【Ray Tracing in One Weekend】（ch9）Dielectrics

Chapter 9: Dielectrics

这章是关于折射的，看不太懂，这里转一篇别人的技术文章，讲的很好：

http://blog.youkuaiyun.com/libing_zeng/article/details/72599121

我在这就贴贴代码和图片吧。

首先是在 Material.h 中，新添如下代码：

float Schlick(float cosine, float ref_idx) {
    float r0 = (1 - ref_idx) / (1 + ref_idx);
    r0 = r0*r0;
    return r0 + (1 - r0)*pow((1 - cosine), 5);
}

//计算折射光线的方向向量。ni_over_nt 为入射介质的折射指数和折射介质的折射指数的比值。
bool Refract(const Vec3& v, const Vec3& n, float ni_over_nt, Vec3& refracted) {
    Vec3 uv = unit_vector(v);
    float dt = dot(uv, n);
    float discriminant = 1.0 - ni_over_nt*ni_over_nt*(1 - dt*dt);
    if (discriminant > 0) {
        refracted = ni_over_nt*(uv - n*dt) - n*sqrt(discriminant);
        return true;
    }
    //根号里面的内容小于零，说明折射光线的方向向量无实根，即没有折射光线，即出现全反射。所以，折射光线函数return false
    else
        return false;
}

class Dielectric : public Material {
public:
    Dielectric(float ri) : ref_idx(ri) {}
    virtual bool Scatter(const Ray& r_in, const hit_record& rec, Vec3& attenuation, Ray& scattered) const {
        Vec3 outward_normal;
        Vec3 reflected = Reflect(r_in.direction(), rec.normal);
        float ni_over_nt;
        attenuation = Vec3(1.0, 1.0, 1.0);
        Vec3 refracted;
        float reflect_prob;
        float cosine;
        if (dot(r_in.direction(), rec.normal) > 0) {
            outward_normal = -rec.normal;
            ni_over_nt = ref_idx;
            //cosine = ref_idx * dot(r_in.direction(), rec.normal) / r_in.direction().length();
            cosine = dot(r_in.direction(), rec.normal) / r_in.direction().length();
            cosine = sqrt(1 - ref_idx*ref_idx*(1 - cosine*cosine));
        }
        else {
            outward_normal = rec.normal;
            ni_over_nt = 1.0 / ref_idx;
            cosine = -dot(r_in.direction(), rec.normal) / r_in.direction().length();
        }
        if (Refract(r_in.direction(), outward_normal, ni_over_nt, refracted))
            reflect_prob = Schlick(cosine, ref_idx);
        else
            reflect_prob = 1.0;
        if ((rand() % (100) / (float)(100)) < reflect_prob)
            scattered = Ray(rec.p, reflected);
        else
            scattered = Ray(rec.p, refracted);
        return true;
    }

    float ref_idx;
};

Main方法中代码修改如下：

    Hitable *list[5];
    list[0] = new Sphere(Vec3(0.0f, 0.0f, -1.0f), 0.5f, new Lambertian(Vec3(0.8f, 0.3f, 0.3f)));
    list[1] = new Sphere(Vec3(0.0f, -100.5f, -1.0f), 100.0f, new Lambertian(Vec3(0.8f, 0.8f, 0.0f)));
    list[2] = new Sphere(Vec3(1.0f, 0.0f, -1.0f), 0.5f, new Metal(Vec3(0.8f, 0.6f, 0.2f),0.3f));
    list[3] = new Sphere(Vec3(-1.0f, 0.0f, -1.0f), 0.5f, new Dielectric(1.5f));
    list[4] = new Sphere(Vec3(-1.0f, 0.0f, -1.0f), -0.45f, new Dielectric(1.5f));
    Hitable *world = new HitableList(list, 5);

所得如图：