TYPE: SOP
TITLE: Spring SOP

The Spring operation deforms and moves the source geometry using "forces"
that simulate the laws of physics.

Parameters:

State
    StartTime            - Time at which the simulation resets.     /timestart
    Preroll Time         - Number of initial frames skipped.        /timepreroll
    Time Inc             - Time per iteration.                      /timeinc
    Accurate Moves       - Particles move more accurately.
    Attractor Use                 - How the attractor points affect particles
        All points                - All points affect each particle
        Single point per particle - Only one point affects each particle

Forces
    External Force  - Force of gravity on points.                   /externalx
							   /externaly /externalz
    Wind            - Wind force acting on points.         /windx /windy /windz
    Turbulence      - Amplitude of turbulence along axes.  /turbx /turby /turbz
    Turb Period     - Inverse variance of turbulence over space.     /period
    Seed            - Seed for random turbulence generator.          /seed

Nodes
    Fixed Points       - Group of points not to be moved by this operation.
    Fixed Points go to Source Positions - Fixed Points are put to
			 their positions in the Source.
    Copy Groups from Source - All Source groups are copied at each frame. Useful
                         if the Fixed Points group contents are animated.
    Add Mass Attribute - Causes point mass to be calculated.
    Mass               - Relative mass of each point.                /mass
    Add Drag Attribute - Causes drag coefficient to be calculated.
    Drag               - Drag of each point.                         /drag
    Spring Behavior    - How the springs will behave
        Hooke's Law            - Use Hooke's law. ~r
                                 Force = displacement x spring constant.
        Normalize Displacement - Like Hooke's law except displacement is
                                 normalized to the original length of the
                                 spring. (Behavior used in Houdini 2.5).
    Spring Constant    - Stiffness of the spring                     /springk
    Initial Tension    - Initial spring tension before deformations. /tension

Limits
    + Limit Plane, -~sLimit Plane - Points die or bounce off limit
		        planes on contact.
                        /limitposx ... /limitposz,   /limitnegx ... /limitnegz
    Hit Behavior      - Whether particles die or bounce on limit planes.
    Gain Tangent      - Energy loss tangent to the collision.        /gaintan
    Gain Normal       - Energy loss perpendicular to the collision.  /gainnorm
