onb.h

#ifndef ONB_H
#define ONB_H

#include "rtweekend.h"

class onb {
    public:
        onb(){}

        inline vec3 operator[](int i) const {return axis[i];}

        vec3 u() const {return axis[0];}
        vec3 v() const {return axis[1];}
        vec3 w() const {return axis[2];}

        vec3 local(double a, double b, double c) const {
            return a*u()+b*v()+c*w();
        }

        vec3 local(const vec3& a) const {
            return a.x()*u()+a.y()*v()+a.z()*w();
        }

        void build_from_w(const vec3&);

    public:
        vec3 axis[3];
};

void onb::build_from_w(const vec3& n){
    axis[2] = unit_vector(n);
    vec3 a = (fabs(w().x()) > 0.9) ? vec3(0, 1, 0) : vec3(1, 0, 0);
    axis[1] = unit_vector(cross(w(), a));
    axis[0] = cross(w(), v());
}
#endif