I can only connect the node immediately to the surface otherwise i don't get any AO effect. I cant use for example as the vray dirt map. So what the purpose of it?

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thanks in advance :)

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    $\begingroup$ An AO node which returns color (yellow socket) and not a shader (green socket) is on the todo list. This will let you use it as a dirt map or whatever you like. $\endgroup$ – gandalf3 Jul 19 '14 at 17:48

The purpose is that you can control how much AO each object has.

The ambient occlusion node gives per-material control for the amount of AO. When AO is enabled in the world, it affects all diffuse BSDFs in the scene. With this option it's possible to let only some materials be affected by AO, or to let it influence some materials more or less than others.

Quote from the Blender Wiki.

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    $\begingroup$ It can also let some materials not be affected by enabling world ambient occlusion. Add AO shader to the diffuse shader and set the color of the AO to the negative diffuse color. It can be done with a vector curves node with all (XYZ) channels with point1:[x: -1.0, y: 1.0] and point2:[x: 1.0, y: -1.0] $\endgroup$ – Iszotic Dec 22 '17 at 3:48

I can only connect the node immediately to the surface otherwise i don't get any AO effect.

It's a shader, you can combine it with other shades before reaching your material output using a Mix Shader just like you would a Diffuse Shader.

  • $\begingroup$ I think that's what the OPs problem is about: you can't really combine the AO shader with other shaders using a mix shader because the result is not good: The AO effect will be mostly gone. You'd also have to plug your color data into the AO shader which makes it appear as multiplied on the material. Although technically your answer is correct, gandalf3 gave the "IRL" answer. $\endgroup$ – bortran Dec 22 '14 at 13:43

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