After testing inside the game, we have just realised that we need to take out all the doors and windows in order to make them interactivity because they are seen to be only a solid object constrained to the building. We are planning to make them openable and also breakable. Thus, I have to remove all the doors and windows in SketchUp, import and reassign all materials again. I found that once I have repeated the process for many times, it becomes easier and takes less time than before.
Figure 1: The Textured Model without Windows and Doors
