constant_medium.h

#ifndef CONSTANT_MEDIUM_H
#define CONSTANT_MEDIUM_H

#include "utils.h"
#include "hittable.h"
#include "material.h"
#include "texture.h"
#include "bvh.h"

struct constant_medium : public hittable {
    constant_medium(std::shared_ptr<hittable> b, double d, std::shared_ptr<texture> a)
    : boundary(b), neg_inv_density(-1/d), phase_function(std::make_shared<isotropic>(a)) {}
    constant_medium(std::shared_ptr<hittable> b, double d, color c)
    : boundary(b), neg_inv_density(-1/d), phase_function(std::make_shared<isotropic>(c)) {}
    virtual bool hit(const ray& r, double t_min, double t_max, hit_record& rec) const override {
        hit_record rec1, rec2;
        if (!boundary->hit(r, -infinity, infinity, rec1)) return false;
        if (!boundary->hit(r, rec1.t+0.0001, infinity, rec2)) return false;
        if (rec1.t < t_min) rec1.t = t_min; if (rec2.t > t_max) rec2.t = t_max;
        if (rec1.t >= rec2.t) return false;
        if (rec1.t < 0) rec1.t = 0;
        const auto ray_length = r.direction().length();
        const auto distance_inside_boundary = (rec2.t - rec1.t) * ray_length;
        const auto hit_distance = neg_inv_density * log(random_double());
        if (hit_distance > distance_inside_boundary) return false;
        rec.t = rec1.t + hit_distance/ray_length; rec.p = r.at(rec.t);
        rec.normal = vec3(1,0,0); rec.front_face = true; // arbitrary
        rec.mat_ptr = phase_function;
        return true;
    }
    virtual bool bounding_box(double time0, double time1, aabb& output_box) const override {
        return boundary->bounding_box(time0, time1, output_box);
    }
    std::shared_ptr<hittable> boundary;
    std::shared_ptr<material> phase_function;
    double neg_inv_density;
};

#endif