Connecting Multiple Rigid Bodies in Ammo.js

This article explains how to connect multiple rigid bodies within the Ammo.js physics engine, addressing whether a single complex constraint can link more than two bodies. It covers the inherent limitations of standard Ammo.js constraints, provides the industry-standard workaround using pairwise constraint chaining, and introduces advanced alternatives like the Featherstone multibody system for complex physical simulations.

The Short Answer: No Single Multi-Body Constraints

In Ammo.js (which is a direct JavaScript port of the C++ Bullet Physics engine), you cannot connect more than two rigid bodies using a single standard constraint. All standard constraints in Ammo.js—such as btPoint2PointConstraint, btHingeConstraint, btSliderConstraint, and btGeneric6DofConstraint—inherit from btTypedConstraint. By design, these classes only support relationships between a maximum of two rigid bodies (or one rigid body and the static world).

If you attempt to pass three or more bodies into a standard constraint constructor, the engine will not accept them, as the mathematical solvers are hardcoded for pairwise interactions.

Solution 1: Pairwise Constraint Chaining

The most common and straightforward way to connect multiple rigid bodies is to chain multiple pairwise constraints together.

For example, if you want to connect three rigid bodies (Body A, Body B, and Body C) to act as a double-jointed pendulum, you must create two separate constraints: 1. Constraint 1: Connects Body A to Body B. 2. Constraint 2: Connects Body B to Body C.

Efficiency and Performance of Chaining

Chaining constraints is highly efficient for simple structures, but it introduces simulation challenges as the chain grows: * Solver Iterations: Ammo.js uses an iterative impulse solver. If you chain many bodies together (like a long rope or a complex ragdoll), the joints may appear “stretchy” or loose under heavy loads. * CPU Overhead: To keep a long chain of constraints rigid, you must increase the physics world’s solver iterations using dynamicsWorld.getSolverInfo().m_numIterations. Raising this value improves joint stability but increases CPU usage.

Solution 2: The btMultiBody API (Featherstone’s Algorithm)

For cases where you require highly efficient, perfectly rigid connections among more than two bodies (such as robotic arms, complex machinery, or precise ragdolls), Ammo.js includes the btMultiBody API.

Instead of treating each link as an independent rigid body connected by loose joints, btMultiBody uses Featherstone’s algorithm to treat the entire multi-body structure as a single system with generalized coordinates.

Why Use btMultiBody?

• Zero Joint Stretch: Because the mathematical equations solve the entire system simultaneously, joints will not stretch or drift apart, regardless of the forces applied.
• High Efficiency: It is computationally more efficient and stable than chaining dozens of standard pairwise constraints at high solver iteration counts.
• Complexity: The primary drawback is setup complexity. Implementing a btMultiBody structure requires defining a parent base link and systematically adding child links with specific spatial inertias, which is significantly more difficult to program than standard rigid body constraints.