How to detect several agents within a radius, what is the best and easiest way to do it?

Question

I have a room with 30 objects which may or not be present, and I want to be able to detect which objects are in the room.

My first idea was to attach a RFID tag to each one of these objects and have a sensor detecting them, but then I have frequency collision problems and since the room is quite large, I also cannot activate tags which are further away. Or I may be missing something...

I would also want to have objects not carrying an active radio transmitter, i.e., no batteries wandering around, just RFID tags or resonant metal pieces.

I could use cameras but image processing would require an Arduino on steroids.

This is for a hobby project since I need an excuse to use my Arduino again :P

How would you solve such challenge?

Answer 1

This: -

I would also want to have objects not carrying an active radio transmitter, i.e., no batteries wandering around, just RFID tags or resonant metal pieces.

And this: -

The room may have several entry points, or it could even be an outside environment. Imagine I want to know how many people are in my terrain which has around 100m of radius.

Are technically incompatible requirements. I suggest you lower your expectations but I hope someone basically proves me wrong.