ffmpeg - extract single frame -- DEINTERLACED

Question

i want to extract the first image from every second of video. i have a command line, based on the video's timecode, that works correctly when the video is progressive, but not when interlaced (only the first field is output in that case, creating a half-height image). i've tried various combinations of bwdif with select but the filter chains i create either cause errors or still return a half-sized image. can someone help me with the syntax ?

here is my call w/a filter that works correctly for progressive video source :
ffmpeg -i $infile -vf "select='if(eq(n\,0),1,floor(t)-floor(prev_selected_t))" -vsync 0 $outfile

the following still return only half-height images for interlaced source :
... -vf "bwdif=0,select='if(eq(n\,0),1,floor(t)-floor(prev_selected_t))'"

... -vf "bwdif=0,select='between(mod(n\,$ips)\,1\,2)'" -- $ips is images per second

... -vf "select='between(mod(n\,$ips)\,1\,2)',bwdif=0"

...and on a related note, is there some truly complete, exhaustive documentation anywhere on the select filter ? (no, i don't consider 'ffmpeg filters' chapter 16.16 exhaustive).

thanks.

Answer 1

it turns out the solution is to use the tinterlace filter first. this causes the half-height frames to be reinterlaced before deinterlacing them as FULL frames with bwdif. the full command line that worked for me was :

ffmpeg -i $infile -vf "tinterlace=mode=merge,bwdif=0,select='if(eq(n\,0),1,floor(t)-floor(prev_selected_t))" -vsync 

many thanks to the responder named Gyan (maybe the same who does ffmpeg development ?) on a thread in stackoverflow.