Main Page Community portal Recent changes Random page Help

Article Discussion Edit History

Stabilize soft body simulations

Article needs peer review

Summary Softbody simulations give unexpected results, with objects exhibiting wierd deformations and transformations.

Steps

To fine tune the stability of a Softbody simulation, we need to adjust the parameter called the "Number of Iterations Per frame" (NIPF). This parameter is located in the SoftBodyOp Property Page, accessed as follows:

1. Select the object to which you have applied the Softbody simulation
2. In the Main Command Area, click the Property button>SoftBodyOp Property Page and access the Simulation tab, where you will find the Number of Iterations per Frame parameter.

Softbody uses an explicit method for calculating dynamics, and NIPF is the simulation's main stabilizing factor.

Note that the NIFP factor is computation intensive, and so the higher the value the slower the calculation.

<P>The Sampling and Stiffness values, found in the SoftBodyOp Property Page's Soft Body tab, also affect the stability of the simulation.

More Information

A case study will better illustrate the influence of each of these parameters:

1. Download the following script: <P><A href="/support/xsi/private/kb/files/softbodyex.vbs">Missing image
   Vbsicon.jpg
   Image:vbsicon.jpg
   
   </A> <A href="/support/xsi/private/kb/files/softbodyex.vbs">softbodyex.vbs</A></P>
2. Open the Script Editor (you can use the button) and select File>Open>softbodex.vbs and click Run.
3. Once the Pick Session is reached (as indicated in the status bar at the bottom of the interface) left-click the grid, to define it as an Obstacle object, and then right-click to end the picking session.

<P>Playback the animation:</P>

<P>Result:</P>

<P>When the sphere hits the grid, it appears to get hit from the right side.</P>

<P>Reason: </P>

<P>The NIPF is too low for this case (sphere hits the grid at a high speed), causing mathematical instability.</P>

<P>Solution: </P>

<P>Raise the NIPF from 6 to 8</P>

<P>To demostrate the next scenario, choose File>New Scene from the main menu and repeat steps 2 and 3 above.

4. Select the sphere Click the Property button>SoftBodyOp Property Page and access the Soft Body tab.

<P>Set the Sampling to 12 and playback the animation: </P>

<P>Result:</P>

<P>When the sphere hits the grid it compresses and then slides forward</P>

<P>Reason: </P>

<P>A high Sampling rate increases instability</P>

<P>Solution:</P>

<P>Set the NIPF value to 10</P>

<P>Once again, choose File>New Scene from the main menu and repeat steps 2, 3 and 4 above.

Lower the Stiffness to value to 15 and playback the animation:

Result:

You will see that the instability will have dissappeared because the high Sampling value is counterbalanced by a lower Stiffness value.  Similarly, the opposite is also true: a low Sampling value of 9, for example, will balance a higher Stiffness of 25.

References Animating guide, Simulation section: Creating a Soft Body Deformation: Sample Workflow

Applies To: XSI 1.0 on NT,Win2K,Irix

Posted: 11/22/2000