When calculating ambient occlusion, transparency is ignored completely. This means that anything positioned behind the transparent object is not visible within the ambient occlusion pass. Most methods for generating ambient occlusion suggest that you hide all your transparent objects before rendering.

In V-Ray 2.0 you can use a method that will allow you to ignore certain objects. This will generate an ambient occlusion pass that is visible behind transparent objects without having to hide anything in your scene. A common method for producing ambient occlusion in V-Ray is to use the VRayDirt texture. Combined with the VRayExtraTex render element, the end result can be saved out as a separate render pass. Although it adds additional render time, it is a simple way for generating ambient occlusion.

In the render elements window add VRayExtraTex. This is similar to material override but instead is just for this element allowing the scene to be rendered normally. Add VRayDirt as a texture and drag an instance over to the material editor.

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VRayDirt is a texture that applies a colouration to the edges of objects; this is useful for simulating dirt or in this case ambient occlusion. Set the radius accordingly as this is scene dependent. The larger the radius the further the dirt will travel away from the edge, too much and the results can start to look unrealistic. For most interior scenes a range between 50mm – 150mm is suffice. The subdivisions parameter controls the noise level so increase this to reduce noise but it will also increase render time. By default the mode is already set to ambient occlusion and there is no need to adjust any of the colours as the resulting pass should be black and white for compositing correctly in post.

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Tick work with transparency and then in the exclude option choose all objects within your scene that are transparent.

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Under refraction set the affect channels to all channels for all the transparent materials in the scene. This makes the ambient occlusion pass become affected by the transparency of the material.

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Now you can render an ambient occlusion pass through transparent objects.

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