[FFmpeg-user] simple bob -- How?

[Mark Filipak (ffmpeg)]

I seek a way to do simple bob (i.e. repeat fields at 2xFPS) with no other filtering. Can you help me?

I suppose I could do something like this:
-filter_complex 
"split[s1][s2],[s1]separatefields[A1][a1],[s2]separatefields[A2][a2],[A1][A2]weave[Ao],[a1][a2]weave[ao],[Ao][ao]merge"
(Kindly forgive any mistakes -- I haven't done anything like this in a couple of months.)
But I thought I'd ask here for an easier way.

These gave pretty good results. They're listed from best to worst:
bwdif
yadif=send_field_nospatial
yadif=send_field

However, I suspect that, in addition to mechanical frame changes, they're also making some 
(unwanted) cosmetic changes.

Based on what's written in the documention, these seemed promising:
separatefields,doubleweave
separatefields,tinterlace=mergex2

But they produced surprising and undesired results.

Optional Reading:

I also tried this:

separatefields,select=eq(mod(n,4),0)+eq(mod(n,4),3),weave

It's not what I actually want, but I tried it because it doesn't make sense to me -- weave together 
fields N%4==0 and N%4==3? (Really? Modulo-4? fields 0 & 3?) -- so, because it doesn't make sense to 
me, I thought I might learn something interesting. So, what happened? "Conversion failed!" is what 
happened. Why it's cited as an example (https://ffmpeg.org/ffmpeg-filters.html#Examples-121) in the 
documentation is a mystery to me.