Function path_trace_step 

pub fn path_trace_step(
    state: &mut PathState,
    bvh: &Bvh,
    triangles: &[Triangle],
    materials: &[Material],
    background: [f64; 3],
    u1: f64,
    u2: f64,
) -> bool

Trace a single path and accumulate radiance.

This is a simplified CPU-side path trace step (one bounce).