Final Gather with Mental Ray (Maya)

My last post described the GI indirect lighting method that uses photons to create the secondary lighting.
Another way to achieve indirect lighting is with Final Gather, that in contrast to GI looks at the information in the regular direct light and based on that it sends out Final Gather Points. These points land where the regular light rays land and from there it sends out their own rays that collects information about their surroundings and adjusts accordingly to how the point is to be lit. In the end, all the points average their values to get an even distribution of light in the scene.
When you use FG in your renders, you can actually use geometry as lights in your scene with a bright surface shader, so that the FG points sends this out when hitting the geometry.
Final Gather gives very nice looking lights which are soft and delicate, but it isn’t really a “true” indirect light. It is kind of fake compared to real indirect light, and therefore it isn’t the most authentic looking light out there. But it is easy to set up and works very well in many situations. Besides it is faster to render than Global Illumination.

With Final Gather the default settings in Maya usually works ok as a starting point. If your render gets noisy you can increase the accuracy in the render settings. If you set this too high, your render times can drastically increase.
Point Density controls how many points getting sent out.
The default usually works well.

One common problem with FG is that you can get noisy areas in the tight shadows in your scene.
You can remedy this by increasing the accuracy and adjusting the Point Interpolation upwards. This makes the points blend better, but at the expense of loss in detail. You can get the details back with the use of Ambient Occlusion, that I’ll explain later.

The Scale sliders (primary diffuse and secondary diffuse) is used to tone down the intensity of the FG light effect. A darker shade gives less brightness. The secondary diffuse implies using the Secondary Diffuse Bounces.
The Secondary bounces works so that the points that land on a surface, spawns another ray that bounces from the original target and creating a new sample point on the next surface.
This increases calculation but gives more even distribution and more authentic lighting.

Keep on lighting!

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