[Ffmpeg-devel] interlace field order

[Dieter]

After googling a bit, I find that:
	Mpeg can be either top-field-first or bottom-field-first.
	DV is always bottom-field first.

When converting mpeg to DV, ffmpeg sometimes outputs the wrong
field order.

So the question is: how can we detirmine when the field order needs
to be corrected?

In libavcodec/mpeg12.c mpeg_decode_picture_coding_extension()
I turned on the printf for top_field_first.  Some files have
top_field_first all 0, some have it all 1, and some have
*interesting* changes, like being 1 for awhile, then changing
to an alternating pattern, with 1, 2 and 3 samples:

top field first=1
top field first=1
top field first=1
top field first=1
top field first=1
top field first=1
top field first=1
top field first=1
top field first=1
top field first=1
top field first=0
top field first=0
top field first=0
top field first=1
top field first=0
top field first=0
top field first=1
top field first=1
top field first=1
top field first=0
top field first=1
top field first=1
top field first=0
top field first=0
top field first=0

I have trouble believing that the field order is supposed to be
changing this frequently.