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Final gather
Final gather is method of simulating global illumination.
On its own, Final Gather is a fast and easy way to achieve good illumination results for architecture visualization and entertainment scenes that require credible, but not necessarily physically accurate, lighting.
When used in combination with global illumination, Final Gather lets you create the most realistic, physically accurate lighting conditions for a scene.
For example, an opening garage door that allows light to spill into the garage, or dawn before the sun rises above the horizon.
To render with Final Gather, see Render with final gather.
When Final Gather is enabled, every object effectively becomes a source of ray-emitting light, mimicking the natural world in which objects influence the color of their surroundings. When one light ray strikes an object, a series of secondary rays are diverted at random angles around it to calculate the light energy contribution from the surrounding objects. The light energy is then evaluated during the ray tracing process to add the effect of the bounced light.
Unlike Global Illumination, Final gather does not use photon maps to calculation of light at a given point in scene. Instead, mental ray for Maya samples the surrounding area above every point in the scene. The illumination at those points is then computed as direct illumination. (If Global illumination is also being used at the same time, Final Gather calculates the total incoming illumination in the scene [called irradiance].)
Final Gather rays are emitted in many directions from a sample point and stop according to the settings in the Final Gathering section of the Render Setting window. Because Final Gather rays do not bounce, secondary surfaces are not taken into consideration. (However, when rays hit geometry, material shaders may cast secondary reflection, refraction, or transparency rays, as long as those secondary rays are specular or glossy, not diffuse.)
Final gathering eliminates the low frequency variation in the global illumination that often results if too few photons are used. (Performance is optimized because mental ray for Maya reuses and interpolates nearby final gathers.)
Final Gather and Global Illumination
You can combine Final Gather and Global Illumination techniques to:
You can reduce the number of Global Illum Photons, the Global Illum Energy levels, and the number of Final Gather Rays resulting in less rendering time, but more realistic lighting.
To create global illumination, see Render with global illumination.