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# functions in rays.i - b

 best_rays ``` best_rays(rays) returns 5-element (x,y,z,theta,phi) 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 5. The "best" coordinate system is the easiest to visualize: (x,y,z) represents any point on the ray, while (theta,phi) represents the ray direction in standard spherical coordinates relative to the +z-axis. Namely, theta is the angle from the +z-direction to the ray direction (between 0 and pi), and phi is the counterclockwise angle from the +x-axis to the projection of the ray direction into the xy-plane, assuming xyz is a right-handed coordinate system. As a specification of a ray, this system is doubly redundant because the point (x,y,z) could be any point on the ray, and the underlying mesh through which the ray propagates is cylindrically symmetric about the z-axis. 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 (x,y,z,theta,phi) 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 40 ``` SEE ALSO: form_rays,   dirt_rays,   internal_rays,   get_s0,   picture_rays