We will Start with a very simple indoor scene similar to our setup for parametric lighting. The floor is blue and the walls are greyish. We will place a light outside the window and make it cast ray-traced shadows.

Now change Maya's renderer to Mental Ray.

The first thing we need to do to start a global illumination setup is to make the light emit photons. Open the lights attribute editor (select the light > CTRL A). Open the Mental Ray folder, then the "Caustic and Global Illumination" subfolder and then check the "emit photons "check box.

Finally in order to get global illumination working we need to enable it in the render globals. Open the render globals, under the Mental Ray tab open the Quality folder, then the "Caustics and Global Illumination" subfolder and check the "global illumination" checkbox. You might also want to enable raytracing for Mental Ray (in the "General" subfolder).

We can adjust the amount of global illumination by varying the exponent and energy levels.
The exponent is like a decay, as the value decreases, the intensity increases. The energy (RGB) increases the colour intensity, red green and blue.

Energy = 8000
Exponent = 3	Exponent = 2	Exponent = 1
Exponent = 2
Energy = 2500	Energy = 5000	Energy = 10000

You will notice that while there is light bouncing, the quality of the bounced light is very spotty. If we increase the number of photons the size of the spot decrease.

Exponent=2, Energy=8000
GI Photons = 10,000	GI Photons = 50,000	GI Photons = 200,000

This decreases the size of the spots, but this is still too messy. Now we can turn to the render globals to adjust the GI accuracy. By default the GI accuracy is 64, but increasing it can reduce the amount of spottiness.

GI accuracy = 1	GI accuracy = 100	GI accuracy = 200

Also adjusting the GI radius can influence the spottiness of the render

GI radius = 0.1	GI radius = 0.5	GI radius = 1

For the GI setup, we will use the following parameters
- Exponent = 2
- Energy (RGB) = 10,000
- GI photons = 100,000
- GI accuracy = 150
- GI radius = 1

Final Gather
Now in order to let the light continue to bounce around the room we can use Mental Ray's final gather. Final gathering is a technique for estimating global illumination for a given point by either sampling a number of directions over the hemisphere over that point (such a set of sample is called a final gather point), or by averaging a number of nearby final gather points since final gather points are too expensive to compute for every illuminated point.

To turn final gather on check the Final Gather checkbox inside the Final Gather subfolder. The accuracy of the render using Final Gather will increase with the number of Final Gather Rays, but the time required to render increases dramatically with each new final gather ray added.

Final Gather Rays = 100	Final Gather Rays = 200	Final Gather Rays = 400

The next step in improving render speed is to work with the Final Gather Max/Min radius. Typically we should aim for around 0.1 of the width of the rendered view for the Max Radius (in our case the rendered view is around 10 units across, so Max Radius should be around 1) The Min radius typically should be 0.1 of the Max Radius, so for our case the Min radius would be around 0.1

FG Rays = 100
FG Max Radius = 1 FG Min Radius = 0	FG Max Radius = 1 FG Min Radius = 0.1	FG Max Radius = 1 FG Min Radius = 0.5

As you increase the FG Min Radius the render time decreases accordingly.

With that we can now increase our render size and in the Sample Quality subfolder of the Mental Ray Render Globals change the Max Samples to 2 and select Jitter.

Final Render

Exponent = 2
Energy (RGB) = 10,000
GI photons = 100,000
GI accuracy = 150
GI radius = 1
FG Rays = 200
FG Max Radius = 1
FG Min Radius = 0.1