Christopher Vedlund Specialization

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Procedural Animations With Inverse Kinematics Using FABRIK

During the second year at TGA, we have a specialization course that allows us to dive deeper into a topic of our choice. For my specialization, I chose to explore how procedural animations can enhance gameplay immersion.

To achieve this, I implemented Inverse Kinematics using the FABRIK algorithm and animated a skeletal structure with it.

My goal was to recreate the subtle and immersive hand animations seen in Uncharted 4: A Thief’s End—the small movements that add a layer of realism and presence to the experience.

The entire project was developed in my own custom C++ game engine using DirectX 11.

Project Specifications
Reference: Uncharted 4
Duration: 8 weeks
Speed: 50%

Why FABRIK?

I chose to use FABRIK because it is well-suited and optimized for game development. Unlike other inverse kinematics (IK) methods, FABRIK offers a balance between efficiency and accuracy, making it ideal for real-time applications where performance is crucial.

However, FABRIK does come with a significant limitation: it only works with positions, meaning that rotations must be handled manually. This can add complexity when implementing it for 3D skeletal animations or other applications requiring precise orientation control.

Despite this drawback, FABRIK also stood out as one of the simplest IK methods to implement, making it a practical choice given the short timeframe of the course. Its ease of implementation allowed for a faster development process while still maintaining the necessary functionality for game-related applications.

FABRIK Implementation

The implementation of the position solving was straight forward. Perform the passes a number of iterations. But I found that a single iteration is enough for my joint chain. But for larger joint chains more iterations was needed.

And you can also set a flag in the chain to decide if it should use the target local to its own space, set it so it uses world space and the models transform.

Passes

The FABRIK algorithm operates in two key passes to adjust joint positions efficiently:

Backward Pass

The chain starts from the end effector and moves backward toward the root. This step ensures that the end effector reaches the target while maintaining segment lengths.

Forward Pass

The chain then moves forward, repositioning joints while keeping the root in place. This step restores the structure while maintaining the target alignment.

FABRIK Forward and Backward Pass

Joint Rotations

This part was the hardest to figure out, I struggled sometime with what space the positions would be calculated in and which space would make the rotation calculations easier. But I solved it by coming up with this solution!

The SolveRotation step in FABRIK adjusts joint rotations after their positions are updated. It iterates through each joint (except the last) to align it with the next joint in the chain. For the last joint, I store a parameter withing the chain where you can chose your own rotations using euler angles.

Using quaternions ensures smooth rotations and avoids problems like gimbal lock, maintaining both correct positions and natural orientations.

Procedural Animation

I took the physics engine I made in project 7 into my own engine and cast a raycast from the the lower right side of the body and just set the target position of the inverse kinematics to the hit point of the ray.

Improvements

Currently, the ray moves at a fixed distance from the body, which makes the animations feel unnatural. Instead, I want the ray's origin to follow at a fixed distance while gradually correcting its position to align with the center of the body over time. This smooth adjustment will create more natural animations by preventing sudden movements and making the transitions feel more fluid.

FABRIK Forward and Backward Pass

Conclusion

Working with FABRIK ended up being a fun and rewarding challenge. It gave life to the character and added that extra bit of polish that makes animations feel more natural and responsive. There were definitely some tricky parts—like figuring out rotations and getting everything to play nicely in the right space—but seeing the joints move naturally in real time made it all worth it. There’s still more I want to improve, especially with making the raycasting behave more naturally, but overall I’m really happy with how it turned out.