CUBICSPLINE resample

[kzhsw]

Defined in gltf spec, or rendered

// tc be the current (requested) timestamp, tk<tc<tk+1;
// (tc−tk)/td be the segment-normalized interpolation factor.
_In_ float amount;
// td=tk+1−tk be the duration of the interpolation segment;
_In_ float td;
// ak, vk, and bk be the in-tangent, the property value, and the out-tangent of the k-th frame respectively.

float squared = amount * amount;
float cubed = amount * squared;
float part2 = -2.0 * cubed + 3.0 * squared;
float part1 = -part2 + 1.0;
float part3 = cubed - 2.0 * squared + amount;
float part4 = cubed - squared;

part3 *= td;
part4 *= td;

return vec_k * part1 + vec_k1 * part2 + outTangent_k * part3 + inTangent_k1 * part4;

Quaternions should be normalized after this.