Interface BodyDefinition

Bit mask of collision types this body can collide with. Defaults to collision.types.ALL_OBJECT.

Bit flag identifying this body's collision type. See collision.types. Adapters translate to their native filter system (Matter collisionFilter.category, Box2D categoryBits).

mass per unit area; defaults are adapter-specific

Disable rotation simulation, so the body keeps whatever angle it has.

Defaults to whatever the underlying engine defaults to, which for both matter and planck is false: a rigid body turns when something turns it. Pass true for anything that must stay upright, such as a platformer actor that should never tip over.

The builtin adapter ignores this field, since its rotation is visual only and its collision shapes never turn either way.

Surface coefficient of friction during contact. Matter's body.friction — 0 = frictionless (objects slide past each other), 1 = high stick. Determines how much tangential velocity is transferred between contacting bodies. Combined with body rotation, this is what produces "throw" between colliding circles and "english" off a wall. Distinct from frictionAir (per-step drag with no contact required). Builtin SAT adapter ignores this.

Per-step velocity damping (Matter's frictionAir). Bleeds velocity off every frame regardless of contact, creating terminal velocity. Number applies uniformly; {x, y} damps each axis independently (melonJS-specific — Matter only supports scalar and will average).

It damps ROTATION as well as translation on the matter and planck adapters, so it is also what stops a body spinning once something has set it turning. Left at 0, a spin never decays. The builtin adapter maps it onto its per-axis body.friction damping vector instead, and has no rotation to damp.

Per-body gravity multiplier. 0 disables gravity for this body; negative inverts it. Matches Matter's body.gravityScale.

the body generates collision events but no physical response

Hard cap on velocity magnitude per axis. melonJS extension — Matter has no direct equivalent and uses frictionAir-induced terminal velocity instead; MatterAdapter implements this by clamping velocity in an afterUpdate hook.

Bounciness (coefficient of restitution): 0 = inelastic (stops on contact), 1 = perfectly elastic (rebound speed equals impact speed). Typically in [0, 1]. Values > 1 produce a super-elastic ("energy-gain") rebound — physically unrealistic but useful for arcade effects like pinball flippers, slingshot boosters, or trampoline pads. The value is not clamped at the interface level; how an adapter handles out-of-range values is adapter-specific.

collision shapes (one or more — compound body support). Already resolved: a definition naming a collision shape file is a BodyDefinitionInit, and the engine resolves it before any adapter is handed it.

simulation kind

arbitrary user data attached to the body