pbrt-perspective-we(Weight emit)
Float GenerateRay(const CameraSample &sample, Ray *) const;
Float GenerateRayDifferential(const CameraSample &sample,
RayDifferential *ray) const;
// 因为 Camera 发射光线是返回权重的,因此每一条射线都是有权重的,因此使用名字 We (Weight emit)
Spectrum We(const Ray &ray, Point2f *pRaster2 = nullptr) const;
src\cameras\perspective.h