Hi folks. Meetings, meetings, meetings. I'm about to go back in, so I didn't have time to develop and test methods #2 and #3. But, I'll present method #1 (using a negative color and transparency) for you. I will not explain the math, unless somebody asks for it. It's not too tricky, just basic algebra. But you don't really need to know it to use it.

I have designed the shader so that we can take all the guess work out of the process. Just follow the steps I outline and you should be all set.

Again, I'm not going to explain here how to generate the shadows in the right perspective. We can talk about that later. 

You can use a square or the ground plane for your catcher. I prefer a square as I can move it and scale it more easily. Here we go.

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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)