Skip to main content

🌊 Committers & Fluid Simulation

Committers are responsible for writing paint data from the tool's output into the surface's texture layers. Simple Painter provides both direct and simulation-based committers.

📦 Committer Hierarchy​

BasePaintCommitter (abstract)
├── DirectPaintCommitter
│   └── StandardCommitter          — Default direct painting
└── SimulationPaintCommitter
    ├── FluidShallowCommitter      — Height-field gravity flow
    ├── FluidViscousCommitter      — SPH viscous solver
    └── FluidParticleCommitter     — Full MLS-MPM simulation
  • DirectPaintCommitter writes paint data immediately to the target texture — no simulation step.
  • SimulationPaintCommitter routes paint data through a fluid simulation before committing to the texture.

✍️ Deposition​

The Deposition property (float, range -1 to 1) controls the committer's write behavior:

// Chế độ deposition — dương để vẽ, âm để xóa, zero để tạm dừng commit
standardCommitter.Deposition = 1.0f;   // Draw — deposit paint onto the surface
standardCommitter.Deposition = -1.0f;  // Erase — remove paint from the surface
standardCommitter.Deposition = 0.0f;   // No commit — stamps are processed but nothing is written
tip

Use intermediate values (e.g., 0.5f) for semi-transparent layering. Negative fractional values create subtle eraser effects that partially reveal underlying layers.

🧪 Fluid Simulation​

Simple Painter offers three simulation tiers, each trading off between performance cost and physical fidelity.

FluidShallow — Lowest Cost​

  • Height-field gravity flow with a single fragment shader pass
  • Uses UNorm8 texture format for minimal memory footprint
  • Supports adhesion and surface tension parameters
  • Ideal for thin paint drips, ink bleeding, and simple gravity effects

FluidViscous — Medium Cost​

  • SPH (Smoothed Particle Hydrodynamics) solver with 7×7 or 9×9 kernels
  • Full color advection — fluid carries pigment as it flows
  • Sub-texel tracking for smooth motion below pixel resolution
  • Signed pressure field enables both expansion and compression behaviors

FluidParticle — Highest Fidelity​

  • Full MLS-MPM (Moving Least Squares Material Point Method) solver
  • Uses R32G32B32A32_SFloat texture format for maximum precision
  • Self-spawn — particles can generate new particles during simulation
  • Full gravity, viscosity, and fluid pressure simulation
  • Best for thick paint, mud, and realistic fluid behavior
info

Each tier runs entirely on the GPU. The simulation textures are internal and never read back to the CPU, maintaining the zero-allocation guarantee.

⚙️ Simulation Pipeline Per Frame​

The simulation runs in 5 sequential steps each frame:

  1. Setup — Initialize simulation textures and bind parameters for the current frame
  2. Simulate — Execute the fluid solver passes (gravity, pressure, advection)
  3. BakeAll — Convert simulation state into paintable color/normal/PBR data
  4. CommitAll — Write the baked results into the surface's target textures
  5. DrainFluid — Apply evaporation and cleanup to prepare for the next frame

📊 SimulationPaintCommitterConfig​

PropertyDefaultDescription
UseTimeouttrueStop simulation after no input for a duration
Timeout5sSeconds of inactivity before simulation halts
FixedTimestep0.016sPhysics timestep — 60 Hz by default
MaxStepsPerFrame2Maximum simulation steps per frame (1 – 8)
ForceScale0.04Global multiplier for all force inputs
Evaporation0.001Mass loss rate per simulation step
EffectScale1.0Spatial scale factor for simulation effects
warning

Setting MaxStepsPerFrame above 4 can cause significant frame drops on mobile GPUs. Profile your target platform before increasing this value.

Previous: Tools & Brushes | Next: API Reference