我在光子映射中用俄罗斯轮盘做错了什么？

Question

我不能让我的对象变得透明，我认为俄罗斯轮盘赌是第4部分出了问题。找出光子继续存在的可能性。

    // Trace a ray from the light

   {
      // 1. Find first object hit by photon.
      // 2. Store the photon if it has already has a specular reflection and if this is a diffuse surface.
   }  

   // 3. Stop if at max depth.
   if (depth == 0) 
      return; 

   // 4. Find the probability that the photon continues.
   vec3 pd = vec3(mat->ks.x + mat->kd.x, mat->ks.y + mat->kd.y, mat->ks.z + mat->kd.z);
   float prob;
   if (pd.x > pd.y && pd.x > pd.z)
      prob = pd.x;
   else if (pd.y > pd.x && pd.y > pd.z) 
      prob = pd.y;
   else 
      prob = pd.z;
   float p_s = prob * (mat->ks.x + mat->ks.y + mat->ks.z) / (mat->ks.x + mat->ks.y + mat->ks.z + mat->kd.x + mat->kd.y + mat->kd.z) ;
   float p_d = prob * (mat->kd.x + mat->kd.y + mat->kd.z) / (mat->ks.x + mat->ks.y + mat->ks.z + mat->kd.x + mat->kd.y + mat->kd.z);

   // 5. Generate random number.  If that shows that the photon dies, return.
   float rnd = randIn01() ;
   if (rnd > prob) 
      return;
   // 6. Probabilistically determine the action (one of diffuse or
   //      specular reflection for all surfaces, plus refracted
   //      transmission for non-opaque surfaces).  Based on the action,
   //      compute the power loss (maybe!) and a direction to follow.

}

我在这里做错了什么？