Data items in the EM_CLASSES category record details
    about the particle classification. 
  Particle classification involves grouping images that are similar, 
  and separating images that are distinct. In practical use, this means 
  that experimental projections that have the same orientation (shape) 
  are placed within the same category for later averaging. In this case, 
  orientation means that the particles are showing the same face to the 
  viewer and the only difference between them is that they can be rotated 
  by some angle in the plane of the image. The experimental projections 
  might also be shifted relative to each other, but the centering of 
  the experimental projections is often done before classification.

  is this required?
  E(e1,e2,e3) = E(w,h,i)

  cos(i)cos(h)cos(w)-sin(i)sin(h)    cos(i)cos(h)sin(w)+sin(i)sin(h)  -cos(i)sin(h)
 -sin(i)cos(h)cos(w)-cos(i)sin(h)    sin(i)cos(h)sin(w)+cos(i)sin(h)   sin(i)sin(h)
  sin(h)cos(w)                       sin(h)sin(w)                      cos(h)