I have implemented a spline interpolation of 3d/4d vectors but my problem is it speeds-slows down every frame steping.

The way I do it is I take 4 points of preframe-frame-endframe-postframe as a Cubic spline, I then derivate it at points 1/3 and 2/3 to obtain derivation vectors at the frame and the endframe.

I then use Cubic Hermite spline from the two derivation vectors, the frame point and endframe point, and I interpolate from 0.0 to 1.0.

It starts motion slowly (from standing still), moves, and then slows down to standing still again, though frame differences exhibit nearly a linear motion.

I wonder how to implement it corectly.