Fitted physics

In this document we give a short introduction into how you can easily add physics to your Avatar using the Fitting Sliders.

This is an extreme demo of what you can make with the physics. The demo was made by SeraphLinden a couple of years ago


Avatar physic is directly attached to Collision Volume Bones in Secondlife. The bones of interest are:

  • BUTT

These bones are automatically animated when the Avatar jumps around (see video above) And all you need to do for getting this to work is adding weight for those bones to your meshes and then wear a physics item in SL

Physics Sliders

Some of the Collision Volumes have been added to simulate gravity (physics). This is implemented for Pecks, Belly and Butt. Furthermore LindenLabs have added some more extra bones for Fitted Mesh: UPPER_BACK, LOWER_BACK, LEFT_HANDLE and RIGHT_HANDLE. Avastar does not automatically create Weightmaps for these bones (Except for the BELLY). But we have added a weight generator for those bones.

This weight generator has unfortunately been named physics generator, although strictly spoken the Handles and the Back bones are no SL physic bones.

The weight generator gets into action when you click the Generate physics button.


After the physics weights have been generated, you get 4 new Fitting Sliders for Butt, Pecs, Back and Handles. You use those sliders to adjust the weights for the specific regions.

Hint: The sliders are split into 2 regions:

  • Left – Center: Increase strength of weights
  • Center – Right: Pinch the weighted area

By default the Handles get no Weight (set to -1, see image) (as recommended by LindenLab )


Important: Remember that only the PECK Bones, the Belly and the BUTT are influenced by Secondlife physics. All other bones are not affected.

Example: Weighting the Breasts

I assume we begin with a mesh that has no weights on the collision Volume bones. When you select the mesh and open the Fitting Panel, then you should see the Button Generate Weights like in the image.

When you click on the button, then the initial weights are generated and the button is replaced by a list of bone sliders…

Initial generated weights for the LEFT_PECK bone

Physics box after Generating physics

 The physics bone sliders explained

The sliders behave a bit in an unusual way:

  • From far left to midpoint the slider increases the amount of weight for the assigned Bone
  • From midpoint to far right the weight distribution gets pinched

Below i have added 3 images to explain visually what  happens on the LEFT_PECK

Slider on far left (no weights)

Slider at midpojnt (max weights)

Slider at far right (Pinched)