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Multibdoy Joints and IK

Godot Rapier has Impulse Joints, which use Constraints-based approach.

It also has Multibdoy Joints, which use the Reduced Coordinates Approach. They are more stable but slower and can be used for IK. Multibody joints are a chain of multiple joints that are continuously connected, and use the Rapier Extension Classes.

"WHICH APPROACH SHOULD I USE?"

The choice of approach depends on the application. For robotics, the reduced-coordinates approach is generally preferred because of its accuracy and ease of use, e.g., for control, inverse kinematics, etc.

Inverse Kinematics​

The multibody joints support IK target nodes. In order to use it, create a Rapier Extension Class of the node you want to use (eg. RapierPinJoint2D) and select joint_type to Multibody:

multibody

Now, the whole chain structure will follow the ik_target:

ik

When setting a multibody joint, the rigidbodies need to not collide with eachother and be massless.