Setting the Final Gathering Trace Depth

Location:

The Render Manager > mental ray (Global Renderer)
The Render Manager > (expand a pass) > mental ray
Set the current pass and choose Render > Render > Renderer Options from the Render toolbar.
Tab: Accuracy [Properties Reference]
Options: Primary Bounce Color, Secondary Bounce Color, and Secondary Bounce Color.

The final gathering Trace Depth settings are similar to the Raytracing Depth settings (mental ray > Optimization), but apply only to final gathering rays. They specify the number of times a given ray is reflected or refracted.

To set the final gathering trace depth

Reflection specifies the maximum number of times a final gathering ray can be reflected in a scene.
Refraction specifies the maximum number of times a final gathering ray can be refracted in a scene.
Combined specifies the number of times a final gathering ray can be reflected and/or refracted in a scene. The Combined value should not be less than the sum of the Reflection and Refraction values.

Setting the Number of Bounces

Final gathering is capable of multiple diffuse bounces as defined by the Trace Depth Diffuse value. By default, only a single bounce is calculated (a value of 0). Raising the number of bounces to values greater than 0 adds secondary bounces. For example, a value of 1 provides a single extra diffuse bounce, for a total of two bounces.

As well, you can set the colors of both the primary and secondary bounces to change the appearance of the final gathering effect. Secondary bounces happen only if you set the Diffuse value to 1 or more.

The overall effect of additional bounces is to brighten the final gathering effect, though the effect is somewhat more localized than the effect of increasing the Sampling Contrast value (see Setting the Sampling Contrast). This is particularly true for scenes with complex geometry with a lot of corners, where the additional bounces are unlikely to travel far before hitting something.

Note

If you are using Sampling Contrast to brighten the final gathering effect, keep in mind that it only applies to the first bounce.
Final gathering bounces are calculated independently from transparency and reflection bounces and do not depend on the overall final gathering trace depth.

To set the number and color of final gathering bounces

Set the Trace Depth Diffuse value. Values greater than 0 mean one or more bounces and increase the brightness of the effect.

A	Bounces = 0
B	Bounces = 1
C	Bounces = 2
D	Bounces = 4

Set the Primary Bounce Color to the desired value. The irradiance part obtained from first bounce final gather is multiplied by the specified color. Note that this affects single bounces only.
If you have set the Diffuse value to 1 or more, set the Secondary Bounce Color to the desired value. The irradiance part obtained from secondary bounce final gather is multiplied by the specified color.