Creative EngineeringWIP

MUJŌ: WebGPU Watercolor Engine

A physics-based Japanese calligraphy and watercolor simulation running entirely in WebGPU compute shaders. Kubelka-Munk pigment mixing, Navier-Stokes fluid dynamics, and 48-bristle brush physics — all on the GPU.

Code

5 pigments · 3 brushes · 3 papers · 60fps GPU

Started Aug 15, 2026

Five Nihonga mineral pigments with curved brush strokes showing unique absorption and scattering

Five authentic Nihonga mineral pigments — each with unique Kubelka-Munk scattering coefficients.

Fast cursive calligraphy strokes in sumi carbon black ink on Kizuki Kōzo washi

MUJŌ watercolor atelier — clean washi paper canvas with Suzuri palette, brush selector, water dilution and canvas tilt controls

Water dilution comparison — dry sumi stroke above, lush Ai bloom below at 90% dilution

Tarashikomi wet-on-wet blending — Shu vermillion dropped into Ai indigo on Kobishi paper

永 (eternity) character close-up showing brush pressure dynamics and fiber absorption

Animated WebGPU watercolor painting with live mineral pigment diffusion and wet-on-wet blending

MUJŌ (無常) started as a question: can you make a digital brush that feels like real sumi ink on handmade washi paper? Not "looks like" in a screenshot — feels like under your hand.

The answer required going deep.

The physics stack

Every simulation pass is a custom WGSL compute shader — no Three.js, no React, no physics libraries.

Layer	What it does	Method
Ink flow	Fluid spreading across paper	Navier-Stokes + Jacobi pressure solver
Pigment mixing	Physically correct color blending	2-flux Kubelka-Munk spectral radiative transfer
Paper absorption	Ink soaking into virtual fibers	Darcy tensor porous flow
Brush dynamics	48 elastic guide bristles	Position-Based Dynamics
Evaporation	Coffee-ring edge darkening	Pinning model with radial deposition

Why build this

The Japanese ink painting tradition is built on impermanence — that's what 無常 means. Each stroke is unrepeatable. The ink reacts to your speed, pressure, and water ratio in ways you can't fully control. Most digital painting tools fake this with procedural textures. MUJŌ simulates the actual physics so the ink behaves — it doesn't just look right, it moves right.

What's in the engine

Five Nihonga mineral pigments: Sumi carbon black, Gofun oyster white, Ai indigo, Shu vermillion, Roku green. Each has measured Kubelka-Munk absorption and scattering coefficients.
Three Japanese brush types: Fude (calligraphy), Menso (detail), Hake (wash). Different bristle counts, stiffness curves, and ink loading capacities.
Three washi paper surfaces: Hosho (smooth), Gasenshi (absorbent), Torinoko (sized). Different fiber densities and absorption rates.
Procedural sound engine: Brush-on-paper audio responds to pressure and speed.

WebGPUShadersSimulationCreative

Related Threads

MUJŌ watercolor app showing five Nihonga mineral pigment brush strokes — Sumi (black), Shu (red), Ai (indigo), Ōdo (ochre), and Rokushō (green) — on Kizuki Kōzo washi paper, with the Suzuri Palette, brush selector, water dilution slider, and canvas tilt controls

Building MUJŌ: WebGPU Watercolor Engine

A physically-based Japanese watercolor and sumi-e simulation running entirely on WebGPU compute shaders — Kubelka-Munk optics, fluid dynamics, 3D elastic bristle physics, and Zen impermanence.

The physics stack

Every simulation pass is a custom WGSL compute shader — no Three.js, no React, no physics libraries.

Layer

What it does

Method

Ink flow

Fluid spreading across paper

Navier-Stokes + Jacobi pressure solver

Pigment mixing

Physically correct color blending

2-flux Kubelka-Munk spectral radiative transfer

Paper absorption

Ink soaking into virtual fibers

Darcy tensor porous flow

Brush dynamics

48 elastic guide bristles

Position-Based Dynamics

Evaporation

Coffee-ring edge darkening

Pinning model with radial deposition

Why build this

What's in the engine

Five Nihonga mineral pigments: Sumi carbon black, Gofun oyster white, Ai indigo, Shu vermillion, Roku green. Each has measured Kubelka-Munk absorption and scattering coefficients.

Three Japanese brush types: Fude (calligraphy), Menso (detail), Hake (wash). Different bristle counts, stiffness curves, and ink loading capacities.

Three washi paper surfaces: Hosho (smooth), Gasenshi (absorbent), Torinoko (sized). Different fiber densities and absorption rates.

Procedural sound engine: Brush-on-paper audio responds to pressure and speed.