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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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