如下圖的旗幟、大樓與路樹：
而在加入質量光後卻會因光線透不過去而破功：
在 CG Talk 上對此有一串討論:
A way to shine volume light rays through alpha masked objects?
Use mental ray, and lights with ray traced shadows. It should work.
See the attached pic I did for a test.
裡面提到改用 mental ray 算圖再將燈光的影子使用 ray traced shadows 可以改善。
後面更有人回應這個現象應該是個 bug，藉由更改 Volume Light 與 Material 的設定可以稍微的減輕此問題。
Looks like its a minor bug. I found a workaround after a bit of wild experimentation.
Pic #1 : Density =1.0, Max Light% = 50. You can see a bright sphere, where there should be none. It must be transperent.
Pic #2 : Density = 1.0, Max Light% = 100. The sphere is gone.
Pic #3 : Density = 1.0, Max Light% = 100, Fog color = (127,127,127). In case you want it less bright, reduce the value of fog color instead of MaxLight or density.
Pic #4 : Density = 0.2, Maxlight% = 100. The sphere reappers, this time a dark one. Changing Maxlight wont do anything.
Pic #5 : Density = 0.2, Maxlight% = 100, Specular Level = 110, Glossiness = 8. Its gone. (Dont ask me why.) This may not work with other nonspherical objects.
