WebGPU: Volumetric Raymarching Guide
Rendering heterogenous fog, atmospheric haze, and dynamic god rays in browser applications requires direct physical simulation of light transport. WebGPU compute shader raymarching integrates Beer-Lambert extinction and Henyey-Greenstein angular phase scattering directly in 3D storage textures.
Radiative Transfer & Henyey-Greenstein Phase
How compute pipelines evaluate single and multiple scattering across view rays:
For each pixel ray $\vec{r}(t) = \vec{o} + t\vec{d}$, the compute shader steps through volume density $D(\vec{x})$ via 3D Worley/Perlin noise lookups. Transmittance is attenuated as $T(t) = \exp(-\int_0^t \sigma_t(s) ds)$, while in-scattered light is modulated by phase function $p(\theta) = \frac{1}{4\pi} \frac{1 - g^2}{(1 + g^2 - 2g\cos\theta)^{3/2}}$ towards primary light vectors.
Rendering Architectures Compared
| Volumetric Pipeline | Scattering Physics | 4K Framerate | Memory Footprint |
|---|---|---|---|
| WebGPU Compute Raymarching + Froxel Grid | Full Henyey-Greenstein + Shadow In-Scatter | 120+ FPS (Temporal Filter) | 16 MB (Froxel Texture) |
| WebGL2 Fragment Shader Raymarching | Single Scattering (No Froxels) | 30–45 FPS | 64 MB (Fullscreen Texture) |
| Alpha-Blended Billboard Particles | Constant Ambient Factor Only | 60 FPS (Overdraw Bound) | 8 MB (VBO Buffer) |
WebGPU Engine Invariants
Architecture standards for real-time volumetric graphics:
- Frustum Voxel (Froxel) Clustering: Discretize camera view frustum into $160 \times 90 \times 64$ volumetric cells to evaluate lighting at decoupled spatial frequencies.
- Temporal Anti-Aliasing (TAA) Jittering: Jitter sample offset along view rays per frame using Halton low-discrepancy sequences to eliminate banding artifacts.
- Early Ray Termination: Break accumulation loop when cumulative ray transmittance $T(t) < 0.001$ to prevent unnecessary compute shader execution.
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