[Feature] 2D joints and constraints #123

Overview

Add joints that connect rigid bodies together with various constraint types
for doors, chains, vehicles, and character rigs.

Current State

No response

Scope

Goals:

• Revolute joint (hinge, rotation only)
• Prismatic joint (slider, translation only)
• Distance joint (fixed distance between points)
• Spring joint (soft distance constraint)
• Motor support on applicable joints

Non-Goals:

• Rope/chain simulation
• Breakable joints
• Gear joints

Proposed API

pub struct Joint2D {
  // handle
}

pub struct RevoluteJointBuilder {
  body_a: RigidBodyHandle,
  body_b: RigidBodyHandle,
  anchor_a: [f32; 2],
  anchor_b: [f32; 2],
  motor_speed: Option<f32>,
  motor_torque: Option<f32>,
}

pub struct DistanceJointBuilder {
  body_a: RigidBodyHandle,
  body_b: RigidBodyHandle,
  anchor_a: [f32; 2],
  anchor_b: [f32; 2],
  rest_length: f32,
  stiffness: f32,
  damping: f32,
}

impl RevoluteJointBuilder {
  pub fn new(body_a: RigidBodyHandle, body_b: RigidBodyHandle) -> Self;
  pub fn with_anchors(self, a: [f32; 2], b: [f32; 2]) -> Self;
  pub fn with_motor(self, speed: f32, max_torque: f32) -> Self;
  pub fn build(self, world: &mut PhysicsWorld2D) -> Joint2D;
}

Acceptance Criteria

• Revolute joint constrains rotation around anchor
• Prismatic joint constrains to linear motion
• Distance joint maintains fixed distance
• Spring joint provides soft constraint with oscillation
• Motors can drive joint motion
• Example demonstrating a swinging door or pendulum

Affected Crates

lambda-rs, lambda-rs-platform

Notes

• Anchors in local body coordinates
• Stiffness/damping for spring behavior