OPG is a freeware utility for Max that allows you to create occlusion passes with the click of a button. The plug in will allow you to control all the relevant parameters and even keep bump / opacity / displacement maps in the calculations.
is an occlusion pass?
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.
WHen to use it?
After you decide you are finished with your scene and have rendered your image.
Use this plugin to render out another pass and then composite it over the render as explained next:
How to use it?
> Download the plugin
Install the plug in by simply placing it in the plugins folder and select it in the utilities tab by clicking more.
When clicking the execute button, the plugin will switch to Mental Ray and make all the necessary scene adjustments so all you have to do next is hit render.
Quick info on the parameters:
Samples - depending on the quality you want and the
time you have, 128 is usually enough to get and high quality result.
Spread - Higher values soften the shadows between the geometry by
expanding them further.
Max distance - when set on 0 it works like skylight, any other value makes
it calculate the occlusion "locally" to a certain distance. this
parameter determines the distance of the local calculation. This is a must for indoor
Bump maps - Enabling this option will include the effect of bump maps in the occlusion pass.
Displacement maps - Enabling this option will include the effect of displacement maps in the occlusion pass. This option is a must when you have displacement maps in your scene
Opacity maps - Enabling this option will include the effect of opacity maps in the occlusion pass. This is VERY useful if you have objects with alpha maps in your scene (for example leafs).
Execute this button makes all the necessary scene adjustments so all you have to do next is hit render. If you decide to change a parameter simply click this button again to execute again. (Note that if you decide to enable the usage of maps after you already clicked it once you will have to get back to the original scene first).
Save and rename - Enabling this option will automatically save your scene to a different file adding _occ to the filename (this is a good way to avoid accidentally saving over your original scene and losing data).
Tip It is often recommended to render out several occlusion passes with different settings (in large images / long animations some parameters work better for some parts while others might look better in other parts).