Corona AO Shader - C4D 

For maximum realism and physical accuracy, we do not recommend using ambient occlusion to enhance shading. Corona Renderer will take care of darkening concave areas without any special tweaks, as that would be also expected in real life. Ambient occlusion can be however used to create advanced materials, such as rusted metal, worn wood, scratched paint, etc, or to provide "artistic" control over the overall scene's appearance.

1. How to enable ambient occlusion effect for a specific material? (dirt, "worn off" effect)

To add dirt or "worn off" effect to your material, simply use CoronaAO map for this:

The CoronaAO map can also be used for some advanced effects. For example it can be used as a mask to blend two or more materials - in the example below, paint worn metal:

Click to enlarge

Click to enlarge

2. How to add global ambient occlusion pass for further post processing?

To enable a pass which can be used to add ambient occlusion to the whole image globally:

1. Go to Corona > Multi-Pass: 

2. Add a Texmap pass, and then select Corona > AO as the map:

3. This will result in having the Texmap pass available in the Picture Viewer apart from the Background pass:

4. The two passes can be then blended together within the Picture Viewer to preview the effect: