【Ray Tracing in One Weekend】（ch8）Metal&Lambertian

ch8：Metal

再回顾一下上一章中求撞击点颜色的Color()方法：

Vec3 Color(const Ray& r, Hitable *world){    hit_record rec;    if (world->hit(r, 0.0, FLT_MAX, rec))    {        Vec3 target = rec.p + rec.normal + RandomInUnitSphere();        //递归，每次吸收50%的能量        return 0.5f*Color(Ray(rec.p, target - rec.p), world);    }    else    {        //绘制背景        Vec3 unit_direction = unit_vector(r.direction());        float t = 0.5f*(unit_direction.y() + 1.0f);        //(1-t)*白色+t*蓝色，结果是一个蓝白的渐变        return (1.0f - t)*Vec3(1.0f, 1.0f, 1.0f) + t*Vec3(0.5f, 0.7f, 1.0f);    }}

这里我们给球体的是Diffuse材质，反射方向是随机的，反射率是0.5，即每次光线射入吸收该光线颜色（三个通道）的50%，所以球体会有灰色的趋向（灰色即RGB三值近似）。现实中的Diffuse材质的物体，都有其自身的颜色，其反射率等参数各不相同，为了方便，现在我们把材质也抽象为一个类：

#pragma once#include "Hitable.h"//通过入射光线，计算反射光线Vec3 Reflect(const Vec3& v, const Vec3& n){    return v - 2 * dot(v, n)*n;}//生成随机方向的标准向量Vec3 RandomInUnitSphere(){    Vec3 p;    do {        p = 2.0f * Vec3((rand() % 100 / float(100)), (rand() % 100 / float(100)), (rand() % 100 / float(100))) - Vec3(1.0f, 1.0f, 1.0f);    } while (dot(p, p) >= 1.0f);    return p;}//抽象出的材质类class Material {public:    virtual bool Scatter(const Ray& r_in, const hit_record& rec, Vec3& attenuation, Ray& scattered) const = 0;};//漫反射材质class Lambertian : public Material {public:    Lambertian(const Vec3& a):albedo(a){}    virtual bool Scatter(const Ray& r_in, const hit_record& rec, Vec3& attenuation, Ray& scattered) const {        Vec3 target = rec.p + rec.normal + RandomInUnitSphere();        scattered = Ray(rec.p, target - rec.p);        attenuation = albedo;        return true;    }    Vec3 albedo;};//镜面反射材质class Metal : public Material {public:    Metal(const Vec3& a, float f) : albedo(a) { if (f < 1) fuzz = f; else fuzz = 1; }    virtual bool Scatter(const Ray& r_in, const hit_record& rec, Vec3& attenuation, Ray& scattered) const    {        Vec3 reflected = Reflect(unit_vector(r_in.direction()), rec.normal);        scattered = Ray(rec.p, reflected + fuzz*RandomInUnitSphere());        attenuation = albedo;        return (dot(scattered.direction(), rec.normal) > 0);    }    Vec3 albedo;    float fuzz;};

将求反射光线的部分放到了材质类的Scatter()方法里，每个材质可以自己定义其反射光线。

同时应注意到我们添加了新的材质类型：Metal，用来表现发生镜面反射的材质。

同时修改Main方法为：

Vec3 Color(const Ray& r, Hitable *world, int depth){    hit_record rec;    if (world->hit(r, 0.001f, FLT_MAX, rec))    {        Ray scattered;        Vec3 attenuation;        if (depth < 50 && rec.mat_ptr->Scatter(r, rec, attenuation, scattered)) {            return attenuation*Color(scattered, world, depth + 1);        }        else {            return Vec3(0.0f, 0.0f, 0.0f);        }    }    else    {        //绘制背景        Vec3 unit_direction = unit_vector(r.direction());        float t = 0.5f*(unit_direction.y() + 1.0f);        //(1-t)*白色+t*蓝色，结果是一个蓝白的渐变        return (1.0f - t)*Vec3(1.0f, 1.0f, 1.0f) + t*Vec3(0.5f, 0.7f, 1.0f);    }}int main(){    ofstream outfile;    outfile.open("ch8Image_fuzz.ppm");    int nx = 200;    int ny = 100;    //采样次数    int ns = 100;    outfile << "P3\n" << nx << " " << ny << "\n255\n";    Hitable *list[4];    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 Metal(Vec3(0.8f, 0.8f, 0.8f),1.0f));    Hitable *world = new HitableList(list, 4);    Camera cam;    //随机数引擎    default_random_engine reng;    uniform_real_distribution<float> uni_dist(0.0f, 1.0f);    for (int j = ny - 1; j >= 0; j--)    {        for (int i = 0; i < nx; i++)        {            Vec3 col(0.0f, 0.0f, 0.0f);            //每个区域采样ns次            for (int s = 0; s < ns; s++)            {                float u = float(i + uni_dist(reng)) / float(nx);                float v = float(j + uni_dist(reng)) / float(ny);                Ray r = cam.getRay(u,v);                //Vec3 p = r.point_at_parameter(2.0);                //将本区域（(u,v)到(u+1,v+1)）的颜色值累加                col += Color(r, world, 0);            }            //获得区域的颜色均值            col /= float(ns);            //gamma矫正            col = Vec3(sqrt(col[0]), sqrt(col[1]), sqrt(col[2]));            int ir = int(255.99*col[0]);            int ig = int(255.99*col[1]);            int ib = int(255.99*col[2]);            outfile << ir << " " << ig << " " << ib << "\n";        }    }    outfile.close();    return 0;}

渲染结果如图：

同样的，我也作死渲了一个高清版：