Well, those words don't quite capture it.

I build a 3D density field. Within that field, there is a lighting gradient. Remember there are no surfaces here at all. Everything is about scattering light within a medium of varying density. Light can reach into the cloud only so far, because it is scattered. Light arriving (directly) on one side of the cloud is stronger than on the other side. However, some of the light that arrives in the middle gets scattered to the edges, so the shadow side is not at all as dark as it would be if there was a real surface with opacity.

Also, when light reaches into the middle part of a cloud, but there is more cloud between you and that part, you have to decrease how much of that you actually see, because the light cannot scatter all the way back out.

I have not fully worked out all the math for this, nor am I happy with the density function itself at the moment, but I'm making progress.

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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)