[FFmpeg-user] Meaning of ffprobe output

Carl Eugen Hoyos ceffmpeg at gmail.com
Thu Feb 21 02:41:27 EET 2019

2019-02-04 23:29 GMT+01:00, Ulf Zibis <Ulf.Zibis at gmx.de>:
> Am 04.02.19 um 01:00 schrieb Carl Eugen Hoyos:
>> 2019-02-03 21:16 GMT+01:00, Ulf Zibis <Ulf.Zibis at gmx.de>:
>>> Am 03.02.19 um 21:06 schrieb Carl Zwanzig:
>>>> It's a shortening of "Top Field First" (TFF). With interlaced video
>>>> you have a choice of whether the top or bottom field (odd numbered
>>>> lines/even numbered lines) comes before the other in the digital stream.
>>> I know this, but Carl Eugen said, that my video is not interlaced
> Now I've checked the files with mediainfo:
> - The vob file is stated as interlaced top first. (I know, this does
> nothing say about the content, only the flag is evaluated.)

Doesn't FFmpeg also report this?
(I believe it should.)

> - The transcoded mp4 is stated as progressive.

Did you specify that you want x264 to encode using
algorithms for interlaced input material?
Should be "-flags +idct"
(As explained before, there is no way for FFmpeg to
know if the input it really interlaced or not except
for running idet which is not done automatically for
performance reasons and which - as you showed -
is not always helpful).

> I don't know, why libx264 doesn't retain the flag, maybe it evaluates

Again (afair, I already wrote that at some point):
x264 neither retains nor drops "the flag", it doesn't know
about the input content, it cannot "evaluate" its content.

> the content and decides to remove it. Carl Eugen stated, that libx264
> makes an interlaced output, if the input is interlaced, so I would
> think, it shouldn't change the interlace state, if I understood right.
>> FFmpeg cannot know if the video is interlaced.
>> (It was encoded interlaced which is not unusual for mpeg2video)
> I still must admid, that I don't really understand what is meant by
> "encoded interlaced", is it the same than "has the interlaced flag set"?
> I see no reason why ffmpeg "cannot know" this flag.

Of course it knows this flag but since the flag is completely useless
(for encoding, it may be needed to decode correctly) it cannot be
used to determine correct settings for encoding.

Allow me to repeat: Except for the intro, the video you provided
is not interlaced, it has artefacts that may or may not be easier
to encode when using interlaced encoding.
(Because of the given quality, this is mostly a moot discussion:
Just use a low quantiser or keep the "originals", if you are really
interested in the movie buy a dvd that will beat the quality of
your recording by far.)

Carl Eugen

PS: Rethinking, it may be a good idea to add the flags for
your x264 encoding, I believe the encoder is smart enough
to decide when / how to use it. I suspect it won't make a
difference for the native FFmpeg video encoders though.

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