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Spring assigns a mass to each point and simulates the effects of forces (gravity, wind, and/or turbulence) on the points. Edges act as springs which try to pull the points back to their original position.
Keep certain points in the mesh at fixed positions using a point group in the Fixed points parameter on the Nodes tab.
Apply directional force (gravity), wind (a directional force with a maximum speed), drag, and turbulence to the points using the parameters on the Forces tab.
Connect a Force SOP to the third input to let you attract or repel the points to/from metaball fields.
The greater the drag value, or smaller the mass, the faster the oscillation dies out.
The Spring SOP is useful for many situations, but for more sophisticated simulations use rigid body, cloth, or wire dynamics.
The Spring op will use point normals as initial node velocity if point normal attributes exist and there are no point velocity attributes in the incoming data. If you add velocity attributes to the points, the point normals are ignored.
Time at which the simulation resets.
Number of initial frames skipped.
Time per iteration.
Particles move more accurately.
How the attractor points affect particles
All points affect each particle
Single point per particle
Only one point affects each particle
Force of gravity on points.
Wind force acting on points.
Amplitude of turbulence along axes.
Inverse variance of turbulence over space.
Seed for random turbulence generator.
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.
Relative mass of each point.
Add Drag Attribute
Causes drag coefficient to be calculated.
Drag of each point.
How the springs will behave
Use Hooke’s law. Force = displacement x spring constant.
Like Hooke’s law except displacement is normalized to the original length of the spring. (Behavior used in Houdini 2.5).
Stiffness of the spring
Initial spring tension before deformations.
+ Limit Plane, - Limit Plane
Points die or bounce off limit planes on contact. /limitposx … /limitposz, /limitnegx …
Whether particles die or bounce on limit planes.
Energy loss tangent to the collision.
Energy loss perpendicular to the collision.
The points to simulate, for example a polygon sphere or mesh.
(Optional) Geometry for the points to collide with. When the points hit this geometry, they can stick or bounce. If the collision object is deforming, collision detection may fail, causing some points to leak through the collision geometry.
(Optional) A Force SOP, which uses a metaball to create a field which attracts or repels the points.
This network utilizes three SOPs (Bound, Spring and Lattice) that commonly work together to simulate certain physical dynamics.
We have created a simple polygonal sphere to act as the source geometry. The sphere is then fed into a Bound SOP which will act as a deforming reference. The Bound SOP also behaves as re-enforcement for the deforming object.
Then the bounding box is wired into the Spring SOP with a group of grids as collision objects. The Spring SOP simulates the dynamics by calculating the proper deformations and behaviours of our source geometry as it collides with other objects. The Spring SOP is where we can apply external forces along with various attributes (characteristics such as mass and drag) which influence how the object deforms.
Finally the Lattice SOP takes the deformation information from the Spring SOP and applies it to the source sphere geometry.
This example demonstrates the three main functions of the Spring SOP.
It shows how the Spring SOP can deform input geometry to create a cloth like effect by creating interactions between two objects, defining motion, and applying forces.
Play the animation to see the Spring SOP in action.
This example shows how a flag can be simulated using a Spring SOP.
Here the Spring SOP applies forces that simulate the laws of physics on the points of a Grid SOP to create the flag effect.
This example demonstrates a way to create dynamic hair using the Spring SOP.
A Line is copied onto the points of a Sphere and input into the Spring SOP as a source. Then a Metaball and Force are input to further effect the motion of the hair.
Here the Spring SOP is used to give a line rubber band-like characteristics. Used in combination with an Xform SOP, the rubber band dances about on the floor.