What Is Final Gathering?

Final gathering is a way of calculating indirect illumination without using photon energy. Instead of using rays cast from a light to calculate illumination, final gathering uses rays cast from each illuminated point on an object's surface. The rays are used to sample a hemispherical area above each point and calculate direct and indirect illumination based on what the rays hit.

This scene was rendered using final gathering, which "collects" the indirect and direct light around illuminated points on an object's surface to simulate real-world lighting.

The overall effect is that every object in the scene becomes a "light source", at least to some extent, and influences the color and illumination of the objects and environment surrounding it.

You can combine direct illumination and/or global illumination with final gathering to increase photorealism in your scenes. Used in the correct way, final gathering can create stunning, photorealistic lighting in a fraction of the render time needed for global illumination alone.

NoteIt is important to remember that the lighting created by final gathering and direct illumination are added together, meaning that a scene which uses both direct illumination and final gathering will be brighter than a scene which uses either one or the other.

Pros and Cons of Final Gathering

Final gathering has advantages and disadvantages:

Advantages

Because it uses fewer photons, scenes render faster and use less memory when creating the Photon Map.
Final gathering is more accurate than a direct photon map lookup. Accuracy is most noticeable on the edges of objects and in corners.

Disadvantages

If you do not use global illumination, photorealistic results take much longer to render.
When using global illumination and high Accuracy settings, the render can take longer than it would without final gathering; however, the slower the render, the higher the quality.