Why Screen Tone Moiré Happens: Angle, Line Count, and Reduced Printing

What moiré actually is: two periodic patterns interfering

A screen tone's dots sit on a strict, regularly spaced grid. When one such grid overlaps a second one, a small mismatch between them gets amplified into a beat — a new pattern neither layer actually contains. That's moiré. It shows up as stripes far coarser than a single dot, or as swirling, rosette-like ripples, so touching up fine details never removes it, and changing the line count or density without finding the cause first is just trial and error.

Every case comes down to the same thing: two periodic patterns overlapping — two tones at different angles, two tones at different line counts, or a tone's grid interfering with a separate grid introduced by printing or reduction. The next three sections walk through each in turn.

Cause 1: Angle offset between overlaid tones

Overlay two tones of the same line count at an angle offset θ, and the moiré period works out to dot spacing ÷ (2 × sin(θ ÷ 2)). As θ approaches 0° (the grids line up exactly) or 90° (a square dot grid repeats every 90°, so 90° behaves like 0°), the period grows without bound. A larger period means a ripple spanning a wider area of the page, and one that's far easier to spot.

• 60 LPI, 5° offset: period about 4.85mm — clearly visible across the page.
• 60 LPI, 10° offset: period shrinks to about 2.43mm, still a large ripple.
• 60 LPI, 15° offset: period about 1.62mm. The tone preview tool's overlay pattern deliberately defaults to this offset as an easy example of visible moiré.
• 30° or 60° offset: period shrinks to about 0.82mm. A dot grid repeats every 90°, so 60° is treated the same as 90 − 60 = 30°, giving the same period.
• 45° offset: period about 0.55mm — the finest, least noticeable period this formula produces.

The rule of thumb that "30° and 60° are safe, everything else is risky" falls straight out of this calculation. Within a few to roughly fifteen degrees of 0° or 90°, the period balloons into a ripple anyone can see. 30° and 60° sit clear of that zone, are easy to remember thanks to the 90° wraparound giving them the same result, and keep the period to roughly twice the dot spacing. 45° gives an even finer period on paper, but 30°-increment angles have stuck in practice, being memorable and forgiving of small registration errors. Put differently, offsets of 5°-15° — visibly rotated, but not rotated enough — are the most dangerous zone of all.

Cause 2: Mixing line counts

Even with matching angles, overlaying two tones with different line counts (different dot spacing) still produces moiré. Two grids with different periods alternate between spots where they line up and spots where they fall out of step, and that alternation is itself the beat period, roughly 1 ÷ |1/P1 − 1/P2| (P1 and P2 being each tone's dot spacing). Overlay a 60 LPI tone (spacing about 0.42mm) with a 65 LPI tone (spacing about 0.39mm) — just 5 LPI apart — and the beat period still reaches about 5.1mm, even with the angles matched exactly.

That's why the tone preview tool's overlay pattern exposes only a single line count and lets you sweep just the angle: mixing line counts isn't a "safe difference" you can dial in like an angle — it's a state to avoid outright. Tone materials pulled from the Material palette, or layers copied from an older manuscript, can easily carry a different line count than the one you're using now. Always check the Frequency value in the Layer Property palette before overlaying anything.

Cause 3: Reduced printing and un-toned grayscale areas

Printing a page smaller changes the effective line count of any tone on it — the line count as actually counted on the printed paper. It equals line count ÷ reduction ratio: printing a B4 (257mm) page at B5 (182mm) gives a reduction ratio of about 0.708, so a 60 LPI tone becomes roughly 84.7 lines (see the companion article on line count and the effective line count under reduced printing for the derivation). Tighter dot spacing also means the tone can newly interfere with a pattern that was never a problem at full size.

The other easy miss is grayscale left untouched. Submit a monochrome manuscript with areas still in grayscale rather than converted to a tone, and most print shops automatically screen those areas during plate-making, adding their own periodic dot pattern. A page carrying both your own tone and this second, separately screened pattern gets the same "two overlapping periodic patterns" as causes 1 and 2 — except it only shows up once the pages come back from the printer, never on screen. This is also why an image exported from the print-look tool should never be submitted as final artwork: dots already baked in get screened a second time at plate-making, doubling up the pattern.

Preventing it in Clip Studio Paint

1. Select each tone layer on every page and check its Frequency and Angle in the Layer Property palette. Unless you have a reason not to, keep one line count across the manuscript (around 60 LPI is typical for monochrome doujinshi body pages).
2. Only add an angle offset when deliberately overlaying two tones. Default to 30° or 45°, and avoid small offsets like 5°-15°.
3. Check for areas still left in grayscale. Either convert them to black-and-white, or turn on the Tone effect in the Layer Property palette before submitting.
4. If printing smaller, check the effective line count after reduction and confirm it doesn't exceed 100 LPI — past that, moiré and clogging risk rises sharply.

Checking ahead of time with the tone preview tool's overlay pattern

You can check how prone a setup is to moiré on screen before anything reaches paper. The tone preview tool's test patterns include an overlay option that layers two screens at an angle offset, showing the moiré period number alongside the preview.

1. Switch the test pattern to overlay.
2. Match the line count and density to the values you plan to use.
3. Sweep the overlay angle from 0° to 90° and watch the preview and the moiré period number change together. The 15° default is deliberately an example of visible moiré.
4. Avoid angles with a large, obviously visible ripple, and pick a smaller-period angle around 30°, 45°, or 60° instead.
5. Enter the angle you settle on into the tone layer's Angle field in the Layer Property palette.

Keep in mind this preview only sweeps the angle offset. Interference from mismatched line counts, or from a print shop's automatic screening, happens for the separate reasons in causes 2 and 3 — check line count consistency and grayscale conversion in the manuscript itself.