Constraints

Constraints move bones for you. Rigby2D has five kinds, all evaluated by the editor and by every runtime:

Constraint	Does	Typical use
IK	Bends a chain of one or two bones so its tip reaches a target bone.	Arms reaching, feet planted on the ground.
Transform	Copies (or adds) a source bone's rotation, position, scale or shear to other bones.	Gears, mirrored parts, a jaw following a chin control.
Path	Places and rotates bones along a path attachment.	Tails, ropes, tentacles, objects following a track.
Physics	Simulates springy follow-through on bones.	Hair, braids, earrings, cloth, antennae.
Slider	Plays an animation at a time set by a value or by a bone.	Facial rigs, dials, a bone that "scrubs" a pose.

#Creating a constraint

In Setup mode, select the bones as the constraint needs them (below).
In the tree's Constraints section, press + and choose the kind.
Adjust the settings in the inspector.

Every constraint has a Name, a Mix (how much it applies, 0 to 1), an Order (Apply earlier / Apply later: constraints apply one after another) and Skin required (active only when the current skin includes it; see Skins).

In Animate mode, selecting a constraint shows its keyable values instead (mixes, IK bend, path position and spacing, physics strength and wind, slider time) with ◆ key buttons. Keys appear in the dopesheet.

#IK

Select 1 or 2 bones for the chain, then optionally the target bone, and create an IK constraint. With no target selected, a new bone is added at the tip of the chain to act as the target. Move the target and the chain follows.

The IK constraint settings for NILA's right arm — IK constraint ik_hand_r: the chain upperarm_r → forearm_r reaching for its target

Setting	Meaning
Bones	The chain (1 or 2 bones). Use selected bones as chain replaces it with the selection.
Target	The bone the chain reaches for.
Mix	0 = the animated pose, 1 = fully solved by IK.
Softness	Distance before full reach where the chain starts slowing, so a straightening arm or leg does not snap.
Bend	The direction the middle joint bends. Flip it if an elbow or knee bends the wrong way.
Compress	Scale the bones down when the target is closer than their length.
Stretch	Scale the bones up when the target is farther than their length.
Uniform	Compress and stretch scale both axes.

Tip

For walk cycles, key the foot targets, not the leg bones: feet stay planted while the hips move. AutoRig adds IK targets (ik_foot_l, ik_hand_r, …) for every two-bone limb.

#Transform

Select the bones to constrain, then the source bone last.

Setting	Meaning
Bones, Source	The constrained bones and the bone they read from.
Local source	Read the source bone in its parent's space.
Local target	Change the constrained bones in their parent's space.
Additive	Add the source's values instead of matching them.
Clamp	Limit mapped values to each mapping's range.
Properties	Which of Rotate, X, Y, Scale X, Scale Y and Shear Y are copied, each with an Offset and a Mix.
Property mapping	Advanced: let one source property drive another target property (Source → target) with an Offset, Scale and Max. For example, the source's rotation can move the target along X.

Offsets from current pose sets the offsets so the constrained bone stays where it is now in the setup pose.

#Path

Add a path attachment first, or let the constraint make one: select the bones to place along the path and create a path constraint. A new path through the bones' origins and the last bone's tip is added.

Setting	Meaning
Path slot	The slot whose path the bones follow.
Position	Fixed (in world units) or Percent (of the path length), with Position value. Key the value to slide the bones along the path.
Spacing	Length (each bone's length), Fixed, Percent or Proportional, with Spacing value.
Rotate	Tangent (along the path), Chain (point at the next bone) or Chain scale (also stretch to reach it).
Rotation offset	Added rotation.
Mix rotate, Mix X, Mix Y	How much of the rotation and position the path controls.

#Physics

Select the bones that should move with physics and create a physics constraint. Each bone gets one. Physics adds motion the animation does not have: when the head turns, a braid swings past, overshoots and settles.

Setting	Meaning
Bone	The simulated bone.
Translate X, Translate Y, Rotate, Scale X, Shear X	Which motions are simulated, and how strongly (0 to 1). Braids and earrings usually only need Rotate.
Limit	The fastest speed the simulation lets a bone move, so fast animation cannot fling it away.
Step	Simulation rate (60 fps by default).
Inertia	How much of the parent's motion the bone resists (0 to 1).
Strength	How strongly it springs back to the animated pose.
Damping	How quickly the swinging dies down (0 to 1).
Mass	Heavier bones swing more slowly.
Wind, Gravity	Constant forces.
Mix	How much of the simulation is applied.

Global next to a value makes it also follow skeleton-wide physics keys, so one key can change the wind or gravity for every physics constraint at once.

Physics runs while an animation plays (in the editor, in renders and in the runtimes) and restarts from the animated pose when playback starts. Renders simulate frame by frame from the start of the animation, so they are repeatable.

Note

Rigby2D's physics uses its own spring model driven by these settings, so the motion can differ slightly from other tools that read the same files.

#Slider

A slider plays an animation at a time you set. Create one when the project has an animation to drive.

Setting	Meaning
Animation	The animation it applies.
Time	Where in the animation (seconds). Key it to scrub the animation from another animation.
Mix	How much is applied.
Loop	Wrap the time around the animation's length.
Additive	Add the animation on top instead of replacing the pose.
Bone	Optional: drive the time from a bone. With a bone set, Property (rotation, X, Y, scale or shear) times Scale gives the time; Local reads it in the parent bone's space.

For example, make an animation smile that goes from neutral (time 0) to a full smile (time 1), add a slider for it, and set Bone to a smile_ctrl bone with Y × 0.01: moving the control up 100 units smiles fully.

#Through the runtimes

The JavaScript and Rust runtimes evaluate all five constraint types with their keys. See Runtimes. Binary .skel export supports all of them; see Export.