Path tracing

Path tracing is a Monte Carlo rendering technique that simulates the transport of light by tracing random paths from the camera through the scene, bouncing off surfaces according to their material properties until they either reach a light source or are terminated by Russian roulette. Unlike simpler ray tracing methods that sample only direct illumination, path tracing accounts for global illumination — the indirect light that bounces between surfaces — by constructing complete light paths. The algorithm is asymptotically unbiased: given infinite samples, it converges to the exact solution of the rendering equation, though in practice the variance reduction techniques known as importance sampling are essential for achieving acceptable convergence rates in finite time.

Path tracing is not merely an algorithm but a philosophical commitment: it asserts that the correct way to render an image is to simulate the physical process that produces images in the real world. This commitment is both its strength and its limitation. The physical world does not care about computational budgets; path tracing, in its pure form, does not either.