; docformat = 'rst'
;+
;This function translates input vector, V, through transformation 
;quaternion, q.
;
; :Author:
;   Brian Boyle: 11-14-1996
;   
; :Examples:
;   res_vec = quat_vect_mult(quat, vec)
;-
;+
; :Params:
;   q : in, required, type=vector of double
;     Single transformation quaternion (Euler parameters).
;   v : in, required, type=vector of double or array of vectors
;     Vector grid to be transformed (dblarr(*, *,  3), vector is 3D.
;     
; :Returns:
;   Returns the quaternion transformed 3D vector or vector array. 
;-
function quat_vect_mult,q,v
;
; Check input parameters...
;
if n_params() ne 2 then begin
  message,/info,'USAGE: result=quat_vect_mult(q,v)'
  return,-1
endif
;
; Compute transformation
;
e4=q[3]
e=q[0:2]
result= v + 2d*( e4*crossp_grid(e,v) + crossp_grid(e,crossp_grid(e,v)) )
;
; Done
;
return,result
end