Lightmap based Ambient Occlusion

Faking FG with sampled Lightmap and AO shader

This tutorial shows a method for generating FinalGather like ambience using the XSIAmbientOcclusion and the Lightmap Color_Sampler shader in XSI.
The basic idea behind this, is to create faster and non-flickering (animation) renderings. Unfortunately this method is not as accurate as FG and doesn't support multiple bounces.

Let's take a look how this works:  
First we are going to create a polymesh sphere which will sample the scene lightning for the ambience. More on this a few steps later.
Make the sphere as small as possible to avoid lights being inside it.
Create a spherical texture projection for the sphere.
Open the visibility options and uncheck primary and secondary rays and shadowcasting, -receiving as well.
Then create a new material and plug a color_sampler node to the Lightmap port of the Material and a Simple Lambert node in the input port of the Color_Sampler.
The material doesn't need to have a node plugged into the surface port.
Make the diffuse of the lambert pure white and the ambience pure black.
Apply this material to the sphere.
Create a new phong material and plug a XSIAmbientOcclusion node into the ambience port of the phong node.
Set the AO spread to 0.9 (you should not raise it higher than this value cause the node will create artifacts) and the Output Mode to Sampled Environment.
Create an Environment node and plug it into the environment port of the material.
Under Clips in the RenderTree menu, choose the Lightmap_Color clip that was created with the Colo_Sampler node in the sphere material. Connect the clip to the environment node.

Create some geometry in the scene and apply the newly created material to the objects.
Add some lights and light the scene as you like. Raise the scene ambience to pure white and render the scene. That's it!

Thank you for the reading!

You can download the video tutorial here: LMbAO_video_tutorial.avi (43mb)
You will need the TechSmith Codec available here:

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