[FFmpeg-user] Detelecine: I got the right result from the wrong command?

[Alex Xu]

Hi everyone,

I'm working to detelecine a video, attached here (12 MB, first 32 seconds
truncated):
https://www.mediafire.com/file/x4ly6moqb8ayfyy/detelecine_input.mkv/file

I wasn't sure what was the right command to get the result I wanted so I
looped over all possible combos like so:

for i in 0 1 2 3 4; do
for j in 23 2332; do
ffmpeg -i detelecine_input.mkv -c:v libx264 -crf 18 -c:a aac -b:a 192k
-pix_fmt yuv420p -vf "detelecine=start_frame=$i:pattern=$j"  $i$j.mkv;
done
done

It was very clear which option was correct, since the correct option
encoded the best and had a significantly smaller file size. I visually
verified it too. In the source, every set of 5 frames had 3 clear frames
and 2 interleaved frames. In the final output, it preserved those 3 clear
frames and combines the 2 interleaved frames into 1, going from 30i to 24p.

But I'm just not sure why the correct result was start_frame=3:pattern=2332.

Examining the video, it seems pretty clear to me the pattern should be 23.
Since it has two interleaved frames. I learned about 23 from
https://en.wikipedia.org/wiki/Telecine#2:3_pulldown

Could someone explain the gap in my knowledge?

Alex