Problem using PrincipalComponents to rotate 3D model data

*To*: mathgroup at smc.vnet.net*Subject*: [mg80849] Problem using PrincipalComponents to rotate 3D model data*From*: will <willpowers69 at hotmail.com>*Date*: Tue, 4 Sep 2007 03:43:46 -0400 (EDT)

Dear Math Forum, I am using PrincipalComponents[] (in Mathematica 6) to perform a rotation operation on some 3D data (effectively to align my data so that the maximum length of my models runs down the x axis,the maximum width down the y axis, and height max on the z axis). However, i have noticed that mathematica repeatedly reverses the normal (flips inside out) of and/or mirrors my models. I find this very odd, and was wondering if anyone else had encountered this problem? it seems to happen consistently with certain models, but i cannot see what is different in these models to those (few) that work as they should. if this problem is to do with my data, can anyone think of a way to rotate the models not using PrincipalComponents[]. below is the code i have been using on STL (3D points and surface polygon data). Needs["MultivariateStatistics`"]; 3Dmodel = Import["3Dmodel.stl"]; Rotated3Dmodel = Graphics3D[{EdgeForm[], Polygon[Partition[ PrincipalComponents[Flatten[3Dmodel[[1, 2, 1]], 1]], 3]]}, Boxed -> False]; Export["Rotated3Dmodel.stl", Rotated3Dmodel]; thank you, William