[FFmpeg-user] should I shoot the dog?

Devin Heitmueller devin.heitmueller at ltnglobal.com
Tue Sep 29 18:44:10 EEST 2020

On Tue, Sep 29, 2020 at 11:29 AM Mark Filipak (ffmpeg)
<markfilipak at bog.us> wrote:
> Oh, dear, that's what "packed" means? ...very misleading name, eh? How are fields handled? Are the
> pixels assumed to be unfielded (meaning so-called "progressive")?

So the topic of how interlaced video is handled in subsampled video is
something we could spend an hour on by itself.  In the Luma space, the
Y samples are organized in interleaved form (i.e. lines of
top/bottom/top/bottom).  Because of chroma subsampling and the fact
that multiple lines can share chroma samples, this gets tricky.  In
the simple progressive case for 4:2:0, you'll have the first Chroma
sample corresponding to the first two luma samples on line 1 and the
first two luma samples on line 2.  If the video frame is interlaced
however, the first chroma sample corresponds to the first two luma
samples on line 1 and the first two luma samples on line 3.  The first
chroma sample on the second line of chroma corresponds with the first
two luma samples on line 2 and the first two luma samples on line 4.

This is known as "interlaced chroma" and a Google search will reveal
lots of cases where it's done wrong and what the effects are.  This is
the article I usually refer people to:


The above article does a really good job explaining the behavior (far
better than I could do in the one paragraph above).


Devin Heitmueller, Senior Software Engineer
LTN Global Communications
o: +1 (301) 363-1001
w: https://ltnglobal.com  e: devin.heitmueller at ltnglobal.com

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