While soft bodies can’t really be true “rigid bodies”, you can get the most rigid results if you set the Stiffness as high as possible and the Sampling as low as possible.
Setting the Soft Body Deformation Behavior
On the Soft Body page in the SoftBodyOp property editor, you can define the soft-body object’s deformation behavior using these parameters.
• Sampling controls the resolution of the lattice around the volume of the object that is used in the calculation of the soft-body deformation.
Higher values cause the lattice to more closely resemble the volume of the object to be deformed. While this creates a more realistic result, higher sampling rates take longer to process.
• Stiffness sets the rigidity of the soft-body lattice applied to the object. High values cause the object to better resist deformations. You can set the object to bounce back after a collision and either retain its original form or maintain the deformed appearance after the event.
Stiffness also has an effect on Body Friction: a higher Stiffness value causes more “bounciness” as the soft-body object hits an obstacle.
• No Stretch activates a filtering process at the end of the simulation step that prevents excessive stretching of the springs.
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While soft bodies can’t really be true “rigid bodies”, you can get the most rigid results if you set the Stiffness as high as possible and the Sampling as low as possible. |
Stabilizing a Soft Body Deformation
Soft-body deformations can sometimes give unexpected results, such as strange shapes or transformations, when certain parameters, such as Sampling and Stiffness, have high settings.
To compensate for most of these problems, you can adjust the value for the Iterations per Frame parameter, found on the Simulation page of the SoftBodyOp property editor. This defines the time step between one frame and the other, subdivides this value, and then executes the soft-body simulation. Unlike cloth, soft body uses an explicit method for calculating dynamics, so this parameter is the main factor for giving stability to the simulation.
Generally, high values of Sampling and Stiffness can cause instabilities that can be fixed by raising the value for Iterations per Frame. Of course, when you raise value, it increases calculation time. Start with a low value and bring it up until you get the realistic simulation look you want.
In some cases, you can use settings as low as 2. Tune each soft-body object differently—they probably won’t use the same settings. This is highly dependent on the properties you’ve set, like stiffness and mass.
• High Sampling values cause the lattice to more closely resemble the volume of the object to be deformed and thereby create a more realistic result, but these may contribute to the instability of the simulation.
If this happens, you can lower the Stiffness value as well as increase the Iterations per Frame.
• High Stiffness values cause the object to better resist deformations, but again, may contribute to instability. If this happens, you can lower the Sampling value or increase the Iterations per Frame.
• Select No Stretch to help stabilize the deformation, especially when the soft-body object seems to “explode” after a collision.
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You can also help stabilize a deformation by increasing the Body Mass, which determines the weight of the object. |
• Body Mass defines the weight of the soft-body object. This value has a bearing on how the object interacts with forces and collisions with obstacles, as well as to the stability of the deformation.
• Body Friction determines how much of a deformed object’s energy is absorbed by the obstacle with which it collides.
The higher this value, the more friction with the obstacle. This also depends on the Friction value you have set for the obstacle (see Setting the Collision’s Physical Behavior).
Threshold defines the plasticity of the object, meaning how much the object can be deformed by soft body. This value sets the point beyond which the effects of the collision can become permanent.
Threshold is the percentage of the original length of the springs. With a value of 1 (100%), if the spring deformation is 100% (both stretching and compression) of the spring’s original length, the effect of the collision becomes permanent.
Damping controls the degree to which the deformed object recovers its original shape if the deformation is over the Threshold value (see above).
A value of 1 causes the object’s deformation to become permanent and 0 simulates a more resilient material that permits the object to return to its original, predeformed shape.
Autodesk Softimage v.7.5