Volumetric path tracing is a method for rendering images in computer graphics which was first introduced by Lafortune and Willems.
It is used for photorealistic effects of participating media like fire, explosions, smoke, clouds, fog or soft shadows.
Like in the path tracing method, a ray is followed backwards, beginning from the eye, until reaching the light source.
The in-scattering is the most expensive part to calculate because it needs an integration over all paths in the scene that consist of radiance.
Therefore, thousands of paths need to be traced to achieve a result with good quality and without much noise.