constant_medium.h

#ifndef CONSTANT_MEDIUM_H
#define CONSTANT_MEDIUM_H

#include "rtweekend.h"

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

class constant_medium : public hittable {
    public:
	constant_medium(shared_ptr<hittable> b, double d, shared_ptr<texture> a)
	    : boundary(b),
	      neg_inv_density(-1/d),
	      phase_function(make_shared<isotropic>(a))
	    {}

	constant_medium(shared_ptr<hittable> b, double d, color c)
	    : boundary(b),
	      neg_inv_density(-1/d),
	      phase_function(make_shared<isotropic>(c))
	    {}

	virtual bool hit(
	    const ray& r, double t_min, double t_max, hit_record& rec) const override;

	virtual bool bounding_box(double time0, double time1, aabb& output_box) const override {
	     return boundary->bounding_box(time0, time1, output_box);
	}

    public:
	shared_ptr<hittable> boundary;
	shared_ptr<material> phase_function;
	double neg_inv_density;

};

bool constant_medium::hit(const ray& r, double t_min, double t_max, hit_record& rec) const {
    // Print occassional samples when debugging. To enable, set enableDebug true.
    const bool enableDebug = false;
    const bool debugging = enableDebug && random_double() < 0.00001;

    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 (debugging) {
	std::cerr << "\nt_min=" << rec1.t << ", t_max=" << rec2.t << '\n';
    }

    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;
    }

    if (debugging) {
	std::cerr << "hit_distance = " << hit_distance << '\n'
		  << "rec.t = " << rec.t << '\n'
		  << "rec.p = " << rec.p << '\n';
    }

    rec.normal = vec3(1, 0, 0); // arbitrary
    rec.front_face = true;	// also arbitrary
    rec.mat_ptr = phase_function;

    return true;
}

#endif