Astro

A real-time GPU N-body gravitational simulation in Swift and Metal, scaling to 500k+ particles via a Barnes-Hut octree and a custom Morton-keyed radix sort.

Screenshots

Astro

Astro old

How it works

1. Morton encoding (MortonCode.metal) — each frame, every particle's position is mapped to a 63-bit Morton code (21 bits per axis), giving fine spatial resolution while preserving z-order locality for the rest of the pipeline.
2. Custom radix sort (RadixSort.metal, RadixSort.swift) — particles are sorted by Morton key with a 64-pass split-based radix sort over ping-pong buffers. Multi-pass prefix sums in auxiliary buffers handle large inputs. The sort runs entirely in Metal compute kernels with 64-thread workgroups tuned for Apple Silicon occupancy. Done from scratch rather than via Metal Performance Shaders so the bit-pass count and key layout match the Barnes-Hut tree exactly.
3. Octree construction (Octree.metal) — the sorted Morton stream lets every level of the octree be built in a single pass. Layers are stored sequentially leaves-to-root in one contiguous buffer; each layer's offset and size are derived analytically from the maximum particle count, so children find their parents by arithmetic rather than pointer chasing. Empirically 7 levels (21 Morton bits) is the sweet spot — most particles end up at their own leaf while the upper levels stay coarse enough to skip in force evaluation.
4. Barnes-Hut force evaluation (BarnesHut.metal) — each particle walks the tree with a stack-based depth-first traversal, accepting an internal node as a single mass when its angular size satisfies the multipole criterion θ < 0.7. Includes softening for close encounters and per-step clamping against numerical blow-up.

The current build targets 500k particles at 120 FPS on M-series GPUs, with all data structures laid out for coalesced GPU access via a single bridging header shared with Swift.

Versions

The repository keeps two recordings:

• Astro new.mov — current build, tuned for scale (post-processing off).
• Astro old.mov — earlier build with gravitational lensing, redshift, bloom, and chromatic aberration in PostProcess.metal / Spheres.metal. Kept as reference; the current build deliberately drops them to spend the budget on the Barnes-Hut pipeline.

Astro old demo

Stack

• Swift / Metal — host code and compute kernels
• Apple Silicon GPU target

License

MIT — see LICENSE.