multithreaded-raytracer/include/material.h

#ifndef MATERIAL_H
#define MATERIAL_H

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

class material {
  public:
    virtual ~material() = default;

    virtual bool scatter(
        const ray& r_in, const hit_record& rec, color& attenuation, ray& scattered
    ) const {
        return false;
    }
};


class lambertian : public material {
  public:
    lambertian(const color& albedo) : albedo(albedo) {}

    bool scatter(const ray& r_in, const hit_record& rec, color& attenuation, ray& scattered)
    const override {
        auto scatter_direction = rec.normal + random_unit_vector();
              // Catch degenerate scatter direction
        if (scatter_direction.near_zero())
            scatter_direction = rec.normal;

        scattered = ray(rec.p, scatter_direction);
        attenuation = albedo;
        return true;
    }

  private:
    color albedo;
};


class metal : public material {
  public:
    metal(const color& albedo, double fuzz) : albedo(albedo), fuzz(fuzz<1? fuzz : 1) {}

    bool scatter(const ray& r_in, const hit_record& rec, color& attenuation, ray& scattered)
    const override {
        vec3 reflected = reflect(r_in.direction(), rec.normal);
        reflected = unit_vector(reflected) + (fuzz*random_unit_vector());
        scattered = ray(rec.p, reflected);
        attenuation = albedo;
        return (dot(scattered.direction(), rec.normal)>0);
    }

  private:
    color albedo;
    double fuzz;
};

class dielectric : public material {
  public:
    dielectric(double refraction_index) : refraction_index(refraction_index) {}

    bool scatter(const ray& r_in, const hit_record& rec, color& attenuation, ray& scattered)
    const override {
        attenuation = color(1.0, 1.0, 1.0);
        double ri = rec.front_face ? (1.0/refraction_index) : refraction_index;

        vec3 unit_direction = unit_vector(r_in.direction());
       double cos_theta = std::fmin(dot(-unit_direction, rec.normal), 1.0);
        double sin_theta = std::sqrt(1.0 - cos_theta*cos_theta);

        bool cannot_refract = ri * sin_theta > 1.0;
        vec3 direction;

        if (cannot_refract || reflectance(cos_theta, ri)> random_double())
            direction = reflect(unit_direction, rec.normal);
        else
            direction = refract(unit_direction, rec.normal, ri);

        scattered = ray(rec.p, direction);
        return true;
    }

  private:
    // Refractive index in vacuum or air, or the ratio of the material's refractive index over
    // the refractive index of the enclosing media
    double refraction_index;
      static double reflectance(double cosine, double refraction_index) {
        // Use Schlick's approximation for reflectance.
        auto r0 = (1 - refraction_index) / (1 + refraction_index);
        r0 = r0*r0;
        return r0 + (1-r0)*std::pow((1 - cosine),5);
    }

};

#endif