FunnyBones

A simple 2D kinematics library for Rust

Motivation and Goals

When looking at the libraries that support inverse kinematics in Rust in 2024, there are several fairly mature solutions that focus on 3D and robotics. For someone interested in 2D only, a lot of these libraries seemed like overkill for basic 2D games.

This library implements a simplified forward and inverse kinematics model that only uses basic trigonometry and can be solved in one pass across the bone structure with no smoothing algorithms necessary. The initial implementation of this library was under 600 lines of code with no required dependencies.

How FunnyBones works

FunnyBones has two main concepts: joints and bones.

• Joints are used to connect a specific end of one bone to a specific end of another bone. Each joint can be assigned an angle which is applied as forward kinematics to create the angle using the two associated bones.

• Bones are one-dimensional line segments that have a required length. Bones can have a desired position for the end of the bone positioned furthest from the skeleton root. If the desired position is set, it is applied as inverse kinematics.

  In FunnyBones, bones come in two varieties:

  • Rigid bones are a single line segment of a fixed length. An example of a rigid bone in a simple human skeleton might be a single bone representing the spine.
  • Flexible bones are two line segments of fixed lengths that bend and rotate automatically (ignoring the connecting joint's angle) to ensure that both leg segments are always the correct length. An example of a flexible bone in a simple human skeleton might be a leg or an arm.

A Skeleton is a collection of joints and bones. The first bone pushed is considered the root bone. When solving for updated positions, the algorithm starts by evaluating all joints connected to both ends of the root bone and continues until all reachable bones have been evaluated. The algorithm is single-pass and produces stable results.

FunnyBones in Action

The skeleton example in the repository uses Cushy to draw and allow changing various settings of a basic humanoid skeleton:

let mut skeleton = Skeleton::default();

// Create our root bone: the spine
let spine = skeleton.push_bone(BoneKind::Rigid { length: 3. }.with_label("spine"));
// Create the right-half of the hips.
let r_hip = skeleton.push_bone(BoneKind::Rigid { length: 0.5 }.with_label("r_hip"));
// Connect the right hip to the spine.
skeleton.push_joint(Joint::new(
    Angle::degrees(-90.),
    spine.axis_a(),
    r_hip.axis_a(),
));
// Create the right leg as a jointed bone with equal sizes for the upper and
// lower leg.
let r_leg = skeleton.push_bone(
    BoneKind::Jointed {
        start_length: 1.5,
        end_length: 1.5,
        inverse: true,
    }
    .with_label("r_leg"),
);

// Connect the right leg to the right hip.
skeleton.push_joint(Joint::new(
    Angle::degrees(0.),
    r_hip.axis_b(),
    r_leg.axis_a(),
));
// Create the right foot.
let r_foot = skeleton.push_bone(BoneKind::Rigid { length: 0.5 }.with_label("r_foot"));
// Connect the right foot to the right leg.
let r_ankle_id = skeleton.push_joint(Joint::new(
    Angle::degrees(90.),
    r_leg.axis_b(),
    r_foot.axis_a(),
));

The example draws a small white circle where a desired location for a joint is. FunnyBones ensures that all bones remain their original lengths while solving the kinematics.

Open-source Licenses

This project, like all projects from Khonsu Labs, is open-source. This repository is available under the MIT License or the Apache License 2.0.

To learn more about contributing, please see CONTRIBUTING.md.