Approximating translucency with inverse ambient occlusion in Blender

Prompted by a question from Superflea if it would be possible to add a translucency map to my WeightLifter add-on I found it was possible to use ambient occlusion baking with inverted normals for this purpose as suggested by this paper. Baking with Blenders built-in baking options is much faster than what is possible using Python and this might be useful for more people so that's why I share it here.

The idea is simple enough: just create a map for each position at the inside of the mesh with a value that represents how much light would be received at that point from nearby surfaces. Sounds a lot like ambient occlusion so the idea is to use ambient occlusion baking but with the normals inverted. The only snag is that inside a mesh it is extremely unlikely that a ray will ever reach the sky (even impossible if the mesh is watertight), resulting in a black map. However, Blenders AO settings have a distance parameter that can be used to tell the AO baking that any ray that does not hit a surface within this distance is considered sky:

(Note that we do not even have to enable AO for our purpose, i.e. baking, just setting the distance to 0.1 or something will suffice)

Now we can create a map that approximates translucency with the following steps:

  • Invert the normals on the mesh (they should all point inward)
  • Bake the ambient occlusion to an image texture (documented here)
  • Make sure you point the normals to the outside again
  • Use the inverted values of the image as a translucency map.
An example image is shown below (the head model is from OscarLeif on BlendSwap)

The noodle that uses this map to illustrate their values with an emission shader is shown below

Now if this is really useful to for example tweak a subsurface scattering shader is up to you :-) If you create a skin shader with it I am eager to see the results.

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