Inverse Kinematics

Grab an end. Drag it. The joints figure themselves out. It's the future and it's here.

FK vs IK — When to Use What

With Forward Kinematics, you rotate each joint manually from parent to child. With Inverse Kinematics, you grab the end of a chain (a hand, a foot) and drag it where you want it. The solver calculates all the joint rotations needed to get the end effector to that position.

Enabling IK Mode

Click the IK toggle button in the viewport controls. When IK is active, red and pink drag targets appear on the character's extremities — hands, feet, and head.

Click and drag any target sphere to reposition the end effector. The entire chain from root to effector responds in real time. The solver uses CCD (Cyclic Coordinate Descent) — a fast, stable algorithm that handles complex chains gracefully.

Biomechanical Constraints

Here's where BlackBox gets serious. When IK is enabled, the solver automatically applies anatomically accurate joint constraints based on medical research from the American Academy of Orthopaedic Surgeons (AAOS) and Gray's Anatomy.

Translation: knees only bend one way. Elbows have realistic limits. Shoulders rotate within anatomical range. Your character won't contort into impossible poses — unless you want them to.

Joint Constraint Data

The Constraint Editor

Want to see and adjust the constraints? Press C while in IK mode to open the Constraint Editor.

White spheres appear at each constrained joint. Click one to see its constraint visualization:

• Colored arcs — show the allowed rotation range for each axis
• Yellow handles — drag to adjust the min/max limits
• Blue line — current bone rotation relative to the constraint
• Press Tab to cycle through X, Y, and Z axes

Joint Types

Swing-Twist Decomposition

Under the hood, constraints use a swing-twist decomposition. Every rotation is split into two parts:

• Twist — rotation around the bone's length axis (like rolling your forearm). Constrained by min/max angles.
• Swing — rotation that changes the bone's direction (bending, abduction). Constrained by a cone radius.

This separation lets the solver enforce anatomical limits without the gimbal lock issues that plague Euler-angle constraints. The format is:

{ twistAxis: Vector3, twistMin: rad, twistMax: rad, swingRadius: rad }

Importing & Exporting Constraints

Spent time perfecting constraints for one character? Export them and reuse on others.

The JSON format includes joint type detection, parent/child relationships, and automatic left/right mirror bone mapping. Import across different models and the system matches constraints to equivalent bones.

IK + FK Workflow

The most efficient animation workflow combines both:

1. Block with IK Enable IK mode and quickly drag end effectors to set up your key poses. Get the hands where they need to be, plant the feet.
2. Refine with FK Switch to FK and fine-tune individual joint rotations. Add personality — a slight wrist rotation here, a spine twist there.
3. Polish timing Use the timeline to adjust keyframe spacing. Move keyframes closer for snappy movements, spread them out for smooth, flowing motion.

What's Next