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 94