[ahartmetz]

AFAIU it is possible to get nearly full resolution and full frame rate by using motion tracking to merge the fields. I.e. if the motion is regular enough, lines from the previous and / or next frame can be inserted instead of doubling the current lines. Which is almost the same process required to achieve best image quality at half the frame rate, just without throwing away frames.

[ErroneousBosh]

You're not throwing away frames, though. You're blending the lines from the odd and even frames so that you get the full vertical resolution, with a certain amount of motion blur on moving vertical edges.

Analogue video is 25 frames per second, 50 fields per second. You could guess at what the "missing" lines in a field are, but that doesn't magically make it 50p video.

[LocalH]

The effective rate for 50i or 60i video, in terms of motion fluidity, is 50 or 60fps. Just because it's half the resolution per field doesn't mean that it's not a "frame" of video in abstract terms, just that it's not a "frame" in the jargon (because fields by definition are half a frame).

It is impossible to decimate a video from 50/60i to 25/30p without losing half of the motion fluidity, even if the properly interlaced source video is technically 25/30fps.