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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.

Should be

Now I use them as textures of planes that are stacked up at different heights in Blender, like this.

enter image description here

Here are the material node settings.

enter image description here

I then render them with Cycles under "full global illumination." Here is what I get.

enter image description here

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?

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Your planes are touching, Cycles can't figure out which is in front of what.

You have overlapping planes with or coinciding geometry in your scene.

Organize your blend file, be thorough move them apart in Z direction a consistent amount of units.

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  • $\begingroup$ Hmmm, by touching do you mean within a certain small range that Cycles cannot distinguish? Maybe here I've produced a bad toy example whereby two planes' z values are exactly the same, but in my real application, there definitely won't be exact touching, and I still have this problem. $\endgroup$ – Sibbs Gambling Dec 30 '16 at 3:08
  • $\begingroup$ In the file I downloaded they are touching, exactly coincident, there needs to be a gap between them, the larger the better. $\endgroup$ – Duarte Farrajota Ramos♦ Dec 30 '16 at 3:10
  • $\begingroup$ OK, so I went amiss making this illustration. This is a great point though -- now I suspect there are exact touchings too in my real application. Let me check and get back to you. :-) By the way, BI can handle this touching correctly? I had the same geometry, and the problem showed up when I swtiched to Cylces $\endgroup$ – Sibbs Gambling Dec 30 '16 at 3:12
  • $\begingroup$ No engine, Internal, Cycles, or any other non-blender related can handle overlapping geometry correctly, as far as I know. Sometimes it causes more issues, other less, but it is always considered a bad practice having coinciding geometry regardless. $\endgroup$ – Duarte Farrajota Ramos♦ Dec 30 '16 at 3:20
  • $\begingroup$ Note that if you use an orthographic camera, the planes don't need to be close to each other. $\endgroup$ – lbalazscs Dec 30 '16 at 3:50

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