What is occlusion ?
To simplify the explanation, just think of a situation when all your objects have a simple white color and your scene is lit equally from all directions by a white light. Basically this should result in a blank white image, but what happens when some objects block a certain amount of rays that should have gotten to other objects ? those rays don't reach the other object and as a result, the parts where the rays were blocked will become darker. The more rays blocked, the darker the surface will be. So basically what we get is a white image with dark areas where geometry intersects with itself. WHy using it ? The resulting occlusion pass gives very accurate and smooth shadows that resemble the result of global illumination. Compositing these shadows over your final render can significantly improve your image in many ways,it can mainly improve your shadows, give more depth to your scene and really helps to better show all the details of your models. where can I make it ? in any major 3d package that has mental ray (or other rendering engines that support it) basically.WHen to use it ? After you decide you are finished with your scene and have rendered your image.
There are two simple ways to get an occlusion pass.Both ways will usually give a similar result, the AO shader method can often be faster and gives much more control but the shader is not included by default in the early versions of some packages. experiment with both so you could decide which works best for you. Note that these steps are demonstrated with max but they are similar in every major 3d package that has mental ray.
Skylight method
1. Save your scene as a new file to avoid losing any data.
2. Set your renderer to mental ray.
3. Set the background color to white and the global light to 1.0 (software's default ).
4. Select all the objects and give them a standard material, set it's color to white.
5.In the rendering options, turn final gather on (more samples for higher quality).
6. delete all the lights in your scene and add a sky light, set it's color to white.
7. render.tip - instead of using a white diffuse color, you can also use the AO shader in the diffuse color slot for more fine tuning.
Ambient occlusion shader method
1. Save your scene as a new file to avoid losing any data.
2. Set your renderer to mental ray.
3. Set the background color to white and the global light to zero (everything is black).
4. select all the objects and give them a standard material, set it's color to white.
5. place an ambient/occlusion map in the self-ilumination slot (more samples for higher quality).
6. delete all the lights in your scene (they are unnecessary for this)
7. render.
