Joint

Joint is a configurable link between two rigid bodies, it restricts relative motion of two bodies. Fyrox provides a fixed set of joints that are suitable for various applications.

• Fixed Joint - hard link between two bodies, it is the same is if two rigid bodies were "welded" to each other with a metal rod.
• Revolute Joint - restricts all translational movement and any rotations around Y and Z axes, but leaves rotation around local X axis free. An example of the joint from real world is a door hinge, it allows the door to rotate around single axis, but not move.
• Prismatic Joint - restricts all rotations, movement is allowed along single axis (local X of the joint). An example of the joint from real world could be a slider that supports drawers on a table.
• Ball Joint - restricts all movement, but leaves rotations unrestricted. An example of a ball joint from real world could be human shoulder.

2D joints does not have revolute joints, because it degenerates into ball joint.

Bodies Binding

When the joint is created and all bodies are set to it, it uses self global transform and bodies global transforms to calculate local frames for bodies. This process is called binding, it happens once when the joint is created, but can be initiated by moving the joint to some other position by changing local transform of the joint.

How to create

To create a joint from code use JointBuilder:

#![allow(unused)]
fn main() {
extern crate fyrox;
use fyrox::{
    core::pool::Handle,
    scene::{
        base::BaseBuilder,
        graph::Graph,
        joint::{BallJoint, JointBuilder, JointParams},
        node::Node,
    },
};
fn create_joint(graph: &mut Graph, body1: Handle<Node>, body2: Handle<Node>) -> Handle<Node> {
    JointBuilder::new(BaseBuilder::new())
        .with_body1(body1)
        .with_body2(body2)
        .with_params(JointParams::BallJoint(BallJoint {
            x_limits_enabled: false,
            x_limits_angles: Default::default(),
            y_limits_enabled: false,
            y_limits_angles: Default::default(),
            z_limits_enabled: false,
            z_limits_angles: Default::default(),
        }))
        .build(graph)
}
}

Once the joint is created, it will bind given bodies, using the process describe in the above section.

To create a joint from editor, use MainMenu -> Create -> Physics -> Joint, select the new joint and find Body1 and Body2 properties. Assign the fields by holding Alt key and drag'n'drop a rigid body to a field. Move the joint to correct position to ensure the binding will happen as intended.

Limits

You can restrict motion on primary joint axis (rotational and translational) by setting a limit to desired axis.

• Ball Joint have three angular limits, one per rotation around an axis. The angle range is given in radians.
• Prismatic Joint have only one limit it is maximum linear distance between two bodies along primary joint axis.
• Revolute Joint have a single angular limit around primary axis. The angle range is given in radians.
• Fixed Joint does not have any limit setting, because it locks all degrees of freedom.

Usage

Joints can be used to create many game entities, such as doors, chains and rag dolls. The most interesting here is rag doll. It is used to create realistic behaviour for humans and creatures in games. In general, it is a set of rigid bodies, colliders and joints. Where each joint configured to match joints of a creature, for example ball joint could be used for shoulders, revolute joints for knees and elbows.