PhysBones

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Physbones (also styled as PhysBones; a pormanteau of 'physics bones') are components, native to the VRChat SDK that allow avatars to display additional motion beyond the normal skeletal movements. PhysBones were implemented into VRChat on April 21, 2022.

Overview

Physics Bones are designed to be a more performant, and free replacement for Dynamic Bones, which are a paid Unity asset. These can be enabled on components like hair, clothing, tails, ears, and more, allowing those features to be affected by movement, gravity, and collisions. With the Avatar Dynamics system, PhysBones can optionally be used as interactions by oneself, or other users, for interactions such as grabbing and posing.

History

Before PhysBones, VRChat added support for Dynamic Bones circa 2017. PhysBones was first introduced as "physics bones" in the April 2021 Developer Stream, during the Avatar Dynamics segment. An update about its development was posted on Medium on December 17, 2021. After the season, Avatar Dynamics launched into Open Beta on March 4, 2022. The Open Beta period ended on April 21, 2022, and subsequently was launched to VRChat's live build on the same day, in the VRChat 2022.1.2 (build 1189) release.

As an expansion to PhysBones, the introduction of SquishyBones was added in 2023.2.2 (build 1304) on May 3, 2023.

Initially, Dynamic Bones were still supported, and even had a user option to be automatically converted into PhysBones, but as of the VRChat 2024.3.1 release, Dynamic Bones are now always automatically converted. VRChat SDK changes were made, so that compatibility with uploading new avatars with Dynamic Bones components are no longer permitted.

SDK

Components for PhysBones, and SquishyBones can be found in the VRChat SDK, and be applied to avatars within avatar projects on Unity.

Avatar performance

PhysBones were introduced to VRChat as an optimized and free alternative to legacy Dynamic Bones for Unity. Compared to Dynamic Bones, more PhysBones can be applied to an avatar on both PC, and Android versions, as they are more cost-performant.

Definitions

Avatar Quality	Quality Description
PhysBones Components	The number of PhysBone components on the avatar.
PhysBones Affected Transforms	The total number of transforms affected by PhysBones components on the avatar.
PhysBones Colliders	The number of PhysBone collider scripts on the avatar.
PhysBones Collision Check Count	The sum of how many PhysBone transforms each collider can affect. This can count transforms twice or more, because a single transform can be affected by multiple colliders.

PC limits

Avatar Quality
PhysBones Components	4	8	16	32
PhysBones Affected Transforms	16	64	128	256
PhysBones Colliders	4	8	16	32
PhysBones Collision Check Count	32	128	256	512

Mobile limits

Avatar Quality
PhysBones Components	0	4	6	8
PhysBones Affected Transforms	0	16	32	64
PhysBones Colliders	0	4	8	16
PhysBones Collision Check Count	0	16	32	64

If the Very Poor value for an avatar is met on mobile, no matter the current "Show Avatar" state of the avatar, all Avatar Dynamics-related components will be removed.

SquishyBones

A graph, demonstrating stretching and squishing of a SquishyBone.

On May 3, 2023, VRChat announced an expanded addition to Physbones 1.1, called SquishyBones, which grant the illusion of a mesh or physbone being stretched or compressed, affecting bone length, compared to being moved around like a standard Physbone.

Patchnotes for this update included the following:

• Squishy Bones update, allowing for bones to squish and length to be affected by motion.
• Gravity now acts as a ratio of how far the bones will rotate when at rest. A positive Pull is required for bones to move in the direction of gravity.
• Stiffness now acts as a ratio which keeps a bone in it's previous orientation.

Debugging

Using the Action Menu, you can use the Avatar Overlay option to show visual representations of both PhysBones and Contacts live in game. These are useful for seeing exactly what is happening, or if objects have been set up properly.

Both PhysBones and Contacts run in the editor as they would in the client. By entering Play mode you are able to simulate these systems and see how your avatar will react without needing to upload your avatar.

Official resources

• 'PhysBones' on the VRChat Creation Docs
• 'Avatar Performance Ranking System' on the VRChat Creation Docs
• 'Debugging Avatar Components' on the VRChat Creation Docs

See also

• Avatars
• Debug Menu