Moiré Is Not Dirt on the Scan, It Is the Scan
Scan something printed — a magazine page, a photo from a book, a leaflet — and you often get a shimmering grid of colored waves across it that was nowhere on the paper. It is tempting to treat that as dirt and reach for a cleanup tool. It is not dirt. The printed page is itself a grid of tiny dots at a fixed angle, your scanner is a second grid sampling it, and where two regular grids disagree slightly they produce a third pattern that neither of them contains. That pattern was created at the moment of scanning, and nothing downstream can subtract it cleanly.
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What it does
- Descreen pass that softens the interference grid
- Edge contrast restored afterwards
- Mild tone adjustment
- No generative model, identical output every run
How to use Scan Enhancement
- 1
Try to fix it at the scanner first
Turn on the scanner's descreen setting, change the resolution, or rotate the page a few degrees on the glass. Any of those can remove the pattern at capture, which nothing here can match.
- 2
Run the descreen
A small blur softens the dot grid, a sharpening pass restores the edges the blur cost, and a mild contrast adjustment recovers the tone.
- 3
Judge it at 100 percent, not zoomed out
Moiré changes appearance with zoom level, so a pattern that vanishes in a thumbnail may still be there. View at full size before deciding it worked.
How it works
There is no model in this workflow at all. The file goes through three deterministic steps.
A small blur, with a radius chosen to be about the size of a print dot cluster at common scanning resolutions. This is the descreen: it attacks the fine regular pattern more than it attacks the picture, because the pattern is finer than most real detail.
A sharpening pass, which puts back the edge contrast the blur cost. Sharpening raises the difference across boundaries; it does not restore texture, so what comes back is crispness rather than detail.
A mild contrast adjustment, because both previous steps flatten tone slightly.
Because nothing here samples, the same input always produces the same output, and there is no seed and nothing to re-roll.
Two grids and the third one they make
The reason a printed page behaves so differently from a photograph is worth understanding, because it tells you which fixes can work.
A printed image is not continuous tone. It is thousands of dots of a few ink colors, laid on regular grids rotated to different angles so they do not overlap badly. Your eye at reading distance blends them into a picture.
A scanner samples on its own regular grid. When two regular patterns of similar spacing meet, they beat against each other and produce a much coarser pattern — the same effect as two mesh fences seen through one another, or a striped shirt on television.
Crucially, that coarse pattern is not added on top. It is the fine detail folded down into a lower frequency, which is the definition of aliasing, and once folded there is no unfolding. The signal that carried the picture and the signal that carries the moiré are now the same signal.
Which is why the honest options are all upstream: sample much finer than the print screen, use the scanner's own descreen, or change the angle so the two grids stop agreeing. This page is what you do when the file is all you have.
The trade this page is making
Descreening is not free, and the cost should be stated rather than discovered.
The blur removes the pattern by removing fine detail, and it cannot tell the difference between an ink rosette and a genuinely fine feature of the picture. Eyelashes, fabric weave, and small type all thin out along with the moiré.
The sharpening afterwards makes the result look crisper than the blurred version, which hides the loss rather than reversing it. That is a legitimate thing to do — apparent sharpness is what a viewer responds to — as long as you know that a descreened scan carries less information than the scan you started with.
So run it on material that has the pattern, and leave it alone on material that does not. On a scan of a photographic print, where there is no dot grid, this chain costs you detail and returns nothing in exchange.
Where it sits among the document pages
Several pages here clean up scans and they attack different defects.
This one is for the interference pattern that scanning printed material creates, which is a sampling problem.
Dust removal is for physical specks on the glass or the original, which are localised objects rather than a global pattern. Document shadow removal is for uneven illumination when a page is photographed rather than scanned. OCR preprocessing is tuned for a machine reader rather than an eye, and pushes contrast much harder than anything a person would want to look at.
Diagnosing which one you have takes a moment at full zoom and saves running three tools in sequence, each undoing the last.
Publication gate
This page ships once the workflow has been run against a magazine page scan with visible rosettes, a newsprint scan, a scan of a photographic print with no screen at all, and a scan containing small text, with the pattern checked at 100 percent and fine detail loss compared against the source in every case.
Examples
Photo scanned from a magazine page
The intended case. The pattern softens noticeably and the picture loses a little fine detail, which is the trade this page is making.
Scan of a photographic print
Little to descreen, since a photographic print has no dot grid. Here the pass is mostly the sharpening and tone steps, and the blur costs more than it returns.
Frequently asked questions
Why can't the pattern just be removed?
Because it is aliasing, which is not an overlay sitting on top of the picture — it is what happens when detail finer than the sampling grid gets folded down into a coarser frequency. The original detail and the false pattern now occupy the same place in the image. Removing one removes the other.
What resolution should I scan a printed page at?
Higher than the print's screen frequency, which for magazines is often around 150 lines per inch, so scanning at 600 dpi or more and then reducing usually beats scanning at 300. Sampling well above the pattern is what keeps it from folding down in the first place.
Does scanning a photo at 1200 dpi give me more of the photograph?
Only up to a point. Past the resolution of the print itself, extra dpi records the paper — its texture, its grain, its surface — rather than more of the image. If the original negative exists, scanning that is a different order of improvement.
Is anything invented here?
No. The chain is a blur, a sharpen, and a contrast adjustment, all deterministic, with no model anywhere in it. Run the same file twice and you get identical bytes. That also means it cannot add detail, only redistribute contrast.
Is the AI Scan Enhancement tool free, and do I need an account?
Yes, free and without registration. Everyone is served the same scan enhancement workflow, at the same quality, with no watermark and no trial that runs out. An upload can be up to 25 MB. The one thing worth knowing is that the result is temporary by design, so download it rather than bookmarking the job and returning tomorrow.