Here's a tube model of the vertical fibers.

In the center we see through only one layer, and there are gaps.

As we approach the edge, the gaps are eliminated, and they start to overlap. We didn't care about this when they were opaque. But now we have to care.

At first, they only overlap a little, but getting closer to the edge, they overlap more. At some point, we start to have double overlap, where there are places we're looking through three or four tubes. Then four or five, and so on. Eventually, at the very edge, we're looking through many tubes, and they get very dark again, almost opaque. But not quite opaque.

The math for this is ... intense. So I won't show it. The interesting thing about it is that in the statistical sense of a lot of fibers, there are some simplifications we can make. I have made these simplifications in my shader and I'll just demonstrate what it looks like.

I still have a few more tweaks to do to it. When it's ready, I'll publish it.

Now I'll make some demo renders.

---

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)