Keyframe

Keyframing comes from hand-drawn animation. A lead animator drew the important poses, the keyframes, and assistants drew the frames in between. In computer animation the computer does the in-betweening, by interpolation.

To make a keyframed motion:

1. Choose the key moments: for a step, perhaps the start, the foot lifting, the foot at its highest, touchdown and settled.
2. Set the pose at each, often by choosing where the feet and hips go and solving the joints with inverse kinematics.
3. Join the poses with smooth curves over time.

Interpolation

The simplest join is a straight line between keys, linear interpolation:

$$q(t) = q_0 + \frac{t - t_0}{t_1 - t_0}\,(q_1 - q_0)$$

It’s continuous, but the velocity jumps at every key, so the motion looks robotic. Smooth curves such as cubic splines fix that. For motion, a monotone cubic spline is a good choice because it doesn’t overshoot the keys.

In this project

The scripted recovery moves (the sideways hop with a counterweight leg, and forward and backward steps) are keyframed. Each has a size dial from 0 to 1 and is stored as a table at 5 sizes in 0.02 s frames, which the environment reads as the reference motion.