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Physics

VRChat PhysBones, VRM spring bones and legacy Dynamic Bone all become Jiggle Physics rigs, the secondary motion system Basis ships. Dynamic Bone is an ordinary Unity asset and VRM is a format, not a platform, so an avatar using either converts whether or not VRChat was ever involved.

This page is about PhysBones and Dynamic Bone. Spring bones have their own page, since the two VRM formats describe a chain differently: see VRM.

One rig is written per chain. A PhysBone covering a whole head of hair is one rig, since jiggle physics walks into every child of the root. Each root bone a Dynamic Bone names becomes a rig of its own.

What carries across exactly​

  • The root bone, the transforms the source ignored, and grab settings. A PhysBone with radius 0 is not grabbable in VRChat, and the rig is locked from grabbing to match.
  • Radius and stretch motion.
  • Colliders: sphere, capsule and plane, with the same three shapes on both sides. A capsule's height is measured end to end on the source side and between the cap centres on the Basis side, and is converted between the two.

What is fitted​

Two PhysBone settings do not mean the same thing on both sides and are fits rather than conversions:

  • Stiffness, from VRChat's pull and stiffness.
  • Drag, from VRChat's spring. Spring's curve is dropped, since drag runs the other way.

Both are exposed under Advanced in the window, as weights to adjust before rescanning.

Approximated, and reported as such:

  • Falloff curves carry over as curves. VRChat samples them by bone index and jiggle physics by distance from the root, so a bone reads a different point: physbone.curves.domain.
  • Gravity. PhysBone gravity blends the rest direction toward down; jiggle scales world gravity. The value is copied as the multiplier. Dynamic Bone gravity is not comparable and is left to the preset: dynamicbone.gravity.
  • Multi Child Type. A root with one child is simulated on both sides. With several, Ignore becomes a motionless root, and bones further down with several children swing where VRChat held them still.
  • Hinge limits become a cone of the same angle.
  • Immobile becomes ignore root motion, which cancels the root's translation only. An immobile bone with almost no pull wanders when its parent turns: physbone.immobile.drift. Raise that rig's stiffness.

Dynamic Bone is derived, not fitted:

  • Stiffness is the square root of Elasticity, scaled by Update Rate over 60.
  • Dynamic Bone Stiffness becomes the angle limit at the same angle; caps wider than 90 degrees are left off.
  • Damping becomes drag and air drag.
  • Blend Weight tightens the cap; 0 writes no rig.

Everything else is a direct mapping.

Values the source does not determine are taken from the jiggle physics package's own presets. The preset per rig is guessed from the bone's name and can be changed in the window.

What does not carry across​

  • Angle limits wider than jiggle physics can express. No limit is written rather than a tighter one.
  • Polar limits, which are approximated to a single angle.
  • Gravity falloff, max stretch, max squish, endpoint positions, and per-axis limit rotations.
  • Is Animated, and anything driven by a PhysBone parameter.
  • A collider inverted to keep bones inside it.
  • A collider marked global, for other avatars' bones. Basis offers an avatar's hands, arms and feet to other avatars on its own; a collider elsewhere collides with its own avatar only.
  • Which way a plane collider faces, when that is not its transform's Y axis. Basis planes face that axis: collider.planeRotation.dropped, collider.planeAxis.dropped.

Checking the result​

Press Test In Editor on the BasisAvatar component. Jiggle physics only runs on a calibrated avatar, so plain Play mode shows nothing moving.