Final Gather and Ambient Occlusion

Autodesk 3ds Max Tutorials > Lighting and Rendering > Rendering with mental ray: Final Gather and Global Illumination >

In architectural rendering, one often needs to present a new image to a customer in a very short time. To get this done in a few minutes instead of hours, you can combine final gather with ambient occlusion (AO).

What is ambient occlusion? Briefly, it darkens occluded areas. For example, at the intersection of a wall and a floor, occlusion is higher than in the middle of the floor. Ambient occlusion does not transfer colors between surfaces; it simply darkens them.

To get a quite good image in minutes instead of hours, use final gather with a low density, such as 0.1. This is very fast and distributes light, including colors, in the scene. To get contact shadows and local details, add ambient occlusion. mental ray has special support for ambient occlusion that makes this fast.

The new mental ray Arch & Design material can do AO directly, in a single pass.

Use material-based Ambient Occlusion:

If not continuing from the previous lesson, from the \tutorials\mental_ray\ folder, open the scene file fg+gi_start.max. Open the Render Scene dialog, and on the Indirect Illumination panel > Final Gather rollout, turn on Enable Final Gather.
On the Indirect Illumination panel > Final Gather rollout, choose the Draft preset and set Diffuse Bounces to 5, if necessary.
Open the Material Editor and modify the floor material: On the Special Effects rollout, turn on Ambient Occlusion. Do the same for the wall material.
Render the scene.

Final gather only

Final gather combined with ambient occlusion

The image with AO has good contact shadows and edges.
A final touch is to reduce the noise on the back side of the sofa. Increase the Interpolate Over Num. FG Points value to 100.

FG and AO with more interpolation

This is the combination of the Draft FG preset with AO. You can also use the Medium preset to improve quality at the cost of rendering time.

General notes on AO

Make sure AO is applied only locally within a small radius, just to bring out the small details, and let FG handle the coarser transport of light. A good rule of thumb is to set the AO radius to about 4 inches (100mm); this is the default setting.

If you think the “darkening” effect of AO is too strong, you can modify the Shadow Color value in the Ambient Occlusion group to a lighter color, such as 50 percent gray (set Value to 0.5).

Photons