lux-aurumque

Lux Aurumque — Latin, "Light and Gold." A minimal transient path tracer in Rust, rendering a small gilded room as a wavefront of light sweeps through it.

Standard renderers compute the steady-state radiance arriving at a sensor — the equilibrium reached after light has bounced around forever. Transient rendering refuses that simplification. Light propagates at a finite speed, and every photon path has a definite duration equal to its total optical length divided by c. By binning each path's contribution into a histogram indexed by that duration, we render not a single image but a movie of light propagating through the scene, frame by picosecond.

This project renders such a movie. Each output PNG is one slice of the time histogram (40 ps wide by default), and stitching them produces a video where you can watch a Gaussian pulse leave the source, reach the gold sphere on a measurable delay, scatter off the warm walls, and finally return to the camera by progressively longer paths.

Quick start

Requirements: Rust 1.85+ (edition 2024), a C linker (build-essential on Debian/Ubuntu), ffmpeg for the final encode.

cargo run --release
ffmpeg -framerate 30 -i frames/frame_%04d.png \
       -c:v libx264 -pix_fmt yuv420p -crf 18 lux-aurumque.mp4

Defaults (640×480, 256 spp, 200 time bins of 40 ps) take a few minutes on a modern laptop and need ~800 MB of RAM. Tune WIDTH, HEIGHT, SAMPLES, NUM_BINS, DT in src/main.rs.

What you'll see

A Cornell-box-style room reskinned in aurum — cream floor / ceiling / back wall, deep copper left wall, antique amber right wall. On the floor: a polished gold sphere (R 1.00, G 0.78, B 0.35 — gold's spectral response in linear sRGB) and a smaller satin-gold diffuse sphere. Near the ceiling sits a small emitter that fires a single 50 ps Gaussian pulse, warm-tinted to roughly 3000 K (R:G:B = 50:38:18).

Camera is at (0, 0.27, 0.85) m; light at (0, 0.50, −0.30) m, ~1.17 m apart. Direct light therefore reaches the camera around t ≈ 1.17 / c ≈ 3.9 ns, i.e. frame ~98 at DT = 40 ps. Things to look for:

1. Direct light arrives first at frame ~98 — the source disc itself.
2. Specular highlight on the gold sphere a few frames later, as the wavefront catches the polished surface.
3. Diffuse satin-gold sphere illuminating with a measurable delay relative to the gold mirror (cosine-weighted scatter, longer effective path).
4. Warm color bleed onto the cream walls arriving later still — each bounce adds a centimeter or two of path length.
5. Late tails in the last ~30 frames where multi-bounce paths trickle in.

Architecture

src/
├── main.rs          entry: scene setup, tile-based parallel render, PNG writeout
├── transient.rs     time-binned framebuffer + time-aware path tracer
├── camera.rs        pinhole camera ray generation
├── hit.rs           HitRecord, Hittable trait, world list
├── sphere.rs        analytic sphere intersection
├── material.rs      Lambertian / Metal / DiffuseLight (pulse emitter)
├── ray.rs           Ray with cumulative path_length
└── vec3.rs          Vec3 conveniences over glam

The single conceptual delta from a textbook path tracer (Ray Tracing in One Weekend and friends) lives in three places:

• Ray::path_length — every ray carries the cumulative optical length of its history. Each bounce adds the segment length to the next ray's starting path_length.
• trace_path deposit step — when a path terminates on an emitter, its contribution is deposited into a window of bins around t_arrival = path_length / c, weighted by the source's Gaussian temporal profile p(τ) = exp(−τ² / 2σ²). The window spans ±4σ (~99.99% of the pulse mass); depositing into only the central bin would silently drop most of the pulse whenever σ ≫ dt.
• Pulse::weight — evaluates the unnormalized Gaussian at the bin-center offset. Energy is in arbitrary units; the global Reinhard exposure pass in main.rs normalizes brightness across all bins.

Everything else — BRDFs (Lambertian, Metal, DiffuseLight), tone mapping (Reinhard + sRGB-ish 1/2.2 gamma), and parallelism — is conventional.

Physics knobs

Parallelism

Tile-based with a single shared global frame:

1. The image is split into TILE_SIZE × TILE_SIZE tiles (32 tiles at 640×480 / 64).
2. Tiles run in parallel via rayon::par_iter. Each thread renders its tile into a small private TransientFrame (~9.8 MB at defaults).
3. After finishing a tile, the thread briefly locks the global frame and merges its partial contribution at the tile's offset (TransientFrame::merge_tile).

Memory peaks at 1 × global frame + active_threads × tile frame — about 780 MB at the defaults with 4 threads. The earlier fold + reduce pattern allocated one full-size frame per rayon work chunk (16+ at high parallelism), which OOM-killed the renderer at modest resolutions on memory-constrained boxes (e.g. WSL2's default 7.5 GB). Tiles are the correct primitive here.

Mutex contention is bounded by tile count, not pixel count, and the critical section copies a few MB of floats — negligible vs. the per-tile render time.

What's deliberately missing

This is a teaching/portfolio implementation. The following are absent in the name of clarity:

• Next event estimation / explicit light sampling. Pure brute-force path tracing. Convergence is correspondingly slow on small lights.
• BVH acceleration. With ~8 spheres, linear iteration is fine.
• Spectral rendering. RGB only. Real transient renderers track wavelength too — see Jarabo et al. for the spectral-transient framework.
• Participating media. No fog, no scattering volumes. Marco et al. (2017) extended transient rendering to media; a natural next step.
• f16 framebuffer. All deposits are f32. At 1080p the global frame reaches ~5 GB; halving the precision (or chunked rendering of bin ranges) would push the resolution ceiling further.
• Inverse rendering. The forward model here is the foundation for non-line-of-sight reconstruction — but the inverse problem is its own project.

Further reading

• Jarabo, Marco, Muñoz, Buisan, Jarosz, Gutierrez. A Framework for Transient Rendering. ACM TOG 2014.
• Velten et al. Femto-Photography: Capturing and Visualizing the Propagation of Light. SIGGRAPH 2013.
• Jarabo's PhD thesis, Femto-Photography: Visualizing Light in Motion, 2015 — the most readable single document on the subject.

License

Dual MIT / Apache-2.0.