Setting up bone constraints for games

Setting Up Bone Constraints for Games

Constraints are the mechanism controlling bone behavior in space: where it points, who it follows, rotation limits. Without properly configured Constraints, rig either requires constant animator intervention or breaks at extreme poses.

Constraint Types and Game Rig Usage

Copy Rotation / Copy Transform is foundation of IK/FK switch systems. Result bone copies transformation from IK or FK chain depending on weight. Influence = 0 or 1 switches mode; fractional values give smooth blend. Critical setting: Space—Local or World. For copying joint rotations—Local Space. For positional constraints accounting parent bone rotation—Pose Space in Blender.

Stretch To / Maintain Volume for limbs that should slightly stretch when IK Target approaches chain length limit. Used cautiously in game rigs: too strong stretch looks unnatural but small value (Rest Length + 5–10%) removes stiff feeling at extreme poses.

Damped Track / Track To makes head and eyes track target. Damped Track is smoother with fewer artifacts during fast target movement. Track To is more precise but gimbal flip occurs at certain angles. For eyes—Copy Location to target with rotation limited through Limit Rotation ±30° on X and Y.

Limit Rotation constrains joint rotation range. Mandatory for anatomically correct rig: elbow doesn't rotate backward, knee doesn't bend forward. In Blender: Limit Rotation Constraint with enabled axes and explicit Min/Max. Example: LeftElbow, Limit Y: -150° to 0°.

Floor Constraint keeps bone above specified object. Used for feet in simple Foot IK without raycast: heel doesn't sink through floor plane. Less flexible than Animation Rigging IK but significantly simpler for mobile projects without Physics.

Copy Scale and Squash/Stretch for Stylized Characters

For stylized games with cartoon deformations Squash & Stretch is implemented through Copy Scale with axis inversion. When bone scales on Y (stretching), X and Z axes decrease proportionally to preserve volume. Formula: Scale.X = 1 / sqrt(Scale.Y).

In Blender this is implemented through Driver: 1 / sqrt(scale.y) on X and Z Scale of target bone. Maintain Volume Constraint does same thing natively without extra bones. For game export: all Constraints must be baked to keyframes before FBX export—engine reads only bone transformations, not Blender-specific Constraints.

Child Of Constraint and Dynamic Parent Links

Child Of lets bone temporarily become child of another object. Application: character picks up weapon—weapon bone becomes child of hand bone. Puts weapon down—returns to world space or attaches to environment object.

Important nuance: when enabling Child Of must press Set Inverse—this saves current world bone position when parent changes. Without Set Inverse bone teleports to new parent's local origin.

For Unity export: bake animation with space switching through NLA Editor → Bake Action with Clear Constraints option. Result FBX contains clean World Space keys without Constraint dependency.

Constraints in Unity Animation Rigging

Animation Rigging package implements part of Blender/Maya Constraints as components:

• Multi-Parent Constraint—analogous to Child Of with multiple sources and weights
• Multi-Aim Constraint—analogous to Track To
• Multi-Rotation Constraint—analogous to Copy Rotation
• Twist Correction—distributes rotation along Twist-bones, analogous to Twist Deformer
• Override Transform—fully redefine bone transformation with blend weight

All components work in runtime and can be managed through C#—this makes them powerful tool. Twist Correction with TwistAxis = X and SourceObject = forearm gives correct Twist without extra animation keys. Single Constraint setup suffices for whole project.

Typical Constraint Setup Errors

Circular Dependency. Bone A through Constraint depends on Bone B which through another Constraint depends on Bone A. Blender and Maya detect this as Cyclic Dependency Warning and give unpredictable result. Solved through hierarchy rebuild or using intermediate Driver variables.

Constraint Stack Order. In Blender multiple Constraints on one bone execute top to bottom. Limit Rotation should be last—otherwise other Constraints can push bone beyond limit after limiting. IK overrides everything below—feature often ignored.

World vs Local Space in Copy Rotation. Copy Rotation in World Space copies absolute angles breaking with any parent bone rotation. For rotation transmission within hierarchy—Local Space or Pose Space.

Timeline

Cost calculated individually depending on rig complexity and animation pipeline requirements.