There are the nodes DU and DV which (indirectly) tell me how far something is from the camera. They are a bit difficult to use, but I have used them. For example, I made a variation of my Loom that does procedural anti-aliasing based on apparent size in a render. This means that I intentionally drop a lot of thread detail when you view something from farther away or you turn it so it is not facing the camera anymore. I never published it, but it made a big difference. Otherwise, you had to drastically increase pixel samples and decrease shading rate a lot to keep up with all that busy detail if you were using thread colors with a lot of contrast.

The gravity sag is very easy. You just use an N node to find out which way the geometry is facing, and adjust displacement accordingly. Controlling the amount of that with a mask would be required. Good idea.

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Renderosity forum reply notifications are wonky. If I read a follow-up in a thread, but I don't myself reply, then notifications no longer happen AT ALL on that thread. So if I seem to be ignoring a question, that's why. (Updated September 23, 2019)