A related problem is automatically figuring out the "natural orientation" of a mesh, so your automatic renderings don't look upside down by accident. Fu et al. use pretty simple machine learning tools: http://www.cs.ubc.ca/nest/imager/tr/2008/upright_orientation...
Maximizing the entropy of projected normals works well, for example, like this paper by Polonsky et al.: http://www.cs.technion.ac.il/~gotsman/AmendedPubl/Oleg/Polon...
A related problem is automatically figuring out the "natural orientation" of a mesh, so your automatic renderings don't look upside down by accident. Fu et al. use pretty simple machine learning tools: http://www.cs.ubc.ca/nest/imager/tr/2008/upright_orientation...