Animation

Layer 1. Pure animation data + procs: keyframes, clips, splines, FABRIK IK, procedural modifiers. Knows nothing about scene nodes or GPU buffers — world is the layer that applies these to skeletons.

Keyframe sampling

Keyframe(T) is a union of Linear, Step, and CubicSpline variants. keyframe_sample dispatches to the right interpolator pair based on the two surrounding keyframes — so a single track can mix interpolation modes per segment (matches glTF authoring conventions).

A Clip is name + duration + []Channel. A Channel is per-bone position / rotation / scale tracks. channel_sample_all returns the trio at time t.

Layers + blending

Animation is layered. Each Layer carries a weight, a BlendMode (REPLACE / ADD / MULTIPLY / OVERRIDE), and an optional bone_mask to restrict its influence to a chain.

• FKLayer — sampled clip + play state (LOOP / ONCE / PING_PONG, speed).
• IKLayer — FABRIK target plus pole hint.
• ProceduralLayer — tail / path / spider-leg / single-bone rotation modifiers.

Layers blend in insertion order. Position uses weighted addition, rotation uses SLERP, scale uses weighted multiplication — same shape as Unity’s animation layer stack.

FABRIK

Two-sweep iterative IK: forward pass anchors the tip to the target, backward pass anchors the root, repeat until tolerance. Cheap, no matrix inversion, no Jacobian, converges quickly for typical bone chains. The pole hint biases the bending plane so elbows and knees don’t flip.

Procedural modifiers

• Tail — bones counter-rotate parent motion with damping. Gives hair, antennas, tails their follow-through.
• Path — bones distribute along a spline that follows a path, useful for tentacles / ribbons tracking a route.
• Spider-leg — per-leg parabolic foot-lift driven by root motion; each leg lifts when its target drifts too far from where the foot currently is.

Spline

Catmull-Rom over any vector type with optional uniform arc-length resampling — used by paths, camera moves, and the path modifier.