I posted in a FB groups the same problem with a clearer example.
I try to repeat it here because it is a very simple scene and I can also share the scene (with all property named, so you can understand anything) so that you can understand better which is the point, what I need and, above all, you can test it if you want.
Here is the problem:
How to render reflections passes on a transparent background?
I mean: how to render reflections (coming from light and emissions objects in the scene) shining on an invisible background.
For example, let’s have this simple scene:
In that photo I’d like to composite some text with different shader…; there is a simple background to match the room of the photo, a text shaded with Emission and a normal Text with simple material. Plus 2 area lights where were the original windows of the photo:
But now I have to make invisible the background (Walls and Floor) to match the CGI with the photo backplate. BUT when I enable Shadow Catcher, it kills all reflections and keep only the shadows!
I tried to create different render layers (View Layer) with different set up, but I didn’t find a solution to get the reflections passes (exactly the DiffuseDir, DiffuseIndir, GlossyDir, GlossyIndir) without alpha channel.
Again, the Shadow Catcher seems to kill all reflections and if you disable it, your CGI background (walls, floors…), won’t be transparent.
Here can you find the elementary simple scene:
Reflections_passes_Cycles_Blender 2.90.zip (264.7 KB)
So, maybe if we work all with the same scene it will be easier to fond a solution. Imho.