Long story short, I have Prim_IDs written into a 32bit exr channel.
I want to be able to delete geo primitives that do not show up in that EXR channel.

I'm still a noob so any tips would be appreciated no matter how high level. Is it even possible to index all the render-present Prim_Ids and then translate them into an index that SOPs or VEX can operate on?

Note: I had built a pretty cool set up that did traces from points on each primitive back back to the camera, but in order to avoid false positives and negatives without an absurd number of traces I thought the raster solution would be worth it.