void reorientate_triangles(const MatrixFr& vertices, MatrixIr& faces,
const VectorF& n) {
assert(vertices.cols() == 3);
assert(faces.cols() == 3);
const VectorI& f = faces.row(0);
const Vector3F& v0 = vertices.row(f[0]);
const Vector3F& v1 = vertices.row(f[1]);
const Vector3F& v2 = vertices.row(f[2]);
Float projected_area = (v1-v0).cross(v2-v0).dot(n);
if (projected_area < 0) {
faces.col(2).swap(faces.col(1));
}
}
void assert_faces_are_valid(const MatrixIr& faces) {
assert((faces.col(0).array() != faces.col(1).array()).all());
assert((faces.col(1).array() != faces.col(2).array()).all());
assert((faces.col(2).array() != faces.col(1).array()).all());
}