Body Type System — Rig-Aware Physics for Physically Unconventional Characters
Summary
I would like to propose a new character-oriented physics framework for Blender: a Body Type System.
The basic idea is to let artists define not only how a character is rigged, but also what its body is physically made of and how that body should behave.
A conventional armature describes the structure and intended movement of a character. A Body Type would describe how the character's physical body responds to that movement, deformation, collisions, pressure, gravity and other forces.
For example, characters could have bodies that behave like:
- conventional solid/organic bodies;
- inflatable or balloon-like bodies;
- bubbles or membranes;
- gelatin or slime;
- water/liquid;
- plush or foam;
- stone or other rigid materials;
- highly elastic materials;
- gas/cloud-like bodies;
- custom or hybrid combinations.
The purpose would not be to replace Blender's existing physics tools or automate character creation. The goal would be to provide a unified, artist-friendly framework for rigged characters whose bodies require significant physical simulation, reducing the need to build complicated combinations of independent systems and corrective workarounds.
The Problem
Blender already has powerful tools for rigging, animation and physics.
However, physically unconventional characters can become difficult to create.
For example, an artist creating a living inflatable creature may need some combination of:
- Armatures;
- Weight Painting;
- Cloth or Soft Body simulation;
- Pressure;
- Vertex Groups;
- Shape Keys;
- Constraints;
- corrective deformation;
- collision and self-collision;
- custom drivers or node setups.
Each tool can be useful individually, but combining them into a character that must remain reliably animatable during extreme deformation can become complicated.
Similar problems can occur with characters made of gelatin, liquid, foam, highly elastic material or other unusual substances.
The problem is therefore not simply:
"How can Blender simulate a balloon?"
It is:
"How can Blender treat a physically simulated object as a character whose anatomy, rig and artistic intent must remain usable throughout the simulation?"
A character may need to deform dramatically while still remaining a character.
Core Concept: Rig Type vs. Body Type
A useful conceptual separation could be:
Rig Type = how the character is articulated and controlled.
Body Type = what the character is physically made of and how its body responds.
These should not depend on each other.
A quadruped could be organic, inflatable, stone, gelatinous or something completely fictional.
A humanoid could use exactly the same range of Body Types.
An artist should also be able to create a completely custom rig and then assign physical body behavior to it.
The armature would continue to represent the artist's intended structure, pose and movement.
The Body Type system would determine how the physical body responds around that structure.
Rig-Aware Physics
One of the most important goals would be keeping the rig and simulated body coherent.
In a conventional simulation, extreme deformation may cause the simulated surface to move in ways that no longer correspond well to the character's intended anatomy.
A Body Type system could treat the rig as a structural reference.
For example, if an inflatable character's thigh expands dramatically, the mesh should be able to become much larger, collide with the other thigh, compress against the torso and deform naturally.
However, the thigh should not simply "escape" from the leg structure because the simulation pushed its mass away from the original armature.
The leg bone still conceptually represents where that leg is.
The physical volume expands and deforms around that structural relationship.
This could be controlled by a property such as Rig Adherence (name only suggested), determining how strongly a simulated region follows its rig.
For example:
- high Rig Adherence — hands, joints, limbs and important anatomical structures;
- medium Rig Adherence — flexible body regions;
- low Rig Adherence — slime, loose membranes or highly fluid regions.
The exact technical solution does not necessarily need to involve literally scaling or moving bones. The important requirement is that the rig remains useful and structurally coherent through major body deformation.
Adaptive Character Structure
Large changes in body volume should not make the rig unusable.
For example, if an inflatable character grows several times larger:
- joints should remain correctly associated with the body;
- limbs should remain poseable;
- hands and fingers should remain controllable;
- facial features should remain correctly positioned;
- rigid features such as eyes, claws, horns or beaks should remain attached;
- the spine should remain bendable;
- animation controls should continue to work.
This should also apply in the opposite direction.
If an inflatable character loses pressure and collapses onto the floor, the surface may become wrinkled, folded and heavily deformed, but the animator should ideally retain meaningful control over important regions of the character.
The rig represents character structure and animation intent, while the physics determines how the body material responds.
Body Type Presets
Body Types could provide useful starting presets.
Possible examples include:
Organic / Standard
A conventional character-oriented starting point with limited physical deformation.
Inflatable
Pressure-driven expansion, compression, volume behavior, buoyancy and flexible surface deformation.
Bubble / Membrane
Thin flexible surface with strong tension and enclosed volume.
Gel / Slime
Soft deformation, viscosity, stretching, jiggle and gradual shape recovery.
Liquid / Water
A more fluid body whose shape can be influenced by the rig while allowing redistribution of mass.
Plush / Foam
Compression, softness and gradual recovery after deformation.
Elastic
Strong stretching and squash-and-stretch behavior with configurable volume preservation.
Stone / Rigid
Minimal deformation with the possibility of flexible joints, cracks or other artist-defined exceptions.
Custom / Hybrid
A completely configurable Body Type built by mixing physical properties and regional behaviors.
More complex body types such as gas, clouds, fire or energy could potentially be explored by developers if appropriate, but they may require substantially different simulation methods.
The Body Type concept should not require all possible physical materials to use the same solver.
Presets Should Be Starting Points, Not Restrictions
Selecting "Inflatable" should not permanently force the artist into one predetermined balloon behavior.
Instead, a Body Type could configure a useful initial set of physical properties.
The artist could then modify them.
This would allow two characters using the same Body Type to behave completely differently.
One inflatable creature could be soft, loose and easily compressed.
Another could be highly pressurized, firm and bouncy.
A third could combine an inflatable torso with rigid horns, flexible wings and extremely soft limbs.
The system should therefore remain useful for fictional materials that do not correspond exactly to real-world substances.
Body Property Paint
A major part of the system could be a painting workflow similar in concept to Weight Paint.
Instead of painting only bone influence, the artist could paint the physical behavior of different regions of the body.
Possible paintable properties could include:
- stiffness;
- stretch;
- compression;
- bending resistance;
- volume preservation;
- physical simulation influence;
- Rig Adherence;
- collision softness;
- recovery speed;
- buoyancy influence;
- rupture resistance;
- inflatability;
- inflation distribution.
Values could transition smoothly between regions rather than being binary.
For example, a horn could be almost completely rigid at its tip but gradually become more flexible near its connection to the head.
Animation vs. Physics Influence
Different parts of a character may require different balances between animation and simulation.
A controllable property could determine whether a region is primarily driven by:
Animation ←→ Physics
A character's hands might strongly follow the animator's pose.
Its belly might react much more strongly to physical forces.
Its tail might sit somewhere between the two.
This influence could potentially be painted and animated.
The goal would be to allow physical secondary motion without forcing the artist to surrender control of the character.
Hybrid Bodies and Regional Behavior
A single character should not necessarily have only one physical behavior.
Artists could assign or paint different Body Types or property combinations across the same character.
Examples:
- an inflatable creature with rigid claws and horns;
- a stone creature with flexible or molten regions;
- a water creature with solid eyes or accessories;
- a bubble-like creature containing liquid;
- a soft toy with rigid internal or external components;
- a fictional creature combining inflatable, gelatinous and elastic properties.
Transitions between regions should ideally be smooth.
This would make the system useful not only for reproducing real materials, but also for creating completely fictional bodies.
Detailed Example: Inflatable / Balloon Body
Inflatable characters are a useful example because they expose many of the problems this system could solve.
An inflatable Body Type could include the following concepts.
Inflation Level
For animation, it may be more intuitive to separate the artistic inflation state from absolute physical pressure.
For example:
Inflation Level = 0 The character is in the normal Rest State created by the artist.
Inflation Level < 0 The character progressively deflates.
Inflation Level > 0 The character progressively inflates beyond its normal state.
This parameter could be keyframeable.
The artist would therefore not need to determine an exact physical pressure value merely to preserve the original modeled shape.
Actual internal pressure and solver parameters could exist separately under advanced physical controls.
Rest State
The original modeled body could act as the character's Rest State.
This is important because "zero physical pressure" and "normal artistic appearance" are not necessarily the same thing.
A small inflatable character and a gigantic inflatable creature should both be able to use:
Inflation Level = 0
to represent their intended normal appearance, even if the solver internally requires very different physical values.
Deflation
Deflation should ideally be more than simply scaling the mesh down.
As inflation decreases:
- internal support decreases;
- the surface becomes less firm;
- folds and wrinkles can appear;
- gravity has greater visible influence;
- limbs and body regions may collapse;
- the character may eventually become a loose, deflated body resting on the ground.
The rig should remain usable as much as possible throughout this state.
An optional artist-defined Deflated Reference Shape could potentially assist the solver, but it should not necessarily be required.
Extreme Inflation
Positive Inflation Level values could allow substantial or exaggerated expansion.
Rather than requiring a universal maximum size, the allowed range could be determined by the artist and by simulation stability.
This would support realistic inflatable objects as well as highly stylized living balloon characters capable of extreme changes in volume.
The rig and physical body should remain coherent during these changes.
Inflatability Paint
The artist could paint how much each region is allowed to expand.
For example:
- belly: 100%;
- chest: 90%;
- thighs: 60%;
- arms: 40%;
- hands: 15%;
- horns: 0%;
- claws: 0%.
When the Inflation Level increases, the solver would respect these regional differences.
Inflation Bias / Volume Distribution
A second property could describe where additional volume is preferentially expressed.
This is different from Inflatability.
A region might be physically capable of expanding significantly but normally receive only a small amount of the additional volume.
For example:
Belly
- Inflatability: 1.0
- Inflation Bias: 1.0
Arm
- Inflatability: 1.0
- Inflation Bias: 0.2
The arm is capable of inflating, but additional volume normally favors the belly.
These values or their overall influences could also be animated.
This would give animators artistic control over where inflation visually develops without requiring a large collection of corrective Shape Keys.
Inflation Chambers
More advanced inflatable characters and objects may contain separate internal chambers.
An optional Chamber system could allow artists to define regions such as:
- Torso;
- Left Arm;
- Right Arm;
- Tail;
- Head;
- Wings.
Connections between chambers could potentially be:
- Open;
- Restricted;
- Closed.
An open connection could allow rapid redistribution.
A restricted connection could transfer volume gradually.
A closed chamber could be controlled independently.
This would be useful for balloon characters, pool toys, inflatable props and other objects with physically or artistically separated volumes.
Soft vs. Firm Inflation
Two inflatable bodies at similar sizes should not necessarily feel the same.
Useful parameters could describe qualities such as:
- internal pressure;
- surface stiffness;
- stretch resistance;
- compression;
- bending;
- damping;
- bounce;
- surface tension;
- recovery speed.
A loosely inflated body could flatten significantly against a wall.
A highly pressurized body could deform less and bounce more strongly.
These parameters should ideally remain artist-controllable rather than forcing a single physically perfect balloon model.
Bending Inflatable Limbs
Inflatable limbs, tails and other tubular structures should preserve believable volume while bending.
For example, when an inflatable tail bends:
- the inside of the curve may compress;
- the outside may stretch;
- the cross-section may deform;
- pressure should help preserve volume;
- local folds may appear depending on stiffness and pressure.
The result should look like a pressurized flexible body rather than a conventional solid mesh simply rotated by bones.
Collision and Self-Collision
Collision is especially important for physically deformable characters.
An inflatable character's:
- belly may press against its thighs;
- arms may press into the torso;
- legs may press against each other;
- tail may wrap around and contact the body;
- body may flatten against the floor or walls.
These contacts should deform the relevant surfaces rather than simply allowing them to interpenetrate.
Self-collision should therefore be an important part of the system where technically practical.
Buoyancy / Effective Density
Inflatable bodies could optionally expose an artist-friendly buoyancy or effective-density control.
This could range from:
- heavy / sinking;
- neutral;
- light;
- floating;
- strongly buoyant.
This would allow both realistic and fictional behavior without forcing every artist to manually build the required force setup.
Optional Rupture
Not every inflatable object should rupture.
Rupture should therefore be optional.
For example:
Rupture: Enabled / Disabled
When enabled, artists could define a threshold based on properties such as surface strain or tension.
The system could potentially support different outcomes:
- gradual leak;
- tear;
- rapid rupture/pop.
When rupture is disabled, the object could continue expanding within the range supported by the artist's settings and the solver.
This is useful for stylized living balloon creatures where extreme inflation is intentional rather than a simulation failure.
Rigid Features on Deformable Bodies
Many physically unusual characters contain parts that should not inherit the body's deformation.
Examples include:
- eyes;
- teeth;
- claws;
- horns;
- beaks;
- valves;
- accessories.
These objects or regions should be able to remain rigid while still following the character correctly.
For example, an inflatable head could expand dramatically while the eyes remain correctly attached and retain their intended shape.
Other Body Type Examples
The same general philosophy could apply beyond inflatable bodies.
Gel / Slime
Possible properties:
- viscosity;
- stretch;
- jiggle;
- compression;
- damping;
- shape recovery;
- weak or strong rig adherence.
The rig provides the intended movement while the material responds slowly and organically.
Plush / Foam
Possible properties:
- compression;
- softness;
- crease/fold behavior;
- slow or fast recovery;
- internal volume preservation.
This could be useful for toys, cushions and stylized soft characters.
Liquid / Water
A liquid character could retain some structural relationship to a rig while allowing much greater redistribution of mass and surface motion.
The exact implementation may require a different solver from inflatable or elastic bodies.
Stone / Rigid Material
A stone creature could remain mostly rigid while still having:
- articulated joints;
- flexible transition regions;
- optional cracking or fragmentation;
- softer or molten regions painted onto the same character.
Bubble / Membrane
A bubble-like character could emphasize:
- surface tension;
- thin membrane behavior;
- enclosed volume;
- extreme softness;
- collision deformation.
Custom / Hybrid Body Type
Perhaps the most important Body Type would be Custom.
Artists should not be limited to substances Blender developers anticipated in advance.
A Custom Body Type could combine the underlying properties freely.
For example, an artist might create:
- high elasticity;
- moderate inflation;
- very high volume preservation;
- low weight;
- strong rig adherence at joints;
- weak adherence on the torso;
- no rupture;
- high collision softness.
The resulting fictional material might not correspond to anything in the real world — and that is useful.
Body Type should describe what a character is physically made of without dictating what the character is allowed to be.
Performance and Simulation Quality
A system like this could potentially become computationally expensive, especially with dense meshes and self-collision.
Performance should therefore be considered part of the workflow.
Possible quality levels could include:
- Preview;
- Standard;
- Final/Bake.
Preview mode could use simplified calculations or simulation geometry for interactive animation.
Final quality could use more accurate collision, deformation and solver settings.
Artists should ideally also be able to use simplified simulation meshes while rendering higher-resolution surface geometry.
The goal is not to guarantee that complex simulation is inexpensive, but to make the workflow scalable enough to remain practical.
Artist Control Is Essential
This system should not attempt to make every character obey strict real-world physics automatically.
Animation often intentionally breaks physical rules.
The goal should therefore be physically convincing behavior under artistic control, not mandatory physical accuracy.
An animator should be able to decide:
- where the body is rigid;
- where it is soft;
- where it can stretch;
- where it can inflate;
- where volume accumulates;
- how strongly it follows the rig;
- how much physics influences animation;
- whether it can rupture;
- how quickly it recovers;
- how strongly it collides with itself;
- and how these properties change during the animation.
Simple Workflow, Advanced Control
The interface could support two levels of use.
For a beginner, creating a soft inflatable character might be as simple as:
- Create or rig the character normally.
- Add Body Physics.
- Choose Body Type: Inflatable.
- Adjust a few high-level controls such as Inflation, Softness and Buoyancy.
- Enable Collision/Self-Collision if needed.
- Animate.
Advanced users could expand additional panels for detailed physical parameters, Body Property Paint, chambers, regional behavior, simulation quality and hybrid materials.
This would make the feature approachable without sacrificing professional control.
Why This Could Be Useful for Blender
Stylized animation increasingly includes characters whose bodies do not behave like conventional solid anatomy.
Artists create:
- balloon creatures;
- inflatable mascots;
- pool-toy characters;
- slimes;
- gelatinous creatures;
- elemental characters;
- living objects;
- plush characters;
- highly exaggerated cartoon bodies;
- transforming creatures;
- fictional materials that have no real-world equivalent.
Blender already provides many of the individual building blocks required to approximate these effects.
The proposed Body Type system would focus on making those kinds of physical behaviors cooperate naturally with character animation.
Instead of asking artists to repeatedly construct a custom technical workaround for every unusual character, Blender could provide a framework specifically designed for the relationship between:
Rig / Animation Intent
and
Physical Body Behavior
Scope and Implementation
The names, UI examples, parameters and technical mechanisms described above are suggestions rather than strict requirements.
The proposal is primarily about the workflow problem and desired result.
Different Body Types may require different solvers, and existing Blender systems could potentially be reused or integrated where appropriate.
The system could also be implemented incrementally rather than requiring every Body Type at once.
For example, an initial implementation could focus on a general rig-aware deformable-body framework and one or two Body Types, with additional behaviors added later if the architecture proves useful.
The exact architecture, mathematical models, interaction with modifiers, nodes, armatures and simulation systems would be better determined by Blender developers.
Main Goal
The goal is not to automate character creation.
The goal is to remove much of the technical workaround currently required to make physically unconventional characters behave like characters.
Artists would still model, rig, animate and make the creative decisions.
Blender would provide a more unified way to tell the software:
"This is a character. This is its structure. And this is what its body is physically made of."
The physics system could then respect both the material behavior and the animator's intent.
Thank you for considering this proposal.
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