INTERFACE PlanetLighting;

(*
  This module provides tools for lighting computations
  on planetary images, such as finding the sun direction
  vector from the position of the terminator, and computing the areal
  illumination and light direction at any point on the planet's
  surface.
*)

TYPE
  T = RECORD
      sunDir: LR3.T;   (* Sun direction, planet coords. *)
      maxLight: LONG;  (* Nominal surface brightness at subsolar point. *)
      minLight: LONG;  (* Nominal surface brightness of dark hemisphere. *)
      bias: LONG;      (* Illumination bias, to shift terminator. *)
    END;
    
PROCEDURE Compute(~)~
(*
  This procedure determines the direction vector "sunDir" of the
  sun, in planet-relative coordinates. This vector is either given
  explicitly by the user (the program merely normalizes it), or
  computed from two points on the terminator (which must be given
  counterclockwise as seen from the lighted hemisphere).  If "D" not
  specified, the sphere is assumed to be illuminated frontally,
  i.e. "sunDir=(0,0,1)".
*)  

END PlanetLighting.