lightmapIndex in prefeb always equals -1 in unity5.01f1

1.Bake a scene object, its render component's lightmapIndex equals 0;
2.Make it to prefeb object;
3.Drag it from project windows to scene;
4.Its render component's lightmapIndex equals -1.
why?

 

Because, I don't know the technical reason for it, I think it's more to do with that the scene keeps track of it all and sets the object what to use rather than the object and that's why it's not tracked when prefabbed. You'd need a script on the object that stores the lightmap index and offset etc and set them on Start(). Even then that'd be scene dependant so you'd want it to track what lightmap index goes with what scene name (meaning you'd have to bake it and set it in every scene that prefab would go in). I do have a script that does that somewhere but it ended up causing weird lightmap issues after a rebake, although I think that was to do with "Generate Lightmap UVs" unchecked in the model import settings. Can't find it at the moment though, but it was basically a List of all the scene names and a List of all the lightmap indexes etc that shared the same [] index.

 

Same operation on unity 4.xxx is correct.