Original .blend file: https://www.dropbox.com/s/dlipv36yu28rk46/SE.blend?dl=0 (~120MB)
I manually segment out human body parts from an image and make each body part a .png image where the alpha channel is 1 for that body part and 0 anywhere else. So for example, I have a RGBA "leg image" as follows.
Now I use them as textures of planes that are stacked up at different heights in Blender, like this.
Here are the material node settings.
I then render them with Cycles under "full global illumination." Here is what I get.
Sorry for the typo -- should be right arm/leg
As you can see some body parts (e.g., right arm) look transparent, and some even completely disappear. Why does this happen, and how can I fix it?