moving_sphere.h

#ifndef MOVING_SPHERE_H
#define MOVING_SPHERE_H

#include "hittable.h"
#include "vec3.h"

struct moving_sphere : public hittable {
    moving_sphere() {}
    moving_sphere(point3 cen0, point3 cen1, double _time0, double _time1, double r, std::shared_ptr<material> m)
    : center0(cen0), center1(cen1), time0(_time0), time1(_time1), radius(r), mat_ptr(m) {};
    point3 center0, center1;
    double time0, time1, radius;
    std::shared_ptr<material> mat_ptr;
    bool hit(const ray& r, double t_min, double t_max, hit_record& rec) const {
        vec3 oc = r.origin() - center(r.time()); // (A + tB - C).(A + tB - C) = r^2 => B^2t^2 + 2ABt + A^2 - r^2 = 0
        auto a = r.direction().length_squared(), half_b = dot(oc, r.direction()), c = oc.length_squared() - radius*radius; 
        auto discriminant = half_b*half_b - a*c;
        if (discriminant < 0) return false;
        auto sqrtd = sqrt(discriminant), root = (-half_b - sqrtd)/a;
        if (root < t_min || t_max < root) {
            root = (-half_b + sqrtd)/a;
            if (root < t_min || t_max < root) return false;
        }
        rec.t = root;
        rec.p = r.at(rec.t);
        auto outward_normal = (rec.p - center(r.time()))/radius;
        rec.set_face_normal(r, outward_normal);
        rec.mat_ptr = mat_ptr;
        return true;
    }
    point3 center(double time) const {return center0 + (time - time0)/(time1 - time0)*(center1 - center0);}
    bool bounding_box(double _time0, double _time1, aabb& output_box) const {
        aabb box0(center(_time0) - vec3(radius, radius, radius), center(_time0) + vec3(radius, radius, radius));
        aabb box1(center(_time1) - vec3(radius, radius, radius), center(_time1) + vec3(radius, radius, radius));
        output_box = surrounding_box(box0, box1);
        return true;
    }
};

#endif