Ambient Occlusion

Ambient Occlusion (AO) is a method spearheaded by the film industry for emulating the look of true global illumination by using shaders that calculate the extent to which an area is occluded, or prevented from receiving incoming light.

• Material/Map Browser > Choose an Autodesk Material > Ambient Occlusion rollout
Note: Autodesk Materials do not appear in the Browser if the active renderer does not support them. Not all Autodesk Materials support ambient occlusion.

Used alone, an AO shader, such as the separate mental ray Ambient/Reflective Occlusion shader, creates a grayscale output that is dark in areas light cannot reach and bright in areas where it can.

The following image illustrates the main results of AO: dark crevices and areas where light is blocked by other surfaces, and bright areas that are exposed to the environment.

An example of AO applied to a scene:

One important aspect of AO is that the user can see how far it looks for occluding geometry.

AO looked up within a shorter radius:

Using a radius creates a localized AO effect: only surfaces within the given radius are considered as occluders. This also speeds up rendering. The practical result is that the AO provides nice "contact shadow" effects and makes small crevices visible.

The following illustration depicts a model helicopter that is lit almost exclusively by indirect light. Note how the helicopter does not feel "grounded" in the left-hand image and the shadows under the landing skids are too vague. The right-hand image uses AO to "punch out" the details and the contact shadows.

Left: Without AO

Right: With AO

Interface

Note: Not all Autodesk Materials support ambient occlusion.

There are four controls for ambient occlusion:

Enable

When on, enables ambient occlusion (AO) and makes the remaining group controls available. Default=off.

Samples

The number of samples (rays) shot for creating AO. Higher values yield smoother results but render more slowly, while lower values render faster but look grainier. Values in the range 16 to 64 cover most situations. Range: 2 to 256. Default=16.

Max Distance

Defines the radius within which the renderer looks for occluding objects. Smaller values restrict the AO effect to small crevices only but are much faster to render. Larger values cover larger areas but render more slowly. Default=4.0.

The following illustrations show the raw AO contribution with two different distances:

Left: Higher Max Distance value

Right: Lower Max Distance value

Tip: To specify an infinite radius, set Distance to 0.0.

Use Color from Other Materials

When on, derives the AO coloring from surrounding materials, for more accurate overall results (also known as color bleeding). For example, a glowing material would return a brighter color than a dark material. Default=off.

Use Color From Other Mats is off.

Use Color From Other Mats is on.

If you find that using AO creates a "dirty" look with excessive darkening in corners, or dark rims around self-illuminated objects, turn on Use Color From Other Mats for a more accurate result.