Joint

The physics system contains a series of Joint components for linking two rigidbodies. Joint components can be used to simulate the interaction between real objects, such as hinges, pistons, ropes, wheels, pulleys, motor vehicles, chains and so on. Learning how to use joint components can effectively help create a truly interesting scene.

Note: joint components are invalid in the Builtin 2D physics module.

The following joints are available in the physics system:

  • Distance Joint - The anchors of two rigidbodies at both ends of the joint will keep a certain distance.
  • Fixed Joint - Hold two points on two objects together according to their initial angles of view.
  • Hinge Joint - Think of it as a hinge or a nail, where the rigid body rotates around a common point.
  • Relative Joint - Control the relative motion between two rigid bodies
  • Slider Joint - The angle between two rigid body positions is fixed, and they can only slide in one specified axis.
  • Spring Joint - Connects objects at both ends of the joint like a spring.
  • Wheel Joint - Consist of Revolute and Prismatic joint, used to simulate motor vehicle wheels.

Common properties of joints

Although each joint has a different behavior, they have some common properties.

  • connectedBody - the rigidbody connected on the other end of the joint.
  • anchor - the anchor of the rigidbody on the same node of the joint.
  • connectedAnchor - the anchor of the rigidbody connected at the other end of the joint
  • collideConnected - whether the rigidbody at both ends of the joint can collide with each other

Each joint need to link two rigidbodies to work. The one on the same node of the joint and the one on another node, which will be assigned to connectedBody property. Usually, each rigid body chooses a location around itself as an anchor point. Depending on the type of joint component, the anchor point determines the center of rotation of the object, or a coordinate point to hold a certain distance, etc.

collideConnected property is used to determine if the rigid bodies at both ends of the joint need to continue to follow the normal collision rules. For example, when ready to make a ragdoll, you may want the upper and lower leg can be partially intersected, and then linked at the knee joint. You will need to set collideConnected property to false. If you are making a lift, you may want the lift platform and floor to collide, then you need to set the collideConnected property to true.

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