the operation whose result cannot be judged in the view most people look at it in. A tool that puts a sharpening slider beside twenty other sliders, on a canvas fitted to the screen, has arranged for its users to fail. Everything about this page — the four steps, the compulsory 1:1 view, the ring that appears around the worst edge — exists to make the judgement possible rather than to make the operation fast.
The second reason is the unblur question. A large number of people arrive at any sharpening tool wanting something the physics does not always allow, and the ordinary commercial response is a slider marked unblur. This site answers it on its unblur page instead: the blur is measured and classified first, and the page says which kind it found. A frame smeared by even, straight movement has had its information transformed, and a transform can be partly inverted; a frame that was badly out of focus has had its information averaged away, and nothing brings that back. The page says so rather than pretending otherwise.
How it is built
The whole engine is TypeScript running in the browser tab. Separable box cascades stand in for the Gaussians, so a radius-80 pass costs no more than a radius-8 one; a guided filter supplies the base layer for local contrast; a hand-written radix-2 transform carries the deconvolution and the motion estimator. The arithmetic runs in a worker so a slider stays a slider, and the inspection patch is cut at full resolution with an apron of real picture around it so that what is inspected and what is exported are the same computation on the same pixels. The specialised routes — a lens-blur solver, a document mode for pages and signatures, descreening for printed photographs, JPEG-aware and transparency-aware sharpening, a noise station — are passes inside that same function, not separate engines, and each is checked tile against frame on the engine report.
What is not here, on purpose
There is no upscaling, no face-aware retouching, no named looks, and no global tonal correction — even though clarity plainly wants a shadow lift beside it. Where a page proposes a starting point, it is derived from measurements of your own photo and shown as numbers you can change; nothing is applied that you cannot see and undo. Each of the missing operations is a different problem with its own failure modes, and putting them on one page would mean doing all of them at the level of an afterthought.
Who publishes it
SharpenPicture is an independent site, written and maintained under its own name. There is no account, no sign-in, nothing to buy; advertising is served by Google AdSense, which is a consequence of the architecture rather than a promotion: there is no server behind this site to hold an account on, and every picture is processed in your own browser tab.
Corrections
The print sharpening figures are conventional starting points from press practice rather than measurements from a calibrated machine, and the motion estimator is asserted against synthetic plates with a known kernel rather than against a body of real photographs. If you have evidence that a number here is wrong, it is worth more than an opinion about whether the page feels right, and it will be acted on.