// Primitive Declarations
class Primitive {
  public:
    // Primitive Interface
    virtual ~Primitive();
    virtual Bounds3f WorldBound() const = 0;
    virtual bool Intersect(const Ray &r, SurfaceInteraction *) const = 0;
    virtual bool IntersectP(const Ray &r) const = 0;
    ///描述该几何图元是否是光源
    virtual const AreaLight *GetAreaLight() const = 0;
    virtual const Material *GetMaterial() const = 0;
    ///初始化曲面上交点处的光散射材质的性质表示,如BSDF BSSRDF
    ///SurfaceInteraction中存放该点的BSDF 和 BSSRDF的指针
    virtual void ComputeScatteringFunctions(SurfaceInteraction *isect,
                                            MemoryArena &arena,
                                            TransportMode mode,
                                            bool allowMultipleLobes) const = 0;
};

bool GeometricPrimitive::Intersect(const Ray &r,
                                   SurfaceInteraction *isect) const {
    Float tHit;
    if (!shape->Intersect(r, &tHit, isect)) return false;
    r.tMax = tHit;
    isect->primitive = this;
    CHECK_GE(Dot(isect->n, isect->shading.n), 0.);
    // Initialize _SurfaceInteraction::mediumInterface_ after _Shape_
    // intersection
    if (mediumInterface.IsMediumTransition())
        isect->mediumInterface = mediumInterface;
    else
        isect->mediumInterface = MediumInterface(r.medium);
    return true;
}