Videos: Please submit videos (described below) that demonstrate the requested (and any additional) functionality of your system. Details on video generation are here.  The current starter code allows you to use the 'e' key to toggle exporting of png frames to the "frames" subdirectory.

Software Dependencies:  The starter code will compile and run using JDK 1.5 or later. I recommend you get Sun's latest JDK6 here. In addition to Java the starter code also uses two other libraries:

1.  Implement Spring Forces:  Three spring forces currently do nothing:

• SpringForce1Particle
  
• SpringForce2Particle
  
• SpringForceBending
  

and need to have their  apply_force()  function implemented.  The first two should use a damped Hookean spring (as in the Witkin course notes), whereas the second can use the bending force derived in class (here is a simple derivation of that bending force).  Following these implementations, your "Create Spring" and "Create Hair" tasks should have more meaningful results. 
Generate a video demonstrating a hair model with these force implementations.

2.  Implement Gravity and Viscous Damping Forces: Currently gravity and viscous damping are "hacked" into ParticleSystem.advanceTime(), and conflate integration and physical modeling. Implement some global Force objects,

• GravitationalForce
  
• ViscousDragForce
  

and add these to the ParticleSystem (immediately after its construction) to properly apply these forces to all particles.
Generate a video demonstrating your gravity and viscous damping forces.  Suggestion: you can change the direction of gravity to be at an angle, or even be rotating to make the dynamics inside your box more interesting.

3.  Implement Numerical Integrators using Witkin's Interface: The current forward Euler implementation is flakey, and hard coded---it doesn't support proper abstractions to enable easy swapping in/out of different integrators. Implement an Integrator interface to provide support for both the following integrators:

• Integrator_ForwardEuler
• Integrator_SymplecticEuler
  
• Integrator_Midpoint
  

As discussed in Witkin's course notes, you will want to have the ParticleSystem implement an interface (perhaps called DynamicalSystem) consisting of the get/setState(), derivEval(), getStateN(), getTime()... functions.  The Integrator interface might abstract timestepping the system using a method such as Integrator.advanceTime(double, DynamicalSystem). Document your choices.

Add GUI controls (e.g, radio buttons, or a list, etc.) to select the active Integrator implementation.  You may want to do the same for the time-step size---which is currently adjusted using the +/- keys. 
Given the same time-step size, which integrator is the least stable or most stable for very stiff hair? (be specific)

3.  Pin constraints were implemented using a post-step velocity and position projection in the hacky integrator. Implement a Constraint or filter class to make pin constraint projections play nicely with your new integrators.

4.  Wall-particle collision detection and response (or "staying inside the box"): The particles are currently not constrained to stay inside the unit computational cell. Implement a simple scheme to detect collisions with the four walls, and (as in Witkin's course notes) filter the post-step state to provide simple collision response. Use a restitution coefficient to attenuate normal velocity (defined in Constants.RESTITUTION_COEFF). Be careful how you update the particle velocity and position or you might find your particles interpenetrating or sinking into the wall. Also add a friction-like drag force.
Generate a video demonstrating collisions.