Description and usage of the Ambient Occlusion Shader

There are many uses for the Ambient Occlusion shader, one being to accentuate details/edges and increase the realism of 3d renders. This occlusion is also known as the 'dirtpass' or 'dirtmap'. The pass renders quite quickly and since it is a Global Illumination (GI) solution, it is an excellent tool when time is of the essence, and GI quality is required.

I will not go into the details of the science behind the magic, simply put; ambient occlusion fires rays into the scene at various angles from various points on the geometry, and determines which rays are blocked by other geometry, and creates a visual representation of the occlusion. The closer the occlusion, the stronger the representation.

Below are 3 spheres over a plane. You can see the strength of the occlusion is dependant on the distance to the plane. If you look closely, you can see the spheres also are 'casting' occlusion upon each other.

Overview:

We will render our image normally, and save the result. This can be done using any renderer, default scanline, radiosity, mentalray etc. Next we will assign all the objects in our scene the occlusion shader. We will composite the two resultant renders in a third party application. In this instance, since it is just a still, we will use Photoshop. Any video editing package will allow you to composite them for animation as well.

Scanline Render	Ambient Occlusion Pass

Comp	Click here for more examples

Step 1: Render Scene
There is no need for any preparation, simply render your scene as you normally would. In this instance I created a simple greeble wave, rendered with default lighting.

Step 2: Assign MentalRay as your renderer.			Step 3: Create a MentalRay Material.

Step 3: Click None in the surface parameter slot, and select Ambient/Reflective Occlusion to create the base shader.			Step 4: You now have your base 'dirtmap' shader. Read on for an explanation of the base parameters relevant to our cause.

Samples: determines the quality of the solution, the more the better. Tradeoff is rendertime. Bright/Dark: sets the colors for the extremes of the shader. For our purposes, we will leave the defaults of black and white. Spread: this value sets the level of diffusion of the occlusion. If the default value leaves your render 'loose' you may reduce this value to 'tighten' the solution. Similar to the level of a Gaussian Blur filter in Photoshop. Type: again, we leave this at default, 0, which is occlusion.

Step 5: Assign the shader to all your scene objects. One sweet way to accomplish this is by using mental rays Material Override. Under the processing tab dwells the Material Override slot. Drag your material from the editor, or select it by clicking the Material Override slot, and selecting it from the editor. The benefits of this technique are that you do not have to edit your scene materials, simply toggle Enable, and that shader overrides all the objects in the scene. "Captain!!! I canna control the materials!!" "INITIATE MATERIAL OVERRIDE!!!!"

Step 6: Set your environment background to white. Depending on your scene specific/quality needs, you may want to raise the Samples per Pixel in the Render Scene dialog. Render the image.

Step 7: Composite In Photoshop, copy the Occluded image and paste it on a layer over the Diffuse pass. Set the Occlusion layer to Multiply. Adjust the opacity as you see fit. Click save. Dance a jig.

This is just one use for this most excellent shader, you may add it to other material slots as well as incorporate it into your lights. It is fast, simulates Global Illumination and is highly customizable. Have fun! Read on for further options and other uses for this most excellent shader. Next Page: Using displacement with the shader

Updated 5.13.05