I've been looking at the recently uploaded files on Commons, and it seems a lot of files render very funny because they have backfaces that really should be visible.

Examples:

• Entire model is "inside out". Ideally, all of the polygons should have their normals inverted, so that they face opposite direction, but in many 3D editors this is apparently not a problem.
  • https://commons.wikimedia.org/wiki/File:Icosahedron.stl (try rotating it in 3D viewer… freaky)
  • https://commons.wikimedia.org/wiki/File:Tetrahedron.stl
  • https://commons.wikimedia.org/wiki/File:Wicipedia_Cymraeg_2.stl (only the old, first version)
• The model is not really a closed model, but just a "surface". (Or alternatively, it has "holes" in it.)
  • https://commons.wikimedia.org/wiki/File:Half_Torus,_Diagonal_Cut.stl
  • https://commons.wikimedia.org/wiki/File:Freddy_tortoise_3D_artificial_shell.stl

These would display in a much more intuitive way if the viewer enabled backface visibility. With three.js, apparently you have to pass side: THREE.DoubleSide to the MeshPhongMaterial constructor. I tested this with this hack in JS console:

mw.mmv.ThreeD.prototype.geometryToObject = function ( geometry ) {
	var material = new THREE.MeshPhongMaterial( { color: 0xf0ebe8, shininess: 5, flatShading: true, side: THREE.DoubleSide } );
	return new THREE.Mesh( geometry, material );
};

…but I'm not sure where else this would have to be changed to apply to server-generated thumbnails too.