Rtgi [upd] -

Calculating where a single ray of light goes is easy. Calculating millions of them, multiple times per second, while accounting for diffuse surfaces (like rough concrete) versus reflective surfaces (like water), is mathematically brutal.

Consider the difference in a single frame: a ceramic coffee mug inside a dimly lit cabin. With rasterization, the handle is dark, a void. With RTGI, the light from the window bounces off the pinewood table (absorbing its amber tone), hits the underside of the mug's handle, and wraps around the ceramic in a warm, soft caress. The shadow is not a black cutout; it is a penumbra, soft at the edges, colored by the bounce light from the ceiling. You don't notice RTGI. That's the point. You notice its absence—a deadness, a flatness—like a room with no echoes. Calculating where a single ray of light goes is easy