A wavelength-resolved renderer. Light is carried as actual wavelengths instead of RGB triples, so it does the things RGB physically cannot: dispersion through a prism, the spectrum fanning out of glass, metamerism, and a forward-traced beam glowing in haze.
The headline scene is a Dark Side of the Moon homage: a collimated white beam enters an equilateral SF11 prism, refracts wavelength by wavelength, and a full spectrum fans out the far side, glowing in single-scatter haze on black. The same scene runs live on the GPU.
| GPU forward + volumetric composite | A water-droplet rainbow |
Spectral transport: n(λ) disperses |
Same scene, n(550) for all λ: the fan collapses to white |
| Dispersive caustic on a surface | Beam through haze |
(more in images/)
Three crates:
spectral-core— the CPU renderer. A spectral path tracer with hero-wavelength sampling, Sellmeier dispersion, a CIE 1931 sensor with swappable illuminants, and a forward light-tracer + single-scatter volumetric pass (volume.rs,lighttrace.rs). This is the reference: every result the GPU produces is checked against it.spectral-gpu— awgpucompute-shader mirror of the CPU renderer. Each kernel is diff-gated against the CPU oracle, so "fast" never costs "correct."spectral-viewer— a livewinit/wgpuwindow: orbit the prism, watch the spectrum sharpen, press SPACE to toggle dispersion on and off.
- Spectral transport. A ray carries radiance at sampled wavelengths, not RGB. Refraction uses a wavelength-dependent index
n(λ)(Sellmeier), so blue bends more than red and the beam fans into a spectrum. Hero-wavelength sampling keeps the wavelength count down without leaving chromatic noise. - Sensor. Spectral radiance is integrated against the CIE 1931 colour-matching functions to XYZ, then to sRGB. Swapping the illuminant (D65, A) shifts colours exactly the way metamerism predicts.
- The DSOTM beam. The visible shaft and its fan are forward light tracing through a participating medium: photons scatter off haze and the single-scattered radiance is connected to the camera. That is why the beam glows in the air rather than only lighting surfaces.
- Live reconstruction. The viewer accumulates cheap point samples per frame and reconstructs smooth volumetric density with a screen-space neighbour kernel (a normalized Gaussian gather), so even a sparse frame reads smooth and the image converges in noise, not brightness.
The spine of the project: every GPU kernel is diff-gated against the CPU oracle, and each gate is mutation-tested (break a term, confirm the gate fails, revert). The RNG is bit-matched across CPU and GPU; per-photon paths agree to ~1e-6 including the total-internal-reflection boundary at the violet edge; the volumetric film matches the oracle to a fraction of a percent. The gates live in spectral-gpu/tests/ and spectral-core/tests/.
cargo test
Render a still:
cargo run -p spectral-core --release --example prism_dsotm # -> prism_dsotm.png
cargo run -p spectral-core --release --example rainbow
cargo run -p spectral-core --release --example prism_caustic
Run the live viewer:
cargo run -p spectral-viewer --release
Controls: drag or arrow keys to orbit, SPACE toggles dispersion (rainbow ↔ white), ESC to quit. It also has a headless capture mode: --capture out.png --mode spectral|rgb --frames N.
Prebuilt spectral-viewer binaries for macOS (arm64 and x86_64), Linux x86_64, and Windows x86_64 are attached to each release. They are built natively per-OS in CI.
- The live viewer's neighbour kernel is a screen-space reconstruction filter: correct at convergence, with a bounded blur on sparse frames. A view-independent 3D photon-density estimate is the path to full correctness and remains a future option.
wgpuselects the platform backend automatically (Metal, Vulkan, or DX12).