internal_rays

internal_rays(rays)
returns 6element (cos,sin,y,z,x,r) representation of RAYS.
The first dimension of RAYS may be length 3, 5, or 6 to represent
the ray(s) in TDG/DIRT coordinates (x,y,theta), "best" coordinates
(x,y,z,theta,phi), or internal coordinates (cos,sin,y,z,x,r),
respectively. The first dimension of the result always has length 6.
The internal coordinates are what Drat uses internally to
describe the ray. The coordinate system is rotated about the
zaxis until the ray lies in a plane of constant y (there are at
least two ways to do this). The point (x,y,z) can be any point on
the ray, and r=sqrt(x^2+y^2) is the corresponding cylindrical radius.
The clockwise angle theta from the +zaxis to the ray direction
(which always lies in the zxplane) determines cos=cos(theta) and
sin=sin(theta).
As a specification of a ray, this system is triply redundant because
the point (x,y,z) could be any point on the ray, both the sine and
cosine of theta appear, and r=sqrt(x^2+y^2).
However, the slimits parameter  used to specify the points along
a ray where the transport integration starts and stops  is
measured from the point (x,y,z) specified as a part of the
(cos,sin,y,z,x,r) ray coordinate. Thus, any change in the point
(x,y,z) on a ray must be accompanied by a corresponding change in
the slimits for that ray.
interpreted function, defined at i/rays.i line 155
