There is currently no easy way to create projective transformations, in particular perspective projection matrices are probably the most common one, we can think about others ones. Two options:

1 - add new methods to the class Transform, e.g.:
 Transform<float,3,Projective> P;
 p.setPerspective(a,b,c,d,e,f,....);

2 - add global functions, e.g.:
  Transform<float,3,Projective> p = Perspective<float,3>(a,b,c,d,e,f,....);

3 - add static functions to the class Transform:
  typedef Transform<float,3,Projective> Proj;
  Proj p = Proj::Perspective(a,b,c,d,e,f,....);


To be consistent with the rest of Eigen, I guess we should implement both 1 and 3.