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Fluid Simulation

PaintSimulation is the base for every GPU fluid solver a SimulationCanvas can drive — it handles the fixed-timestep clock, an inactivity timeout, and warming up its own shaders once at scene load. Add exactly one concrete solver as a sibling component to a SimulationCanvas.

The three solvers​

Fluid Viscous​

MLS-MPM viscous paint. Thin films and small droplets stay pinned in place until accumulated mass exceeds an adhesion (yield) threshold, then flow downhill, blending velocity between neighbouring texels for viscosity, and pulling dense clusters together via cohesive pressure.

  • Adhesion / yield pinning — nothing moves below the yield threshold.
  • Viscosity — velocity is blended between neighbouring texels.
  • Cohesive pressure — dense clusters are pulled together.
  • Gravity via flow field — once mass exceeds the threshold, paint flows downhill along the surface's gravity direction, supplied by the seam-fixing flow field.

Fluid Ink​

An Eulerian grid solver (adapted from the "Chimera's Breath" technique) whose signature feature is vorticity confinement — re-injecting the small-scale curl a grid solver would otherwise smear away, giving swirling, low-viscosity ink/smoke tendrils instead of a smooth blob.

  • Viscosity and buoyancy — a density-gradient pressure that drives the ink's outward bloom.
  • Vorticity confinement — the signature knob; 0 behaves like thick oil, higher values give curling filaments.
  • Capillary spread — lets ink creep into dry ground independent of velocity.

Fluid Film​

A height-field cellular automaton — no velocity state, no pressure solve, by far the cheapest of the three. Paint pools on flat faces, runs down steep ones, and dries into a permanent stain behind the run.

  • Retention — a slope-dependent mass threshold; only mass above it can move.
  • Squared flow axis — collapses the flow onto the dominant direction, so a drip stays a narrow streak instead of diffusing into a blob.
  • Capillary spread and exponential fade — optional sideways creep and drying.
Colour and coverage come from what you painted

All three solvers carry colour directly from the painted data rather than synthesising it, so what you paint is what flows — no separate colour parameter to keep in sync.

Shared workspace​

All three read the same shared flow field (gravity direction, optional gust + micro-noise turbulence, optional normal-map influence — see Seam Fixing) and write into a common SimulationWorkspace: a Visual buffer (advected colour + painted absorption) and a Dynamics buffer (velocity + mass), both fed directly by the brush's VisualInk / DynamicsInk stamps.

Derived PBR channels​

Each of the canvas's SimulationChannel entries bakes the shared workspace into its own layer using a per-channel ThicknessResponse curve — dry ↔ wet value or normal strength, with its own gamma — at an independently tunable commit ratio. This is how, for example, a Scalar "wetness" channel and a Normal "relief" channel can both be derived from the same simulated paint thickness.

// Pick a solver by enabling exactly one on the SimulationCanvas GameObject.
fluidViscousSimulation.enabled = false;
fluidInkSimulation.enabled = true;

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