functions in hex.i - p

    rays= pic3_rays(xpict, ypict, ray)  
 or rays= pic3_rays(xpict, ypict, ray, q_up)  


Like picture_rays, but returns rays in the [p,q] representation  
appropriate for hex5_track.  
(XPICT,YPICT) are 2D arrays of pixel corners in the image plane;  
RAY is the central ray (0,0) in (XPICT,YPICT) coordinates, given  
in [p,q] representation (i.e. RAY is a 3-by-2 array).  The  
optional Q_UP is a 3-vector specifying the orientation of the  
y-axis in the picture plane (see theta_up, phi_up in picture_rays  
for a description of default orientation).  Q_UP must not be  
parallel to RAY(,2).  
Interpreted function, defined at i0/hex.i   line 96