How the detector decides
The hard part of removing dust is not removing it. It is deciding what is dust. Every plausible test for “this pixel is damage” also fires on something legitimate: a test for thin bright lines catches telephone wires, a test for local outliers catches film grain and catchlights, a test for hard edges catches the whole photograph. Running one test well is not the answer. Running two that fail on different things and only believing them when they agree is.
| Pixel | Narrower than the element? | Far from its local median? | Marked |
|---|---|---|---|
| A dust speck | Yes | Yes | Yes |
| The edge of a roof | Yes, at the edge itself | No — the median follows the edge | No |
| Film grain | Often not | Yes | No |
| A hairline scratch | Yes | Yes | Yes |
Sensitivity is a threshold, not a re-reading
The expensive part — the two tests — runs once when the scan opens and produces a score from nought to two hundred and fifty-five for every pixel. The slider picks the line through those scores. That is why the overlay redraws while you drag it rather than a second after you let go, and it is why the useful range of the control is not linear: the interesting territory on a real scan is a narrow band, and the slider is shaped to spend most of its travel inside it.
Two tidying passes you do not control
Isolated single pixels are dropped before the mask is drawn. At a useful sensitivity a scan produces thousands of them, they are grain rather than damage, and filling them softens the photograph for nothing. What survives is then grown by one pixel in each direction, because the dark shadow beside a scratch is part of the scratch, and a fill that stops at the bright core leaves a visible outline of where the scratch was.
Editing the mask is the work
The brush is not a fallback for a detector that failed. It is the point of the design. No threshold on any measurement can tell a scratch across a face from an eyelash, because at the level of pixels they are the same event, and the difference is entirely a matter of what the photograph is of. So the tool marks what it can defend and then hands you the two brushes.
Marking and unmarking are stored apart from the detected mask, which is what lets you change sensitivity after you have brushed without losing the brushwork. Unmarking is applied last and always wins. Both survive into the save, resampled from the preview grid to the full-resolution one by nearest neighbour — brushwork is a decision rather than a measurement, and half a decision is not a thing.
- Scratches, tears and creases are three different problemsOne taxonomy, three fills, one of them built — and what a model does instead.
- Restore an old photographOpens on fade correction, for a print that has gone pale and warm.
- What restoration cannot bring backSilvering, mould, missing faces, and the honest end of the list.
