In [1] they describe a clever way of rendering the thickness of an object in a single pass. The method only correctly works for convex objects but this limitation isn’t that bad, the method can often be used to get the approximated thickness of concave objects as well. For example, [1] uses it to fake the light scattering in clouds rendered as billboards. The methods works like this:
The object is rendered and the distance from the near plane is saved in a color channel R. Also, the distance to the far plane is saved in channel G. By rendering with the blend color mode MIN, one will get the minimum distance from the near plane in R, and the minimum distance to the far plane in G. By using these two distances, one can easily calculate the thickness of the rendered object with the following formula (1-G) – R (if distance is scaled so one is the the distance between the clip planes). Alpha can be saved as well in the same render pass, by outputting it to the A channel. And selecting blend alpha mode ADD (color and alpha can have different modes). This will add up the alpha.
All this is done in only one pass. Just remember to clear to white before rendering.
The image below shows the thickness of the popular Hebe mesh rendered with this method. This model is not convex, and the problem areas are for example the arm holding the bowl. As one can see, the algorithm believes that the bowl and the shoulder are connected, and therefore believes that part of the object is the thickest.
Image may be NSFW.
Clik here to view.
[1] The Art and Technology of Whiteout
http://ati.amd.com/developer/gdc/2007/ArtAndTechnologyOfWhiteout(Siggraph07).pdf
