Path tracing
A rendering method that follows light rays along random bounce paths through the scene and computes global illumination directly.
It shoots rays from the camera and lets them bounce onward in random directions at each surface, each bounce weighted by the material, until they hit a light or give up. Averaging thousands of such paths per pixel (Monte Carlo integration) resolves reflections, soft shadows, colour bleeding and caustics in one pass — everything global illumination otherwise needs separate tricks for.
Jim Kajiya formulated the rendering equation in 1986; path tracing is its direct numerical solution. Long confined to film (Pixar, Arnold), hardware RT and AI denoising made it run in real time around 2018 — Quake II RTX and Cyberpunk 2077's overdrive mode. The price is noise that only falls with the square root of the ray count.