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

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>

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.