Final Gather

The final gather method involves computing incoming illumination at various points in a scene and storing this data in a cache. Information from the cache is used to determine how illumination from other parts of the scene contribute to the illumination at the point being shaded.

Key applications of final gather

One Bounce GI - Final Gather can be thought of as a "one-bounce" global illumination method. In practical terms this means that shining a spotlight at the floor in an otherwise dark room and rendering with final gather will illuminate the ceiling and walls with bounced light. Light will not be bounced back onto the floor as this would take 2 bounces so areas of floor outside the spot highlight will remain black.

Accelerating dome light types - The illuminance values from "dome" light types can be accessed directly from the final gather cache. A feature of final gather is the way illuminance values are interpolated allowing much lower sampling rates to be defined for dome lights while still maintaining image quality. Undersampling dome lights in this way results in much faster renders of environment, simple sky and sky lights compared with scanline or raytraced render methods.

Rendering Radiosity solutions - Final gather can take values from a radiosity solution as input. This means that multiple inter-reflections of light can be modelled using radiosity, then final gather will shade pixels in the image based on these illumination values.

This application of final gather allows multiple light bounces and removes the artefacts often associated with radiosity processing. In a similar way to under-sampling dome lights outlined above, a coarse but fast radiosity solution can be used as input to final gather. With appropriate radiosity and final gather settings, accurate and well-lit images can be produced very quickly.