Now that’s puzzling… I already tried RGBA and so-forth, although I am rendering to AVI-Jpeg. What I found is that the blue world-color apparently had alpha so nothing showed through. So did a jet-black color (0/0/0).
Reading the tutorial you cite, I focused on the preceding paragraph:
First, select the background plane, and go to the Material buttons. Press “Env” in the Mirror Transp panel. This makes it so that the background plane will render transparent to the background color, but still obscure anything behind it. Go to the World buttons and set HoR, HoG, and HoB to 0.
Could that, the bold-faced text above, be the “magic” I’m looking for?
The differences between your approach and mine appear to be: (a) you have an explicit background-plane; and (b) you fiddle with the ‘Env’ setting. Is the ‘Env’ setting significant to my problem?
If it doesn’t, then I guess I still don’t understand. You see, I did try both a blue and a zero-black World background color. I did set RGBA. And I tried all sorts of ways with AlphaOver. What I consistently found is that the entire overlay-picture was treated as containing alpha, so the output always consisted only of the … overlay picture. There was no masking taking place.
I also read, in the same tutorial:
Go to the Texture buttons, and add a new texture. […] Press “UseAlpha”. Then go back to the Material buttons. Decrease the Alpha of the material to 0, and press “Alpha” in the Map To panel. Press “ZTransp” in the Mirror Transp panel.
Once again, you’re doing a lot of magic here with stuff like UseAlpha, Map-to-Alpha, and ZTransp and, frankly, whizzing by this stuff at such speed that I perceive that what you’re doing works but you do not pause to really explain why. I feel that there is some very essential ingredient in your soup that is missing in mine, without which things simply do not work, and I have not yet reached that “ah hah” moment…
Please elaborate even more. Thanks very much in advance.