When to Use a Weld Joint in Planck.js?

In 2D physics simulation using planck.js (the JavaScript rewriting of Box2D), developers use a Weld Joint to rigidly bind two rigid bodies together, preventing them from moving or rotating independently. This article explores the specific scenarios where a Weld Joint is necessary, how it behaves under the physics engine’s constraints, and practical alternatives when performance or structural rigidity is a concern.

Understanding the Weld Joint in Planck.js

A Weld Joint is designed to lock the relative position and angle between two bodies. Once welded, they behave largely as a single physical entity. However, because planck.js is an iterative constraint solver, welds are not completely “infinitely rigid.” Under heavy forces or high-velocity collisions, a weld joint can flex, stretch, or sag slightly before snapping back into place.

Developers can fine-tune this behavior using two key properties:

• FrequencyHz: Defines the stiffness of the joint (higher values mean a more rigid weld).
• DampingRatio: Controls how quickly the joint stops oscillating or flexing when subjected to external forces.

Key Use Cases for a Weld Joint

While it might seem intuitive to just create a single body with multiple fixtures, there are distinct architectural and gameplay reasons to use a Weld Joint instead:

1. Destructible Environments and Breakable Objects

If you want an object to break apart into smaller pieces upon impact (like a wooden crate, a bridge, or a spaceship), you cannot easily do this if it is a single rigid body. By connecting multiple smaller bodies with Weld Joints, you can monitor the reaction forces. When the collision force exceeds a specific threshold, you can destroy the Weld Joint, causing the object to realistically split into independent, physics-driven fragments.

2. Dynamic Assembly of Multi-Part Entities

In games where players can build mechanisms, vehicles, or structures in real-time, Weld Joints are essential. When a player attaches a new part—such as adding an armor plate to a rover or building a tower out of blocks—a Weld Joint instantly locks the new component to the existing structure without requiring you to delete the old bodies and recreate a complex, single multi-fixture body.

3. Combining Dynamic and Kinematic Behaviors Temporary

If a character or object needs to grab, carry, or fuse with another object temporarily, a Weld Joint provides a quick solution. For example, if a sticky projectile hits a wall or an enemy, welding the projectile’s body to the target body ensures it stays perfectly attached and moves in tandem with the target’s physics.

When to Avoid Weld Joints (Alternatives)

Because Weld Joints introduce extra overhead to the physics solver and can suffer from visual “softness” under extreme stress, they are not always the best choice.

• Multi-Fixture Bodies: If two shapes are permanently attached and will never break apart, do not use a Weld Joint. Instead, attach multiple Fixture objects to a single Body. This is perfectly rigid, reduces CPU overhead, and simplifies collision handling.
• Prismatic or Revolute Joints: If you need some restricted movement (like a piston or a hinge), use the specialized joints rather than trying to configure a soft Weld Joint to mimic them.