I am using cluster deformer to simulate paper folding. As far as folding concerned, there is no problem. Everything works fine. Here I am showing a corner folding using cluster deformer.

The problem is when I render it, the back side shows dark area (I expect that it should be white in all area). There is no overlapping between paper since the rotation is only 179 degrees. I am using an ambient light, so there is no shadow involved here.

Any comments would be very much appreciated. Thanks.