I’m not sure that’s helpful.
By a corresponding calculation I recognize surface, edge or corner. There is indirectly an epsilon ( as divider).
Note: in the current revision, the @Mugen87 workaround for raycasting multimaterial is no longer necessary.
Mouse and Input:
https://discourse.threejs.org/t/modify-indexed-buffergeometry-mouse-or-input/1674
http://threejs.hofk.de/modifyGeo/modifyGeo.html
https://github.com/hofk/THREEf.js/tree/master/THREEf_90
Mouse only:
http://sandboxthreef.threejs.hofk.de/modifyRaycastMultiMaterial.html