5. Fill

Press Fill frames between labels. Every frame between your first and last label gets a position; your own labels are never changed, and frames outside that span stay empty.

Backend	When	Cost
Spline (default)	smooth motion, labels close together	instant
Optical flow	fast turns, wingbeats — anything that is not a smooth curve between two labels	about video speed

Neither needs a GPU. Start with the spline and switch to optical flow if the filled points cut corners.[1]

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[1]

Spline: monotone cubic interpolation (scipy.interpolate.PchipInterpolator). Optical flow: pyramidal Lucas–Kanade (cv2.calcOpticalFlowPyrLK), tracked forwards and backwards across each gap.