Spring SOP
Summary
The Spring SOP deforms and moves the input geometry using spring "forces" on the edges of polygons and on masses attached to each point.
Geometry is deformed using forces that simulate simple physics on the points and edges. A simulated "mass" is assigned to each point. Its primitive edges act as "springs" which oppose the forces, and pull the points back toward their original positions. When the springs are stretched by the forces, they try to pull the points back. The points do not stop moving when they return to their original positions, however, but continue to oscillate because of their mass, until the oscillation dies out.
Forces which act upon the points are as follows:
 external force (gravity)
 wind (similar to external force)
 turbulence (chaotic forces)
The greater the drag value, or smaller the mass, the faster the oscillation dies out.
Contents
Parameters  State Page
Preroll Time timepreroll
 How many seconds of the simulation to bypass, after the reset time is reached. For example, if you put the number 33 into this field (and reset is at $TSTART
), frame one will show the simulation that was at a time of 33 seconds. In other words, the first thirtytwo seconds have been bypassed, and the time at thirtythree seconds is shifted to frame one. The first thirtytwo seconds must still be calculated in order to compute the status of the points, so you will notice some delay upon reset.
Time Inc timeinc
 The Time Inc parameter determines how often to cook the SOP. By default, this parameter is set to 1/$FPS
. This means that the SOP will cook once for every frame. When complex dynamics are involved, the SOP may require more frequent cooking for increased mathematical accuracy. To get subframe accuracy in the cooking, set the Time Inc to something smaller than 1/$FPS
. For example, setting the Time Inc to 0.5/$FPS
will mean that the SOP gets cooked twice for every frame.
Note: Never set this parameter greater than 1/$FPS
.
Accurate Moves accurate
 This option makes the nodes move more accurately between frames by calculating their trajectories for fractional frame values.
Attractor Use attractmode
 ⊞ 
 All Points
all
 All point attractors affect all particles (or points).
 Single Point per Particle
single
 When enabled, each particle is assigned a single attractor point, and is affected by only that point. Assignment is done by point number modulo the total number of attractor points.
Reset reset
 While On resets the spring effect of the SOP.
Reset Pulse resetpulse
 Instantly reset the spring effect.
Parameters  Forces Page
External Force external
 ⊞  Forces of gravity acting on the points. When drag is zero, the points can accelerate with no limit on their speed.
 X
externalx

 Y
externaly

 Z
externalz

Wind wind
 ⊞  Wind forces acting on the points. Similar to external force. Using wind (and no other forces, such as turbulence), the points will not exceed the wind velocity.
 X
windx

 Y
windy

 Z
windz

Turbulence turb
 ⊞  The amplitude of turbulent (chaotic) forces along each axis. Use positive values, if any.
 X
turbx

 Y
turby

 Z
turbz

Turb Period period
 A small period means that the turbulence varies quickly over a small area, while a larger value will cause points close to each other to be affected similarly.
Seed seed
 Random number seed for the simulation.
Parameters  Nodes Page
Fixed Points fixed
 This is a point group. All points in the point group will remain unaffected by the forces. Also see the Group SOP for notes on how to specify point ranges.
Fixed Points go to Source Positions revertfixed
 Determines whether or not points in the Fixed Points group should be moved to the positions of the corresponding points in the Source geometry.
Copy Groups from Source copygroups
 Determines if the Spring SOP should copy groups from the Source geometry at each frame. This lets you specify the name of an animating group in the Fixed Points field, and the contents of this group will be kept up to date.
Add Mass Attribute domass
 When selected, the Mass is computed for the deforming geometry.
Mass mass
 Mass of each point. Heavier points take longer to get into motion, and longer to stop.
Add Drag Attribute dodrag
 When selected, the geometry is deformed by the Drag attribute.
Drag drag
 Drag of each point.
Spring Behavior springbehavior
 ⊞  How the springs will behave:
 Hooke's Law
hooke
 Springs will work according to Hooke's Law. This is generally more stable than Normalized Displacement.
 Hooke's Law: Force = Displacement Spring constant
 Normalize Displacement
normalize
 Similar to Hooke's Law except that the displacement is normalized to the original length of the Spring.
Spring Constant springk
 The spring constant. How tight the springs are. Increase this value to make the springs tighter and thus make the object more rigid. As this number becomes higher, the springs can oscillate out of control. Decrease the Time Inc if this happens.
Initial Tension tension
 The Initial k constant of the geometry before being deformed by the spring operation.
Parameters  Limits Page
+ Limit Plane limitpos
 ⊞  The points will bounce off the limit planes when it reaches them. The six limit plane fields define a bounding cube. The default settings are one thousand units away, which is very large. Reduce the values to about one to see the effect.
 X
limitposx

 Y
limitposy

 Z
limitposz

 Limit Plane limitneg
 ⊞  The points will bounce off the limit planes when it reaches them. The six limit plane fields define a bounding cube. The default settings are one thousand units away, which is very large. Reduce the values to about one to see the effect.
 X
limitnegx

 Y
limitnegy

 Z
limitnegz

Hit Behavior hit
 ⊞  Control over what happens when the geometry hits either the six collision planes or the collision object. The options are:
 Bounce on Contact
bounce
 Geometry will bounce upon contact with the Collision input.
 Stick on Contact
stick
 Geometry will stick to the Collision input upon contact.
Gain Tangent gaintan
 Friction parameters which can be regarded as energyloss upon collision. The first parameter affects the energy loss (gain) perpendicular to the surface. 0 means all energy (velocity) is lost, 1 means no energy is lost perpendicular to surface. The second parameter is the energy gain tangent to the surface.
Gain Normal gainnorm
 Friction parameters which can be regarded as energyloss upon collision. The first parameter affects the energy loss (gain) perpendicular to the surface. 0 means all energy (velocity) is lost, 1 means no energy is lost perpendicular to surface. The second parameter is the energy gain tangent to the surface.
Operator Inputs
 Input 0 
 Input 1 
 Input 2 
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