first_proj_axis

EXPRESS specification:

FUNCTION first_proj_axis
(* Modified for LPM/6 *)(z_axis, arg : direction) : direction; LOCAL x_axis : direction; v : direction; z : direction; x_vec : vector; END_LOCAL; IF (NOT EXISTS(z_axis)) THEN RETURN (?) ; ELSE z := normalise(z_axis); IF NOT EXISTS(arg) THEN IF ((z.direction_ratios <> [1.0,0.0,0.0]) AND (z.direction_ratios <> [-1.0,0.0,0.0])) THEN v := dummy_gri || direction([1.0,0.0,0.0]); ELSE v := dummy_gri || direction([0.0,1.0,0.0]); END_IF; ELSE IF (arg.dim <> 3) THEN RETURN (?) ; END_IF; IF ((cross_product(arg,z).magnitude) = 0.0) THEN RETURN (?); ELSE v := normalise(arg); END_IF; END_IF; x_vec := scalar_times_vector(dot_product(v, z), z); x_axis := vector_difference(v, x_vec).orientation; x_axis := normalise(x_axis); END_IF; RETURN(x_axis);
END_FUNCTION;