Path Tracing

Path Tracing

Path tracing is an extension of raytracing that simulates global illumination, the way light bounces around a scene and illuminates surfaces indirectly. While basic raytracing only handles direct light from light sources and a few bounces for reflections, path tracing continues bouncing rays until they eventually reach a light source. This produces much more realistic lighting because it accounts for all the ways light travels in the real world.

The path tracing algorithm works by shooting a ray from the camera through each pixel. When the ray hits a surface, the algorithm randomly chooses a direction to bounce based on the surface's material properties. It continues bouncing until the ray hits a light source or goes off into infinity. This creates a path from the camera to a light source. The color of the pixel is the accumulated light along that path.

Because the bounce directions are random, a single path is very noisy. To get a clean image, you need to shoot many paths per pixel and average them. This is called Monte Carlo integration. The more paths you shoot, the cleaner the image becomes. The noise reduces at a rate proportional to the square root of the number of samples, which means cleaning up the last bit of noise takes a very long time.

Path tracing naturally produces effects that are difficult with other techniques. Color bleeding, where a red wall casts a red tint onto a nearby white wall, happens automatically because the rays bounce off the red wall and carry its color. Caustics, the focused light patterns you see through a glass of water, are produced because the rays are refracted through the glass and converge on a surface.

Almost all modern VFX renderers use path tracing. Arnold, RenderMan, Cycles, Redshift, and V-Ray are all path tracers at their core. The differences between them are in how they optimize the path tracing process, what sampling strategies they use, and how they handle difficult cases like very bright light sources or highly reflective surfaces. Path tracing has become the standard because it produces the most realistic results with the least manual setup.