I'm trying to figure out how to use yadif / yadif-2x with ffmpeg to convert 50i source material into 50p output that preserves the motion of all fifty fields per second. Right now, I am using the following command, which creates a frame-doubled version, essentially the same as 25p output.
ffmpeg -i input.m2t -f:v yadif=1 -c:v prores output.mov
Using yadif=0 makes a deinterlaced 25p file. Using yadif=1 makes a deinterlaced 50p file with doubled 25p frames. I need to make a deinterlaced file with 50 unique frames per second, and I'm pretty well certain this is possible. Is that what yadif(2x) is supposed to do?
[UPDATE] It turns out that the video I was attempting to double-deinterlace was actually 25p that was encoded into a 50i video file in order to conform to an older AVCHD standard. Apparently this is a common practice for tapeless cameras from the mid-late 2000s. So of course when I attempt to deinterlace I only end up with, at most, the original source 25 frames per second because there is no motion between the fields.
f:v yadif=1 does exactly what is described - it takes true interlaced 50i footage and turns it into astonishingly high-quality 50p output using a motion-weighted bob algorithm, just the same as the "yadif 2x" realtime filter does in VLC. The output is of nearly double the effective vertical resolution over frame-discarded deinterlace. It is now one of my favorite video filters ever because practically nothing else appears to offer this capability.