I note that you have boxed yourself into the front-back paradigm when you don't even want that. The "front" of your leaf seems to be facing down at times, and this isn't what you are after. Instead, you should be shading one way or another based on up or down. The top of the leaf is whichever face is on top, right? ...

Now that you've pointed it out...

Using the N node rang a very loud bell - Eventmobil's 'dust-on-top' [nudity warning - not that post, but the thread, which was about "slightly soapy water"] and snarlygribbley's snow machine. I've even used the N-node like that myself, but never thought it could be relevant here. I'll try that tomorrow.

I've run into a separate problem with back-lit one-sided meshes (as opposed to single faces, like my leaf). The mesh lines appear in the render (back-viewed or front-viewed) but only if I use ray-traced shadows. Depth-mapped shadows seem okay. I'm sure that must have been covered many times, but I can't find anything.

(P.S. I don't seem to be getting notification messages for new posts now.)

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The 3Dcheapskate* occasionally posts sensible stuff. Usually by accident.
And it usually uses Poser 11, with units set to inches. Except when it's using Poser 6 or PP2014, or when its units are set to PNU.

*also available in ShareCG, DAZ, and HiveWire3D flavours (the DeviantArt and CGBytes flavour have been discontinued).