RTIOW-ch12：Where next

制作书籍的封面图： cpp:

#include
#include
#include
#include “sphere.h”
#include “hitable_list.h”
#include “camera.h”
#include “material.h”

using namespace std;

//获得反射射线
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;

}

vec3 Color(const ray& r, hitable* world, int depth)
{
//这个“rec”会在sphere::hit ()中带上来被撞击球的材料属性（指向一个材质对象的指针mat_ptr）。
//根据这个指针可以获取材料对象的成员方法scatter()和成员变量albedo（反射衰减向量）
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))
{
//获取反射光线向量scattered和反射衰减向量attenuation
return attenuation * Color(scattered, world, depth + 1);
//反射光线的强度需要乘以反射衰减向量（对应坐标相乘作为新的向量）。
//然后反射光线就扮演之前“原始光线”的角色。如果再次撞击到小球，就再次反射，直到不再撞击到任何球为止
}
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);

    //线性混合，t=1时蓝色，t=0时白色，t介于中间时是混合颜色
    //blended\_value = (1-t)\*start\_value + t\*end\_value
    return (1.0f - t) \* vec3(1.0f, 1.0f, 1.0f) + t \* vec3(0.5f, 0.7f, 1.0f);

    //注意这里，原始光线和反射光线最后都会跑到这里来。
    //背景的颜色：原始光线的方向向量的映射
    //漫反射材料和镜面材料的颜色：最后一次反射光线的方向向量的映射 \*  所有反射衰减系数的乘积。
    //漫反射和镜面反射的区别在于，漫反射的每次反射方向是随机的
}

}

hitable* RandomScene()
{
int n = 500;
hitable** list = new hitable*[n + 1];
/*定义一个包含n+1个元素的数组，数组的每个元素是指向hitable对象的指针。然后将数组的指针赋值给list。所以，list是指针的指针。*/
list[0] = new sphere(vec3(0, -1000, 0), 1000, new lambertian(vec3(0.5, 0.5, 0.5)));
/*先创建一个中心在（0，-1000，0）半径为1000的超大漫射球，将其指针保存在list的第一个元素中。*/
int i = 1;
for (int a = -11; a < 11; a++)
{
for (int b = -11; b < 11; b++)
{
/*两个for循环中会产生（11+11）*(11+11)=484个随机小球*/
float choose_mat = (rand() % (100) / (float)(100));
/*产生一个（0，1）的随机数，作为设置小球材料的阀值*/
vec3 center(a + 0.9 * (rand() % (100) / (float)(100)), 0.2, b + 0.9 * (rand() % (100) / (float)(100)));
/*” a+0.9*(rand()%(100)/(float)(100))”配合[-11,11]产生（-11，11）之间的随机数，而不是[-11,11)之间的22个整数。使得球心的x,z坐标是（-11，11）之间的随机数*/
if ((center - vec3(4, 0.2, 0)).length() > 0.9)
{
/*避免小球的位置和最前面的大球的位置太靠近*/
if (choose_mat < 0.8)
{
//diffuse
/*材料阀值小于0.8，则设置为漫反射球，漫反射球的衰减系数x,y,z都是（0，1）之间的随机数的平方*/
list[i++] = new sphere(center, 0.2,
new lambertian(vec3(
(rand() % (100) / (float)(100)) * (rand() % (100) / (float)(100)),
(rand() % (100) / (float)(100)) * (rand() % (100) / (float)(100)),
(rand() % (100) / (float)(100)) * (rand() % (100) / (float)(100)))));
}
else if (choose_mat < 0.95)
{
/*材料阀值大于等于0.8小于0.95，则设置为镜面反射球，镜面反射球的衰减系数x,y,z及模糊系数都是（0，1）之间的随机数加一再除以2*/
list[i++] = new sphere(center, 0.2,
new metal(vec3(0.5 * (1 + (rand() % (100) / (float)(100))),
0.5 * (1 + (rand() % (100) / (float)(100))),
0.5 * (1 + (rand() % (100) / (float)(100)))),
0.5 * (1 + (rand() % (100) / (float)(100)))));
}
else
{
/*材料阀值大于等于0.95，则设置为介质球*/
list[i++] = new sphere(center, 0.2, new dielectric(1.5));
}
}
}
}

list\[i++\] = new sphere(vec3(0, 1, 0), 1.0, new dielectric(1.5));
list\[i++\] = new sphere(vec3(-4, 1, 0), 1.0, new lambertian(vec3(0.4, 0.2, 0.1)));
list\[i++\] = new sphere(vec3(4, 1, 0), 1.0, new metal(vec3(0.7, 0.6, 0.5), 0.0));
/\*定义三个大球\*/
return new hitable\_list(list, i);

}

//And add some metal spheres
int main()
{
ofstream outfile;
outfile.open(“IMG.ppm”);

int nx = 2000;
int ny = 1000;
//采样次数
int ns = 100;
outfile << "P3\\n" << nx << " " << ny << "\\n255\\n";

hitable\* world = RandomScene();

vec3 lookform(13.0f, 2.0f, 3.0f);
vec3 lookat(0, 0, 0);
float dist\_to\_focus = (lookform - lookat).length();
float aperture = 0.0f;

camera cam(lookform, lookat, vec3(0, 1, 0), 20, float(nx) / float(ny), aperture, 0.7 \* dist\_to\_focus);

//随机数
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;

}

最终运行效果： 200乘100大小，每个像素点10条采集光线，运行时间2分钟： 2000乘1000大小，每个像素点100条采集光线，运行时间14小时：虚拟世界真是有趣，再放一张没有水印的：我们已经有了一个很酷的光线追踪器，接下来做些什么呢？ 1. Lights. You can do this explicitly, by sending shadow rays to lights. Or it can be done implicitly by making some objects emit light, 2. biasing scattered rays toward them, and then downweighting those rays to cancel out the bias. Both work. I am in the minority in favoring the latter approach. 3. Triangles. Most cool models are in triangle form. The model I/O is the worst and almost everybody tries to get somebody else’s code to do this. 4. Surface textures. This lets you paste images on like wall paper. Pretty easy and a good thing to do. 5. Solid textures. Ken Perlin has his code online. Andrew Kensler has some very cool info at his blog. 6. Volumes and media. Cool stuff and will challenge your software architecture. I favor making volumes have the hitable interface and probabilistically have intersections based on density. Your rendering code doesn’t even have to know it has volumes with that method. 7. Parallelism. Run N copies of your code on N cores with different random seeds. Average the N runs. This averaging can also be done hierarchically where N/2 pairs can be averaged to get N/4 images, and pairs of those can be averaged. That method of parallelism should extend well into the thousands of cores with very little coding. 参考书籍：《Ray Tracing in One Weekend》 RTIOW系列项目地址：GitHub RTIOW系列笔记： RTIOW-ch1：Output an image RTIOW-ch2：The vec3 class RTIOW-ch3：Rays, a simple camera, and background RTIOW-ch4：Adding a sphere RTIOW-ch5：Surface normals and multiple objects RTIOW-ch6：Antialiasing RTIOW-ch7：Diffuse Materials RTIOW-ch8：Metal RTIOW-ch9：Dielectrics RTIOW-ch10：Positionable camera RTIOW-ch11：Defocus Blur RTIOW-ch12：Where next