# pdfcraft-prc-render

A headless CPU renderer for PRC 3D models read by `pdfcraft-prc`: placed meshes in world space, a fitted camera, a
depth-buffered triangle fill and headlight shading, into a straight-RGBA image.

- **Layer:** L3. Depends on `pdfcraft-prc` only.
- **Licence:** MIT OR Apache-2.0.
- `#![forbid(unsafe_code)]`. Never panics on input: image size, triangle count and every index are checked, and
  damage returns `RenderError`.

## API

```rust
assemble(&PrcFile) -> Result<Vec<WorldMesh>, RenderError>   // every placement's mesh moved into the world and coloured
place(&TriangleMesh, &Matrix, colour, triangle_colours) -> WorldMesh
Camera::fit(&Bounds, direction, up, perspective, aspect) / Camera::fit_meshes(&[WorldMesh], ...)
render(&[WorldMesh], &Camera, &RenderOptions) -> Result<Image, RenderError>
nav::Orbit::named(NamedView, perspective, Upright) / nav::Orbit::looking(direction, up, perspective)
orbit.turn(yaw, pitch) / pan_by([right, up]) / zoom_at(&Bounds, reach, aspect, factor, [right, up]) / anchor_at(&Camera, point) / centre_on / refit
orbit.camera(&Bounds, nav::reach(&[WorldMesh], &Bounds), aspect) -> Result<Camera, RenderError>
```

- `nav` is a viewer's orbit control without the GUI: the fitted view holds the sphere round the model's centre that
  holds every drawn vertex, so turning or reshaping the picture never changes the model's size. Once `anchor_at` picks a
  model point the view turns about it, and the point keeps its place on the picture. Pitch stays 0.01 rad short of
  the up axis, zoom within 0.05–50 of the fitted view, pan within 1,000 model radii; non-finite input is ignored.

- With no product tree, each tessellated mesh is drawn where it lies, in the default colour.
- A triangle's own colour wins over its mesh's, which wins over `RenderOptions::colour`.
- Opaque triangles are drawn first, then translucent ones blended over them; alpha 0 is not drawn.
- `Lighting::Headlight` (default) shades by the angle to the view direction; `Lighting::Unlit` draws flat colour.
- Perspective triangles are clipped at a near plane 1/10,000 of the eye-to-target distance from the eye; orthographic views are not clipped.
- `MAX_RENDER_PIXELS` (2^24, about 16 MP) bounds the image: at most 64 MB of RGBA plus 128 MB of depth.
  `MAX_ASSEMBLED_TRIANGLES` and `MAX_ASSEMBLED_VERTICES` (2^24 each) bound one `assemble` call. `MAX_RENDER_WORK` (2^31) bounds
  the pixel centres one `render` tests, summed over each triangle's clipped bounding box; past it the render returns
  `RenderError::TooMuchWork`. That is still seconds of CPU at the top, so callers on a UI thread should render small or off-thread.
- Not yet: shadows, ambient occlusion, specular light, scene lights, textures, wires and markups.