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

[Nik Armstrong]

On 2023-12-17 13:22, Bouke / Videotoolshed wrote:
>> 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 But again, that’s a guess which may (also) be outdated, since I last 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 So no, it’s NOT complicated, and no, it’s NOT outdated.
> 
> Bouke
> 
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Avid media: Interesting. That is what this command is being used for. 
However, when imported as interlaced (default by nature of the ildct 
flag) it causes problems if any time warps are applied (softens the 
image considerably). But when specifically instructed to import as 
progressive, it's not a problem.

Nik