Class Room Demo Scene Black Windows? I just wonder if anyone know how to fix this issue? When I render the Class Room Scene, the windows often shown up as black (or checker boxes)? Apparently the ray tracing part is not hitting anything at those regions? 

However, when I look at the demo image from the website they have corrected those? Any ideas?
Ok, I figured it out. The black is actually Alpha. So the quickest way to fix it is to patch it up with white. 

Also, I notice the final result used the composite feature! It was not enabled by default. And that's why it looked different.