Colour mixing — paint that picks up what it is laid on

Status: being built, 2026-10-09. Q232 holds the four decisions; this is the design they produce, and the investigation that led to them, written for the next session rather than for this one.

Why this and not a better simulation

The owner's complaint, 2026-10-09: the simulated media "look and perform like ass" next to Clip Studio Paint, and they would "give up the simulation aspects if we can visually create more realistic paint." Measured against the suite's own contact sheets (scripts/visuals.sh media) the complaint held: the simulated watercolour is a pale tube with a seam, oil and gouache are flat blobs with no bristle or chisel in them.

The cause is not the physics but where the simulation sits. MediumSimulator reads only the stamped mark's alpha, seeds one uniform stroke colour, runs the lattice and overwrites the scratch — so every per-dab colour the engine computed (JitterColor: hue, saturation, value, a second colour) is thrown away, and a bristle or chisel tip survives only as coverage, which the opacity map 1 − e^(−3·mass) then saturates: a 60 % bristle scratch reads α 0.83 against 0.95 for the solid. With a medium on, granulation, the stamped wet edge and the paper-texture pass are skipped as well. And at the time of writing the shipped Watercolor's EdgePull was 0.06 (B35), so the lattice's one unique effect was nearly off — B431 found why it had to be, fixed the term, and put it back to 0.45.

Clip Studio Paint has no fluid simulation. Its wet media are textured material tips, a paper texture multiplied in per plot, a whole-stroke watercolor edge (what ApplyWetEdge already is), and — the thing Lightbox lacked entirely — colour mixing in the Ink settings: each dab picks up the colour already on the layer and mixes it with the brush's paint, with an amount of paint, a colour stretch for how long the pickup persists, and a perceptual mixing mode. Its oil brushes imitate impasto with the same mixing; there is no height field. All of it is per dab, forward, single pass.

So the pass is: give the paint path per-dab colour mixing; keep tips, texture, jitter and the stamped wet edge on under a medium; and make the lattice take the colours it is given rather than discarding them. The lattice stays, as an opt-in that composes with the dabs instead of replacing them.

The record

BrushSettings.Mixing, a nullable ColourMixing block. Null is off, and null writes nothing — the camera's rule, and the optional-settings skill's two traps avoided: the block is nullable (not inert at a default), and there is no convenience getter beside it to reintroduce the key.

Field	Meaning	CSP's name
Amount 0..1	how much of a dab is the brush's own paint; the rest is what it picked up	Amount of paint
Length 0..1	how much of the carried colour survives each dab — the smudge option's word for the same idea	Color stretch
Reach 0..1	how far around the dab the ground is sampled, as a fraction of its radius	—

Pressure does not drive the amount yet, deliberately. A pressure curve is stored under a BrushDynamic key, and a key an older build does not know makes that build's brush store unreadable — which it then saves over, empty. The owner runs older builds daily, so the one mixing control an artist would reach for by pressing waits for Q193's version field (the sensitivity guardian's finding). Invariant 4: the block is on the stroke, so a preference change never alters existing art.

The engine

Per dab, in StampDabRange, before the dab is drawn:

1. Sample the ground under the dab: the scratch so far (the stroke's own earlier dabs) composited over the layer beneath, a five-point average at radius × Reach — the same sampler the dulling smudge uses.
2. Carry. The first dab's carried colour is the ground. Each later dab mixes the ground into what it carries by Length: at 0 the sample is replaced every dab and colour barely travels, at 1 it is dragged the length of the stroke.
3. Deposit the brush's colour (after its own jitter) mixed toward the carried colour by (1 − Amount) × carriedAlpha. Nothing under the brush means nothing to mix with, so on a blank canvas a mixing brush is exactly the same brush without mixing — a test pins that identity.
4. Mix in Kubelka–Munk (Pigment.Mix on K and S, mass tone back out), so yellow into blue goes green. Alpha is lerped; only the colour goes through the pigment model.

The deposit then takes the ordinary dab path — tip, roundness, flow, the footprint ceiling — so everything a paint brush has stays. A mixing brush is excluded from DrawsAsOneSilhouette and SubdividesForFidelity for the same reason colour jitter is: its dabs are not interchangeable.

The live path stamps settled dabs once and re-stamps a tail every event after restoring a backup, so dab k always sees dabs 0..k−1 in the scratch — the same ground the commit sees. The carried colour is checkpointed at the settled boundary, as SmudgeCarry is, and the tail continues from a copy. The layer beneath is the frame's cached bitmap, as the medium pass already borrows it.

Symmetry copies reuse the identity pass's mixed colours rather than sampling at their own position. The copy is the mark, moved — Hash01 seeds every other dynamic from authored coordinates for the same reason — and a copy sampling its own ground would make two mirrored halves of one stroke disagree. One known gap, filed as B426: the live path stamps the copies into the same scratch the drawn mark samples, while the commit gives each copy its own, so a mixing stroke crossing its axis picks up its reflection live and not on commit.

Where a mixing stroke is refused, the way a smudge is: per-tile rasterisation (CanTile), a region repaint on undo (RepaintRegion) and a layer merge all replay strokes over pixels a full render would not have shown them, so each takes the whole-frame route for a mixing stroke.

Bounded (invariant 6): one five-point sample of two bitmaps and one mix per dab, nothing proportional to the canvas.

The medium takes the colours it is given

MediumSimulator now seeds each cell's pigment colour from the downsampled scratch — the colour the dabs actually laid there, unpremultiplied — rather than from one stroke colour. WriteBack already recovers colour from the deposit's mass, so nothing else changes. Existing media documents whose brushes carried colour jitter re-render with that jitter restored. Two honest qualifications from review: no render fingerprint in the suite covers a medium stroke, so nothing was re-recorded and nothing would catch a further change; and a plain single-colour stroke's rim cells — alpha of a few levels, above the seed threshold — read back a colour quantised by premultiplied storage, so those bytes move by a few levels too. Invisible at that alpha, and said here rather than claimed away.

Cost

Mixing reads the canvas back per dab, so BrushCostOf puts a mixing brush in the Expressive tier with the smudge and the media, and the picker badge says why. The four media presets already sat there.

What this is not

Not a height field, not pickup of a patch (that is the smearing smudge, and it is what a palette-knife brush would be built on later), not a change to the fluid lattice's physics. The lattice's edge pull, bleed and glazing remain the things only it can do; whether the shipped presets should lean on them again is a later decision, made by looking at the contact sheets once the dabs carry real colour.

Evidence

ColourMixingTests (Raster): yellow over blue goes green not grey; a blank canvas leaves a mixing brush identical to itself without mixing; length carries a picked-up colour further; deterministic; not a silhouette. ColourMixingRecordTests (Core): absent from the file until used; round-trips; clones deep. LiveMatchesCommittedTests.AMixingBrushLooksTheSameLiveAndCommitted (App). MediumSeedColourTests (Raster): the lattice keeps per-dab colour.