Rigid and Soft Bodies

Owned by thomas (Unlicensed)
Last updated: 07 Nov, 2016
2 min read

Every polygon object – native or imported – can act as a rigid or soft body. You can choose from three different modes under Node Params > Node > Simulation:

  • Passive rigid body. The body acts like an obstacle for other rigid/soft bodies, but cannot be moved. Its mass is considered infinite.
  • Active rigid body. The body can be moved by other rigid/soft bodies, but also by fluids and RealWave waves. Active bodies are influenced by force-based daemons.
  • Soft body. The body can be moved by other rigid/soft bodies, but also by fluids and RealWave waves. Soft bodies are influenced by force-based daemons and they are deformable.

 

Once the object has been tagged as rigid or soft body, you will find a new entry in the node's "Node Params" panel. This is where you adjust the body's physical properties.

Important Parameters

Mass

For active rigid and soft bodies this is one of the most important parameters:

  • On Earth mass and weight are used equally, but on other celestial bodies mass has to be multiplied with a specific factor to get a body's weight.
  • Very heavy objects require higher forces to be moved and create stronger splashes when they interact with fluids.
  • An object's mass is calculated automatically by RealFlow. Decrease mass if you want to make an object float on a RealWave surface or a standard particle volume.
Object Friction and Elasticity

When bodies interact with other rigid or soft bodies, friction and elasticity play an important role:

  • Friction decelerates a body – the higher the friction value, the quicker the speed falloff. The friction values of the interacting bodies are considered as well.
  • Elasticity makes a body bounce. The higher the value, the stronger the bouncing effect. The elasticity values of the interacting bodies are considered as well.
Centre of Gravity ("CG", active rigid bodies only)

By changing the centre of gravity you can prevent a floating object from tipping, or create a displaced rotation:

  • The parameter is adjustable for its individual XYZ components.
  • To make the body's centre of gravity visible, enable Node Params > Display > Show CG. A small yellow cube appears (hit 8 to change to wireframe mode).
Initial Velocity ("Velocity") and Rotation ("Rotation W")

Active rigid bodies and soft bodies can have an initial velocity and rotation:

  • The parameters are adjustable for their individual XYZ components.
  • These value will be applied immediately when the simulation starts.

Soft Body Properties

Resolution

In many simulations you get a warning that the "Softbody "[ ... ]" is under-tessellated for its resolution. Possible incorrect behaviour.". As long as the simulation behaves correctly, this message can be ignored.

  • RealFlow internally subdivides a soft body into cells and "Resolution" influences the number of cells.
  • This is necessary to bend and deform objects with large triangles correctly.
  • If the number of cells is too high, and the triangles are too big you will get this warning. Decrease "Resolution" if necessary.
Stiffness

These parameters define, how rigid (high values) or soft (lower values) a body is and this influences the amount of deformation.

Autocollision

When soft bodies are being deformed it might happen that polygons interpenetrate each other. To avoid this, activate the "Autocollision" option, but prepare for longer simulation times.

Damping

Without damping, the typical "wobbling" effect of soft bodies will last very long. To make them rest faster, increase this value.