I do have a technique to solve the shadow problem, but I don't like it because it is very slow.

Here's the technique.

1. A refract node with IOR = 1 basically computes what transparency sees.

2. Use transparency as you want, but never 100%. (This is to let light pass through, but still use some of the shader node values, not just what's behind.)

3. Compute the inverse of your opacity. For example if at a given point the opacity is .02, then the inverse opacity is 50.

4. Plug in a Refract IOR=1 with negative inverse opacity - for my example the amplitude would be -50.

The resulting image transparency data is exactly counteracted by the Refract IOR=1. We now have an object that is black, but lets light pass through as if it is transparent.

Now you just add in the shader you would have used (such as my simple glass shader). Yes that means a second Refract node, now with IOR > 1.

It's a hack but it works.

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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)