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Tracing light paths from the camera and sorting rays drastically help to reduce rendering times. Nevertheless, the global illumination process remains a computation-intensive task. Hundreds to thousands of light path samples per pixel (spp) might still be necessary to obtain accurate results.

In the case of Path Tracing (and Monte-Carlo methods in general), insufficient light path samples translate to noise in the final image. Such noise can be removed by generating more samples per pixel, thus increasing the rendering time. This can be observed in the following examples (results obtained using a workstation powered by 3 NVIDIA RTX 2080 graphics cards):

The correct amount of path samples required to obtain noise-free images depends on the scene content and, more specifically, on the complexity of the paths that light follows before reaching the camera.

For instance, indoor scenes take longer to render when most light comes from the external environment and must cross windows to lit the objects. In the same vein, caustics effects due to refractions or shiny surface reflections might require more time to render smoothly as they involve two or more bounces between the camera and the light source. This can be observed in the following examples:

As can be seen, noise remains visible in the caustics areas at 512 spp (middle image). In contrast, at equivalent spp, the result obtained in the previous example with the textile material is almost noise-free.