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Global Illumination and Final Gather in Mental Ray for Maya

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Date Added: 9th December 2009
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Final Gather

Up to this point an accurate Global Illumination model has been used. To improve indirect illumination and to create a Radiosity effect, Final Gather is introduced. In the Render Globals Settings, Final Gather is turn on and Final Gather Rays are reduced to an initial value of 100. The following is the Maya manual extract.

Final Gather Rays

Controls how many rays are shot in each final gathering step to compute the indirect illumination. The default is 1000 per sample point, but this tends to be high for test renders (renders can take hours). Test rendering at lower values, usually 100 or 200, is sufficient; higher values are required for final renders. Increasing the value reduces noise but also increases the rendering time

This is the resulting image with Final Gather turned on and Final Gather Rays set to 100. The image is currently appearing much darker. This is because there are only really two surfaces for the light to reflect off. Final Gather also works in response to a surface's Diffuse and Incandescence values.

Here a separate material has been applied to the reflector board above the set. The Color has been set to pure white and the Diffuse set to 1. Incandescence has been set to mid-grey and gradually increased to add light. The backdrop has also been set to pure white and a diffuse value of 1. It's Incandescence is a low dark-grey. Adjusting the Incandescence value gradually allows Final Gather to introduce more light.

With more light, the Final Gather Rays are increased to 300 to improve quality.

The last Final Gather adjustment to make is the Min/Max Radius. Here is the Maya manual explanation:

Min Radius, Max Radius

Max Radius and Min Radius control the size of the sampling region within which Final Gather rays search for irradiance information from other surfaces.

With the default values, Maya calculates values that seem appropriate based on scene dimensions to speed up the render, but this calculation doesn't allow for complex geometry. Generally, enter a value that is 10% of scene's overall dimension for the Max Radius, then enter 10% of that for Min Radius. Make further adjustments based on scene geometry detail, how the geometry is arranged in the scene, and how the render looks. For example, use these settings to achieve better diffuse detailing in nooks and crannies in your scene.Mental Ray automatically calculates the Min/Max Radius when the value is 0. Increasing these values can help reduce render time.

Here the Radii are equal with a Min Radius of 0.1 and a Max Radius of 0.1. It leaves a spotty effect.

Here a Min Radius of 0.1 is used and a Max Radius of 1. Render time has been reduced notably.

As a final touch, the Spot lights are converted to Mental Ray area lights. This removes the hard shadow from the black boards and adds softness to the image.

This is the image with area lights. The type is set to Rectangle and both Sampling increased to 6x6. To increase brightness, the light's intensity has been increased slightly to 80 and the Incandescence of the wall and reflector board material has also been increased.

The Final Render

After some tweaking, this final image was rendered. The camera was moved closer, the sphere's color material changed, a key light added and render quality increased.

If you would like to experiment with this set-up the files are available for download.

Initial set-up

Final set-up

Part of this guide has been adapted from Inter-Station 3d

Quotes from the Alias Maya Help are Copyright © 2000-2004 Alias Systems, a division of Silicon Graphics Limited.

Thank you

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