[FFmpeg-user] Set MPEG-2 interlace flag without re-encoding

Nicolas Gaullier nicolas.gaullier at cji.paris
Thu May 12 13:08:37 EEST 2022

>On 5/9/22 2:44 AM, Nicolas Gaullier wrote:
>> Note that if you reencode, you might consider processing the chroma in between (convert to 4:2:2 back and forth) since the vertical sampling filtering is affected.
>What does this mean? I only have basic knowledge of how video compression works so I don't understand what you're talking about.

I suppose your file is a common distribution format, so 4:2:0. This implies a vertical filtering of the chroma and vertical processing with interlaced scan is always very picky.
I have not experienced this myself, maybe someone could comment how to handle this with care in a qualified filter graph; I think converting to 4:2:2 back and forth (weaving in 4:2:2) without precautions may be sufficient though.
But well, now, sincerely, compared to the interlacing artefacts you noticed, the difference here should be very subtle, especially if the original content does not show high vertical frequencies in the chroma. This is why I said you "might" consider processing the chroma, I don't think of it as a requirement. Comparing visually the results of the two workflows and checking the differences would be very interesting.

For more information on this subject, you can look at : https://www.cockam.com/vidbug2.htm
The artefacts reported there concern progressive-encoded content decoded as interlaced, this is more dramatic but clearly it is NOT your case.


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