Use hand motion to influence particle motion

Use hand motion to influence particle motion

We upload the hand motion (position and velocity) and use it to move
particles. The area of influence is based on the velocity of the hands:
faster moving hands affect a wider area, slower affect a smaller area.
This is intended to simulate the wake of the hand in a fluid, and has
the bonus effect that when your hands aren't moving they don't affect
many particles.

The particles are accelerated by mixing their velocity with the hand's
velocity, with the weight for that depending on the distance and the
velocity of the hand. Using mixing instead of direct accelerating
prevents you from accelerating particles faster than your hands move,
which simulates the feeling of them being pulled along in a current in
the fluid.