问光线追踪器，阴影光线产生黑色圆圈？

EN

int pos = 0;

float intersect(const ray &r, vector<unique_ptr<object>> &obj)
{
    //gives closest object hit point and position;
    float closest = numeric_limits<float>::max();
    for(int j = 0; j < obj.size(); j++)
    {
        float t = obj[j]->intersect(r);
        if(t > 1e-6 && t < closest)
        {
            closest = t;
            pos = j;
        }
    }
    return closest;
}
vec color(const ray& r, vector<unique_ptr<object>> &shape,  vector<unique_ptr<Light>> &lighting, int depth)
{   
    vec background_color( .678, .847, .902);
    vec total{0.0, 0.0, 0.0};
    vec ambient{0.125, 0.125, 0.125};

    float t_near = intersect(r, shape);

    if(t_near == numeric_limits<float>::max())
            return background_color;
    else
    {
        total += ambient;
        for(int i = 0; i < lighting.size(); i++){
        total += shape[pos]->shade(lighting[i]->position(), t_near, r);//gives specular + diffuse
        vec shadow_dir = unit_vector(lighting[i]->position() - r.p_at_par(t_near));
        ray shadowRay(r.p_at_par(t_near), shadow_dir);
        float dist = shadow_dir.lenght();
        float a = intersect(shadowRay, shape);
        if(a != numeric_limits<float>::max())
                return vec(0.0, 0.0, 0.0);
        }
        return total;
    }
}