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//! **Joints** are a way to connect entities in a way that restricts their movement relative to each other.
//! They act as [constraints](dynamics::solver::xpbd#constraints) that restrict different *Degrees Of Freedom*
//! depending on the joint type.
//!
//! ## Degrees Of Freedom (DOF)
//!
//! In 3D, entities can normally translate and rotate along the `X`, `Y` and `Z` axes.
//! Therefore, they have 3 translational DOF and 3 rotational DOF, which is a total of 6 DOF.
//!
//! Joints reduce the number of DOF that entities have. For example, [revolute joints](RevoluteJoint)
//! only allow rotation around one axis.
//!
//! Below is a table containing the joints that are currently implemented.
//!
//! | Joint | Allowed 2D DOF | Allowed 3D DOF |
//! | ------------------ | ------------------------- | --------------------------- |
//! | [`FixedJoint`] | None | None |
//! | [`DistanceJoint`] | 1 Translation, 1 Rotation | 2 Translations, 3 Rotations |
//! | [`PrismaticJoint`] | 1 Translation | 1 Translation |
//! | [`RevoluteJoint`] | 1 Rotation | 1 Rotation |
#![cfg_attr(
feature = "3d",
doc = "| [`SphericalJoint`] | 1 Rotation | 3 Rotations |"
)]
//!
//! ## Using joints
//!
//! In Avian, joints are modeled as components. You can create a joint by simply spawning
//! an entity and adding the joint component you want, giving the connected entities as arguments
//! to the `new` method.
//!
//! ```
#![cfg_attr(feature = "2d", doc = "use avian2d::prelude::*;")]
#![cfg_attr(feature = "3d", doc = "use avian3d::prelude::*;")]
//! use bevy::prelude::*;
//! fn setup(mut commands: Commands) {
//! let entity1 = commands.spawn(RigidBody::Dynamic).id();
//! let entity2 = commands.spawn(RigidBody::Dynamic).id();
//!
//! // Connect the bodies with a fixed joint
//! commands.spawn(FixedJoint::new(entity1, entity2));
//! }
//! ```
//!
//! ### Stiffness
//!
//! You can control the stiffness of a joint with the `with_compliance` method.
//! *Compliance* refers to the inverse of stiffness, so using a compliance of 0 corresponds to
//! infinite stiffness.
//!
//! ### Attachment positions
//!
//! By default, joints are connected to the centers of entities, but attachment positions can be used to change this.
//!
//! You can use `with_local_anchor_1` and `with_local_anchor_2` to set the attachment positions on the first
//! and second entity respectively.
//!
//! ### Damping
//!
//! You can configure the linear and angular damping caused by joints using the `with_linear_velocity_damping` and
//! `with_angular_velocity_damping` methods. Increasing the damping values will cause the velocities
//! of the connected entities to decrease faster.
//!
//! ### Other configuration
//!
//! Different joints may have different configuration options. Many joints allow you to change the axis of allowed
//! translation or rotation, and they may have distance or angle limits along these axes.
//!
//! Take a look at the documentation and methods of each joint to see all of the configuration options.
//!
//! ## Custom joints
//!
//! Joints are [constraints](dynamics::solver::xpbd#constraints) that implement [`Joint`] and [`XpbdConstraint`].
//!
//! The process of creating a joint is essentially the same as [creating a constraint](dynamics::solver::xpbd#custom-constraints),
//! except you should also implement the [`Joint`] trait's methods. The trait has some useful helper methods
//! like `align_position` and `align_orientation` to reduce some common boilerplate.
//!
//! Many joints also have joint limits. You can use [`DistanceLimit`] and [`AngleLimit`] to help store these limits
//! and to compute the current distance from the specified limits.
//!
//! [See the code implementations](https://github.com/Jondolf/avian/tree/main/src/constraints/joints)
//! of the implemented joints to get a better idea of how to create joints.
mod distance;
mod fixed;
mod prismatic;
mod revolute;
#[cfg(feature = "3d")]
mod spherical;
pub use distance::*;
pub use fixed::*;
pub use prismatic::*;
pub use revolute::*;
#[cfg(feature = "3d")]
pub use spherical::*;
use crate::{dynamics::solver::xpbd::*, prelude::*};
use bevy::prelude::*;
/// A trait for [joints](self).
pub trait Joint: Component + PositionConstraint + AngularConstraint {
/// Creates a new joint between two entities.
fn new(entity1: Entity, entity2: Entity) -> Self;
/// Sets the joint's compliance (inverse of stiffness, meters / Newton).
fn with_compliance(self, compliance: Scalar) -> Self;
/// Sets the attachment point on the first body.
fn with_local_anchor_1(self, anchor: Vector) -> Self;
/// Sets the attachment point on the second body.
fn with_local_anchor_2(self, anchor: Vector) -> Self;
/// Sets the linear velocity damping caused by the joint.
fn with_linear_velocity_damping(self, damping: Scalar) -> Self;
/// Sets the angular velocity damping caused by the joint.
fn with_angular_velocity_damping(self, damping: Scalar) -> Self;
/// Returns the local attachment point on the first body.
fn local_anchor_1(&self) -> Vector;
/// Returns the local attachment point on the second body.
fn local_anchor_2(&self) -> Vector;
/// Returns the linear velocity damping of the joint.
fn damping_linear(&self) -> Scalar;
/// Returns the angular velocity damping of the joint.
fn damping_angular(&self) -> Scalar;
/// Applies a positional correction that aligns the positions of the local attachment points `r1` and `r2`.
///
/// Returns the force exerted by the alignment.
#[allow(clippy::too_many_arguments)]
fn align_position(
&self,
body1: &mut RigidBodyQueryItem,
body2: &mut RigidBodyQueryItem,
r1: Vector,
r2: Vector,
lagrange: &mut Scalar,
compliance: Scalar,
dt: Scalar,
) -> Vector {
let world_r1 = *body1.rotation * r1;
let world_r2 = *body2.rotation * r2;
let (dir, magnitude) = DistanceLimit::new(0.0, 0.0).compute_correction(
body1.current_position() + world_r1,
body2.current_position() + world_r2,
);
if magnitude <= Scalar::EPSILON {
return Vector::ZERO;
}
// Compute generalized inverse masses
let w1 = PositionConstraint::compute_generalized_inverse_mass(self, body1, world_r1, dir);
let w2 = PositionConstraint::compute_generalized_inverse_mass(self, body2, world_r2, dir);
// Compute Lagrange multiplier update
let delta_lagrange =
self.compute_lagrange_update(*lagrange, magnitude, &[w1, w2], compliance, dt);
*lagrange += delta_lagrange;
// Apply positional correction to align the positions of the bodies
self.apply_positional_lagrange_update(
body1,
body2,
delta_lagrange,
dir,
world_r1,
world_r2,
);
// Return constraint force
self.compute_force(*lagrange, dir, dt)
}
}
/// A limit that indicates that the distance between two points should be between `min` and `max`.
#[derive(Clone, Copy, Debug, PartialEq, Reflect)]
#[cfg_attr(feature = "serialize", derive(serde::Serialize, serde::Deserialize))]
#[cfg_attr(feature = "serialize", reflect(Serialize, Deserialize))]
#[reflect(Debug, PartialEq)]
pub struct DistanceLimit {
/// The minimum distance between two points.
pub min: Scalar,
/// The maximum distance between two points.
pub max: Scalar,
}
impl DistanceLimit {
/// A `DistanceLimit` with `min` and `max` set to zero.
pub const ZERO: Self = Self { min: 0.0, max: 0.0 };
/// Creates a new `DistanceLimit`.
pub const fn new(min: Scalar, max: Scalar) -> Self {
Self { min, max }
}
/// Returns the direction and magnitude of the positional correction required
/// to limit the distance between `p1` and `p2` to be within the distance limit.
pub fn compute_correction(&self, p1: Vector, p2: Vector) -> (Vector, Scalar) {
let pos_offset = p2 - p1;
let distance = pos_offset.length();
if distance <= Scalar::EPSILON {
return (Vector::ZERO, 0.0);
}
// Equation 25
if distance < self.min {
// Separation distance lower limit
(-pos_offset / distance, (distance - self.min))
} else if distance > self.max {
// Separation distance upper limit
(-pos_offset / distance, (distance - self.max))
} else {
(Vector::ZERO, 0.0)
}
}
/// Returns the positional correction required to limit the distance between `p1` and `p2`
/// to be within the distance limit along a given `axis`.
pub fn compute_correction_along_axis(&self, p1: Vector, p2: Vector, axis: Vector) -> Vector {
let pos_offset = p2 - p1;
let a = pos_offset.dot(axis);
// Equation 25
if a < self.min {
// Separation distance lower limit
axis * (self.min - a)
} else if a > self.max {
// Separation distance upper limit
-axis * (a - self.max)
} else {
Vector::ZERO
}
}
}
/// A limit that indicates that angles should be between `alpha` and `beta`.
#[derive(Clone, Copy, Debug, PartialEq, Reflect)]
#[cfg_attr(feature = "serialize", derive(serde::Serialize, serde::Deserialize))]
#[cfg_attr(feature = "serialize", reflect(Serialize, Deserialize))]
#[reflect(Debug, PartialEq)]
pub struct AngleLimit {
/// The minimum angle.
pub min: Scalar,
/// The maximum angle.
pub max: Scalar,
}
impl AngleLimit {
/// An `AngleLimit` with `alpha` and `beta` set to zero.
pub const ZERO: Self = Self { min: 0.0, max: 0.0 };
/// Creates a new `AngleLimit`.
pub const fn new(min: Scalar, max: Scalar) -> Self {
Self { min, max }
}
/// Returns the angular correction required to limit the angle between two rotations
/// to be within the angle limits.
#[cfg(feature = "2d")]
pub fn compute_correction(
&self,
rotation1: Rotation,
rotation2: Rotation,
max_correction: Scalar,
) -> Option<Scalar> {
let angle = rotation1.angle_between(rotation2);
let correction = if angle < self.min {
angle - self.min
} else if angle > self.max {
angle - self.max
} else {
return None;
};
Some(correction.min(max_correction))
}
/// Returns the angular correction required to limit the angle between `axis1` and `axis2`
/// to be within the angle limits with respect to the `limit_axis`.
#[cfg(feature = "3d")]
pub fn compute_correction(
&self,
limit_axis: Vector,
axis1: Vector,
axis2: Vector,
max_correction: Scalar,
) -> Option<Vector> {
// [limit_axis, axis1, axis2] = [n, n1, n2] in XPBD rigid body paper.
// Angle between axis1 and axis2 with respect to limit_axis.
let mut phi = axis1.cross(axis2).dot(limit_axis).asin();
// `asin` returns the angle in the [-pi/2, pi/2] range.
// This is correct if the angle between n1 and n2 is acute,
// but obtuse angles must be accounted for.
if axis1.dot(axis2) < 0.0 {
phi = PI - phi;
}
// Map the angle to the [-pi, pi] range.
if phi > PI {
phi -= TAU;
}
// The XPBD rigid body paper has this, but the angle
// should already be in the correct range.
//
// if phi < -PI {
// phi += TAU;
// }
// Only apply a correction if the limit is violated.
if phi < self.min || phi > self.max {
// phi now represents the angle between axis1 and axis2.
// Clamp phi to get the target angle.
phi = phi.clamp(self.min, self.max);
// Create a quaternion that represents the rotation.
let rot = Quaternion::from_axis_angle(limit_axis, phi);
// Rotate axis1 by the target angle and compute the correction.
return Some((rot * axis1).cross(axis2).clamp_length_max(max_correction));
}
None
}
}