Okay, I've made some progress. What I've done is render a seperate object buffer for the sphere and the rotating objects, made the sphere completely transparent, and set it up like this:
rotating object buffer (hidden)
sphere buffer (hidden)
2d image (luma matte "sphere buffer")
full rendered image
I used text as a placeholder (for the 2d image I'll add later) and it seems to be working as intended except the text's brightness flashes. I looked at it frame by frame and the flashes do not correspond with any of the objects passing in front of or behind the text, it just flashes randomly...
I looked at it frame by frame and the flashes do not correspond with any of the objects passing in front of or behind the text,
They probably do due to how you set it up in C4D... Even on 100% transparent objects, you still havce specualr and other shading, resulting in different brightnesses. Simply use the Invisible to Camera checkbox in the compositing tag to make your objects disappear. That aside, you are oversimplifying. to get correct obscuration, you will still need to separate your 3D stuff by depth and that will require rendering a depth map of sorts or using e.g. the Proximal shader to hide them based on distance to the camera. in eitehr case you will probalbly need two scenes to be able to sandwich things together correctly in AE...
Thanks for the tip about the compositing tag, I don't know why I didn't think of that.
As for the sandwiching idea, I've pondered on it and abandoned it. The animation has the same objects moving in front of and behind the 2d layer, so making duplicate layers won't work (although I don't know anything about the depth map and all that).
edit: unchecking the "seen by camera" box in the compositing tag seems to make the object buffer null too. So unfortunately it doesn't help .
I think I got it lol. It's kind of a clusterf#&$, but here's what I did. I made the sphere in the center 99% transparent (100% seems to affect the object buffer) and then rendered out twice. Once with the "seen by transparency" box unchecked in the compositing tag and once with it checked. This made the objects dissappear behind the transparent sphere in one of the renders and look normal in the other.
I then set up after effects like so:
(edit: render 1 has the objects disappearing behind the sphere, render 2 is the normal render)
rotating objects buffer from render 1
full render image of render 2 (luma matte to rotating objects buffer)
sphere object buffer
text (alpha matte to sphere buffer)
full render image of render 1
I'm 100% sure there is a better and easier way to do this lol, but I got it and I'm leaving it as is.