Here's a render with the camera pulled back to reveal the setup. I've intentionally messed up the edge foam. My sky dome here is not transparent, so the ray bias is picking up the brown bottom of the sky dome (because it skipped the sand). I made the ray bias 20 here instead of 1 to show you how you can easily see the edge effect from the ray bias. Ignore the gritty sand - I accidentally made the bump map too strong in this one.

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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)