The user wants a Gray-Scott reaction-diffusion simulation. This is a classic pattern formation system described by two coupled PDEs:

∂u/∂t = D_u ∇²u - uv² + F(1-u)
∂v/∂t = D_v ∇²v + uv² - (F+k)v

Where:
- u and v are chemical concentrations
- D_u and D_v are diffusion rates
- F is the feed rate
- k is the kill rate

I need to:
1. Create a canvas-based simulation
2. Implement the Gray-Scott equations using a numerical method (likely Euler with finite differences for the Laplacian)
3. Allow mouse interaction to seed new patterns
4. Make it visually appealing with good colors
5. Keep everything in a single HTML file

Key implementation details:
- Use two buffers/grid arrays for u and v (current and next states)
- Laplacian calculation using 3x3 stencil with weights (typically center -1, neighbors 0.2, diagonals 0.05, or similar)
- Parameters F and k can be adjusted to get different patterns (coral, spots, stripes, etc.)
- Render using putImageData or direct pixel manipulation
- Mouse click/drag adds v chemical to seed growth

Let me build this step by step.

I'll create a Gray-Scott reaction-diffusion simulation with mouse interaction for seeding patterns. This will be a self-contained single HTML file.

I've created the Gray-Scott reaction-diffusion simulation. Let me verify the file was created properly and describe what it does.

I've created a Gray-Scott reaction-diffusion simulation in `index.html`. ## Features **Simulation** - Real-time PDE solving using Euler integration with 3x3 Laplacian stencil - Dual-buffer grid system for A (activator) and B (inhibitor) chemicals - Spatial downsampling (2x) for performance while maintaining visual quality **Interaction** - **Click/drag** anywhere to seed new growth patterns - Preset buttons for classic Gray-Scott patterns: Coral, Fingerprint, Spots, Worms - Adjustable parameters: Feed rate (F), Kill rate (k), diffusion rates, timestep **Visuals** - Organic cyan/teal coloring against dark background - Turing patterns emerge naturally from the reaction-diffusion dynamics **Controls** | Parameter | Default | Effect | |-----------|---------|--------| | Feed (F) | 0.0545 | Rate of A replenishment | | Kill (k) | 0.062 | Rate of B removal | | Diffusion A | 1.0 | Activator spread | | Diffusion B | 0.5 | Inhibitor spread | Open `index.html` in a browser to see the patterns grow. Click and drag to create new seeds—the patterns will self-organize into coral-like structures, spots, or mazes depending on the F/k parameter values.