get screen coordinates from c4d

hi.
i'm trying to tell fusion where [in 2d screen space] a c4d object is located in the render from c4d.
does that make sense? i'm exporting an exr sequence from c4d. the c4d scene has lots of objects in it. i want to pin an effect [like, a circle mask, as an example] to the spot on the screen where a given c4d object is being seen by the c4d camera in the c4d render.
back in the aftereffects days, i could do this with the c4d external compositing tag, but this approach doesn't seem to work for fusion...or at least i can't figure out how to get it to work.
any help would be appreciated.
thanks.

You can import your c4d scene as alembic, or fbx, just a null with the position + your camera is enough as well. And then use the locator in Fusion to place it in the same spot. Now you can connect anything to that location in screen space.

Do you have a sample frame with that external compositing tag? Something that verifiably works as expected in AE? It might be possible to reverse engineer something.

But as UserNoah said, Alembic might be a better way to go. Fusion has a full 3d system, unlike AE's janky 2.5d environment, so you have more options.

There are some tricky bits to the C4D alembic -> Fusion pipe, though. First, Cinema's angle of view value is not the same as Fusion's. After you import the Alembic (use file > import > alembic scene. Don't just add an Alembic node), you will probably need to unlock the camera and change the Angle of View Type from Vertical to Horizontal.

There's a camera tag available in C4D that invisible modifies the angle of view. I want to say the camera calibration tag? Anyway, if you have that, you'll need to remove it to get a correct match.