How to mix raytrace ambient occlusion with indirect lighting

Question

As the tilte says, how do I mix a rendered image that has raytraced AO and Environment Lighting with an approximated Indirect Lighting pass? My attempts at putting the images together in Paint.NET results in a washed out image and/or bad color saturation. How does Blender combine Indirect Lighting with the rest of the rendered image (add, multiply, color dodge)?

Answer 1

You need to use two scenes for this as far as I know, or two blend files, but I prefer to use scenes.

You have two choices:

1. The main scene has everything except the Indirect lighting, and the second scene has only the Indirect lighting.
2. The main scene has everything, including the indirect lighting, but not the AO. Then the second scene has just the AO.

Either way is fine in my experience, but I prefer to add the indirect lighting separately, so here's the first method:

Create a new scene and link all the objects from the current scene. This should keep all your animation, meshes, materials and textures in sync. (Adding a new object may be problematic, and I'd like to hear anyone's solution to that)

Unlink the World settings by clicking the number (in this case '2') next to the datablock name, otherwise when you turn off AO and enable Indirect lighting, it'll do the same to the other scene.

Now in the Render Layers tab of the new scene, uncheck Combined and Z, and check Indirect and Color.

Lastly you just need to join the two scene's renders together. You can do this in the compositor or any 2D/Paint program that supports layers.

In the compositor, simply multiply the Color pass by the Indirect pass (the Indirect stores the light color and intensity, but the Color stores the color of the surfaces which we also need), then Add this to the render of the main scene with ray-traced AO.

Here are the renders and passes in my example:

Main scene (raytraced AO):

Color Pass:

Indirect Pass:

Indirect multiplied by Color:

The final composite:

If you want to crank up the indirect lighting effect, simply increase the Fac on the Add node.

This method can also be used to get the Indirect lighting from Cycles if you want. You can even render a low number of samples in the Cycles scene and then use the Bilateral Blur node to even out the noise.