alt Hacker News"Estimate of weight and rigidity" is exactly the kind of thing that's performed rather well by low-dimensional sensing of actuators themselves.
Backdrivable actuators know how much current are they sinking - and thus, how much force are they facing. So even the simplest two finger gripper can easily "know" that one object is squishy and the other is not. The response curves are different. Same can apply to arm actuators for weight estimation.
Now, what this doesn't actually get you is an estimate of how hot/cold an object is, and how moist an object is.
Where cameras come in is specifically the "guide a grasp" part. Force sensing alone can tell a robot "the grasp is failing". A camera pointed at the grasped object can tell "why".
In theory yes but in practice backlash and stiction make drive currents much less useful than the raw force at point of contact for any realistic gearing. And an important part of weight estimation is that force lateral to the point of contact. Now, your arm motors will be drawing more current as they move so again in theory you can distinguish that, you've still got the weight of the whole gripper added in.
I spent years of my professional life on these topics[1] and from my experience motor drive current usually isn't enough to narrow down the weight of an object in a typical picking task even to within a factor of 2 to detect when you've accidentally picked up two items from a bin instead of one.
[1]https://patents.google.com/patent/US12612266