# Principal component analysis

I performed a plain principal component analysis, so there is not much to say here. The only twist was that I downsampled the nodes of the mesh before performing the analysis—I had to, for computational reasons. There are two files pertinent to this process.

- node_subset.txt—This file contains the indices of the subset of the 46,490-node mesh that were used in the components analysis. There are 960 indices (one for each node). The way to read this file is, “The 26th node is first, the 42nd node is second, etc.” It's referring to the 26th node specified in ValidationMesh21.inp (or ValidationMesh21_node.ascii). Note that it's assuming that those nodes are 1-indexed.
- 2kverts.txt—This file contains the triangles you can use to turn the 960 nodes into a nice 3D model. Note that (somewhat confusingly, given the above) in this file the nodes are 0-indexed. Therefore the way to read this file is, “The first triangle is made of the 934th, 927th, and 933rd vertices, etc.”
*3D graphics people only beyond this point.*For each triangle, the vertices are ordered so that the surface normals can be computed automatically. The surface normal is the cross product of the (third vertex minus the first vertex) and (second vertex minus the first vertex). Apologies for the rough description; it's done properly with symbols on page 272. And if you get the order wrong, you'll only get it wrong once. ;-)