[FFmpeg-user] ffmpeg and use of the ildct flag in progressive encoding

Bouke / Videotoolshed bouke at videotoolshed.com
Sun Dec 17 23:22:42 EET 2023

> On 17 Dec 2023, at 20:44, Nik Armstrong <nik.a at smpstaff.org> wrote:
> On 2023-12-16 15:01, Erik Dobberkau wrote:
>> The ildct flag appears only to be making the output file "think" it is
>>> interlaced, when it is, in fact, progressive. It does not seem to be doing
>>> anything to change the footage (quality-wise and it's still progressive).
>>> Since someone other than myself set the flag on this command, I am trying
>>> to figure out what effect, if any, taking it off will do to the output.
>> IIRC, it only has an effect when you are creating an interlaced output
>> using -top 1, (and from here on I‘m guessing) it might instruct the encoder
>> or an automatically inserted filter to apply the DCT in a different fashion
>> compared to a progressive input.
>> But again, that’s a guess which may (also) be outdated, since I last did
>> these kinds of transcodes quite a while ago.
>> Good luck.
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> Thanks Erik. Much appreciated.

Mind you:
Sometimes it’s wanted / needed to have all files ’the same’.
If that is interlaced, it’s interlaced. Codec should say ‘interlaced’, no matter the actual content.

For Avid media, that might (depending on the version of MC) be important for a ‘fast import’, or omitting adapters you don’t want / need.

In broadcast, over here interlaced is mandatory, even if the content is progressive.

So no, it’s NOT complicated, and no, it’s NOT outdated.


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