weave filter produces video with incorrect field order

[alexz]

Summary of the bug:

weave filter produces interlaced video with an incorrect field order

How to reproduce:

1. Create sample video (horizontally moving box) with a double frame rate (50)

   ffmpeg -f lavfi -i "color=color=blue:size=352x288:rate=50:duration=3" \
       -vf "color=white@1:size=50x50:rate=50:duration=3 [box]; [in][box] overlay=n*hsub:10" \
       -codec:v ffv1 -level 3 output-50p.mkv
   

2. Make an interlaced video with the first field top using weave filter

   ffmpeg -i output-50p.mkv -aspect 4/3 \
       -vf "weave=first_field=t" \
       -codec:v ffv1 -level 3 output-25i.mkv
   

Despite mediainfo correctly identifies the resulting video (output-25i.mkv) as interlaced with the top field first, video playes "jerky". This is points to an incorrect field order.

Also the incorrect field order is visible in the frame's pictures, extracted with

ffmpeg -i output-25i.mkv %03d.png

Attached patch fixes the issue.

[alexz]

weave filter patch

[Elon Musk]

Fixed in 16efcfe4138f9e60e01e3d477fd5c2721e060ee0.