xfy
f4e937f35b
Background stars looked square even with anti-aliasing on, and toggling AA just made them bigger rather than round. Two compounding causes, both in star_color(): 1. Square shape: the fast path measured cell distance with max(f.x, max(f.y, f.z)) — Chebyshev distance, which draws an axis-aligned cube, not a sphere. The visible star was literally a faded cube face. 2. AA "just makes it bigger": the Gaussian AA path used Euclidean distance, but only inside ONE cell. hash13(cell) returns a different value across a cell boundary, so a star's radial falloff could never reach zero — it was hard-clipped at the cell edge. The star's outline was therefore the cell's bounding box regardless of the radius parameter; bumping the gain/radius just filled more of that box. Fix: scan the 3x3x3 neighborhood of cells so a star whose center lies in an adjacent cell still contributes across the whole cell it lives in, and take the brightest contributor. Distance is Euclidean in both paths, so both are now genuinely round. star_aa selects soft Gaussian glow (on) vs. a tight smoothstep disk (off) — a real shape choice instead of square-vs-clipped-square. Validated: naga type-checks the preprocessed shader; app runs 20s on Metal (Apple M4) with no shader/compile/validation errors; cargo test (14 physics tests) passes.
singularity-rs
A real-time, physically-motivated black-hole renderer in Bevy 0.19. Each pixel geodesic-ray-traces curved spacetime, producing gravitational lensing, the Einstein ring, a Doppler-beamed accretion disk, a lensed starfield, and optional lensed planets, a spacetime-curvature (Flamm) grid, and a cubemap skybox. Runs on desktop and the web (WebGPU).
The visual target is Gargantua from Interstellar: a black shadow surrounded by a tilted, glowing accretion disk whose back side is lensed up and over the hole (and down underneath), with one side brighter from relativistic Doppler beaming.
The integrator is a spinning (Kerr) geodesic: a dimensionless spin parameter χ ∈ [0,1] drives frame-dragging (Lense-Thirring) asymmetry and pulls the disk's inner edge inward along the Kerr ISCO. At χ = 0 it degenerates exactly to the Schwarzschild (non-spinning) case.
Run
Desktop (Vulkan / Metal / D3D12):
cargo run --release
Web (WebGPU — Chrome, Edge, recent Firefox/Safari):
cargo install --locked trunk
rustup target add wasm32-unknown-unknown
trunk serve
# open http://127.0.0.1:8080
Release web build: trunk build --release. On a browser without WebGPU, the page shows a fallback message instead of a blank canvas.
Controls
- Drag (mouse) — orbit the camera around the hole.
- Scroll — zoom (changes distance / impact parameter).
- Controls panel (top-left) — live-tune every parameter:
- Camera — distance, yaw, pitch, FOV.
- Black Hole — spin (χ), with live ISCO (disk inner edge) and horizon (r+) readouts.
- Accretion Disk — outer radius, tilt, brightness, rotation speed (inner radius is spin-derived ISCO).
- Doppler — enable + strength of the relativistic beaming asymmetry.
- Renderer — integrator step count (quality/perf lever) and render scale (sub-resolution offscreen target).
- Background — procedural star intensity, optional cubemap skybox intensity.
- Grid — toggle the lensed Flamm-paraboloid curvature grid + density.
Orbit input is automatically disabled while the cursor is over the panel.
How it works
A single full-screen quad carries a custom Material2d whose fragment shader, for every pixel:
- Generates a primary ray from the camera basis + FOV.
- Integrates the ray through Kerr spacetime with an adaptive Dormand-Prince RK45 loop, applying the discretized bending acceleration
a = -1.5·Rs·h²/r⁵ · pos + 2·M·a/r³ · (spin_axis × dir)(h= angular momentum,a = χ·Mthe Kerr spin length). At χ = 0 the frame-dragging term vanishes and this is exactly the Schwarzschild bending. - At each accepted step tests the bent segment against the accretion disk (equatorial plane), lensed planets (storage buffer), and the Flamm paraboloid grid surface — compositing hits front-to-back. Rejected steps (error above tolerance,
dtstill above its floor) retry at a smaller step without consuming the ray's step budget. - Terminates on capture (
r < r₊(χ), the spin-dependent horizon; at χ = 0 this isRs, and the shadow emerges naturally at the critical impact parameterb_crit = 3√3/2·Rs ≈ 2.598) or escape (samples the procedural starfield / cubemap along the bent final direction).
The disk is tilted relative to the camera, so the back of the disk is lensed over the top and under the bottom of the shadow — the characteristic "halo." Doppler beaming brightens the approaching side. At spin > 0 the disk's inner edge tracks the Kerr ISCO (shrinking from 3 Rs at χ = 0 toward Rs/2 at extremal spin) and frame-dragging shears the halo off the line-of-sight axis.
The CPU-side physics (src/physics.rs) mirrors the integrator — both the single-step Kerr derivative and the adaptive RK45 loop with its spin-dependent capture radius — and is unit-tested: b < b_crit rays are captured, b > b_crit rays escape, spin = 0 degenerates to Schwarzschild, and higher spin does not enlarge the capture set.
Performance
Defaults target ~60 FPS on a discrete/integrated GPU (developed on Apple M4). The Kerr + adaptive RK45 integrator costs roughly an order of magnitude more per pixel than fixed-step RK4, so the default render_scale is 0.75 on desktop (the quad renders into a sub-resolution offscreen target that is then upscaled to the window). On web, defaults drop to steps=200 and render_scale=0.5 for interactivity. If you need more FPS, lower Steps or Render scale in the Controls panel.
Project layout
src/
main.rs app entry, plugin wiring
camera.rs orbit controller (yaw/pitch/zoom) + WantsPointer
params.rs BlackHoleParams (tunable, mirrored to GPU each frame)
physics.rs CPU mirror of the geodesic integrator: Kerr deriv, adaptive RK45 loop, ISCO/horizon (unit-tested)
ui.rs egui Controls panel (collapsible sections)
web.rs wasm glue: WebGPU detection + fallback message
scene/planets.rs Planet component + storage-buffer upload
render/ BlackHolePlugin, material, offscreen + upscale cameras (render_scale)
assets/shaders/ WGSL: ray gen, Kerr RK45, disk, stars, planets, grid, skybox, upscale blit
docs/superpowers/ design spec + implementation plan
Status
Phase 1 (Schwarzschild) — shipped: shadow, tilted Doppler accretion disk with lensed Einstein halo, lensed starfield, lensed planets, lensed Flamm grid, optional cubemap skybox, live egui controls, desktop + web/WebGPU.
Phase 2 (Kerr) — implemented, pending visual/performance validation. The Schwarzschild fixed-step RK4 integrator was replaced by a Kerr geodesic with an adaptive Dormand-Prince RK45 loop: a spin parameter χ ∈ [0,1] drives frame-dragging and a spin-dependent capture radius (Kerr horizon), the disk inner edge tracks the Kerr ISCO, and render_scale is now wired through an offscreen render-to-texture + upscale pass. The CPU mirror (src/physics.rs) covers the Kerr derivative, the RK45 step, and the adaptive loop, and the unit tests assert the spin = 0 degeneracy plus capture/escape behavior at spin > 0. What remains is the human-in-the-loop checklist: spin = 0 visual regression against Phase 1, frame-dragging asymmetry at spin > 0, and desktop/web FPS at the new defaults — see docs/superpowers/plans/2026-07-13-interstellar-blackhole-phase2-kerr.md (Task 8).
Phase 3 (future work). Full exact Kerr Cartesian pseudo-Hamiltonian (Σ/Δ/Carter-separable form) for sub-percent photon-orbit accuracy at high spin, retrograde spin, tilted spin axis, and adaptive integrator order. See docs/superpowers/specs/2026-07-13-interstellar-blackhole-phase2-kerr-design.md §9.