一、先上图：
左边整个渲染过程耗时：1小时
右边整个渲染过程耗时：35小时（未用多线程）
还是右边好看点（tmd当然了）

二：代码

Vector3f Scene::castRay(const Ray &ray, int depth) const
{
     Vector3f L_dir = { 0,0,0 };
     Vector3f L_indir = { 0,0,0 };
 
     Intersection intersection = Scene::intersect(ray);
     if (!intersection.happened)
     {
         return {};
     }
     //打到光源
     if (intersection.m->hasEmission())
     return intersection.m->getEmission();
 
     //打到物体后对光源均匀采样
     Intersection lightpos;
     float lightpdf = 0.0f;
     sampleLight(lightpos, lightpdf);//获得对光源的采样，包括光源的位置和采样的pdf
 
     Vector3f collisionlight = lightpos.coords - intersection.coords;
     float dis = collisionlight.squrenorm();
     Vector3f collisionlightdir = collisionlight.normalized();
     Ray objray(intersection.coords, collisionlightdir);
 
     Intersection ishaveobj = Scene::intersect(objray);
     //L_dir = L_i * f_r * cos_theta * cos_theta_x / |x - p | ^ 2 / pdf_light
     if (ishaveobj.distance - collisionlight.norm() > -EPSILON)//说明之间没有遮挡
     L_dir = lightpos.emit * intersection.m->eval(ray.direction, collisionlightdir, intersection.normal) * dotProduct(collisionlightdir, intersection.normal) * dotProduct(-collisionlightdir, lightpos.normal) / dis / lightpdf;
 	 打到物体后对半圆随机采样使用RR算法
     if (get_random_float() > RussianRoulette)
     return L_dir;
 
     Vector3f w0 = intersection.m->sample(ray.direction, intersection.normal).normalized();
     Ray objrayobj(intersection.coords, w0);
     Intersection islight = Scene::intersect(objrayobj);
     // shade(q, wi) * f_r * cos_theta / pdf_hemi / P_RR
     if (islight.happened && !islight.m->hasEmission())
     {
         float pdf = intersection.m->pdf(ray.direction, w0, intersection.normal);
         L_indir = castRay(objrayobj, depth + 1) * intersection.m->eval(ray.direction, w0, intersection.normal) * dotProduct(w0, intersection.normal) / pdf / RussianRoulette;
     }
 
     return L_dir + L_indir;
}