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I have tried to add a new scene (Merge) and in the compositor I've added the renderlayers from each scene and then merged them with Color>Mix>Screen.

It seems like there should be a better way.

I also tried to use the File Output node without success.

Nodes

Here is some test using the suggested methods, still testing:

Mix Lighten Add Value

Here is the Alpha with the invisible selected. I think this is it!

Alpha with Invisible selected

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  • $\begingroup$ Did you tried with "Add" instead of "screen" ? $\endgroup$ – Polosson Dec 1 '13 at 23:07
  • $\begingroup$ I have edited your question to try and make it less opinion based, please rollback if you disagree with my edits. $\endgroup$ – gandalf3 Dec 4 '13 at 6:39
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If you set Alpha to Transparent in Render Settings > Shading for the BI scene:

enter image description here

The render will have an alpha channel: (Note that you have to enable RGBA in Render Settings > Output to be able to save it with the alpha channel)

enter image description here

(I made the spot lamp red so it would be visible here)

You can then use the Alpha Over node to mix this with a Cycles render:

enter image description here

You could also use the alpha channel as a Factor:

enter image description here

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  • $\begingroup$ Yup, this is exactly what I was trying to get at... except that it's better ;-) $\endgroup$ – Matt Dec 4 '13 at 15:01
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Multiply would be another good option, but what it looks like you really want is an alpha combine (I think you'd plug your alpha output into the factor of the mix node, not sure). Be sure that your world is being rendered to alpha (straight alpha or pre-multiplied alpha, depending on what you're doing), and that should make it much easier to put one image on top of another.

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