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# Donut Documentation
Deeper documentation for Donut, the real-time Schwarzschild black-hole renderer.
The top-level [`README.md`](../README.md) is the overview; the two documents here
go into how it actually works.
- **[physics.md](physics.md)** — the physics and maths. Tracing light through
curved spacetime: the Schwarzschild metric, null geodesics and the equations of
motion, the numerical integrator and its adaptive step, the event horizon /
photon sphere / ISCO, the Novikov–Thorne accretion disk, gravitational and
Doppler redshift with relativistic beaming, and the observable quantities the
renderer can measure. Tied throughout to
[`assets/shaders/geodesic.slang`](../assets/shaders/geodesic.slang).
- **[architecture.md](architecture.md)** — how the program is built. The code's
layering (Application / Scene / RenderPath / UILayer), the portable RHI that lets
the same rendering run on OpenGL and Vulkan, the frame loop, the unified
scene-and-simulation world, the rendering pipeline (progressive resolution,
supersampling, environment lighting, tone-mapping), the docking UI, the export
pipeline, and the build system.
For each pixel, Donut casts a ray from the camera and follows it backward through
the curved spacetime around Sagittarius A\*. Rays that fall past the horizon are
the shadow; rays that graze the photon sphere wind around it into the bright ring;
rays that strike the hot orbiting disk pick up its shifted glow; rays that escape
read the background sky. That trace is one GPU shader, fed by a small portable
graphics layer so it runs identically on both backends.
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