Dejudder

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Dejudder

Mark Filipak
This is a most important filter. I thoroughly understand judder caused
by hard-telecine. I understand pulldown. I simply seek clarification of
the dejudder filter's behavior.

Re: https://ffmpeg.org/ffmpeg-filters.html#dejudder

"If the original source was partially telecined content then the output
of pullup,dejudder will have a variable frame rate."

What does the author mean by "pullup,dejudder"? That is: Why did the
author jamb the words together separated by a comma? What is the author
trying to 'say'?
I suspect that "of pullup,dejudder" would best be left out of the text,
but I can't know because the current text somewhat mystifies me due to
errors in language usage.

"May change the recorded frame rate of the container."

Under what specific circumstances?
And (with tongue in cheek) how is the "recorded frame rate of the
container" changed when the container is on a DVD-ROM? Should the
sentence read: "Will change the frame rate of the output"?

"Aside from that change, this filter will not affect constant frame rate
video."

Does that mean,
1, That the dejudder filter is ignored for CFR video (and can thereby be
specified universally and indiscriminately)? or
2, That dejudder is incapable of changing CFR video?


In other words, why wouldn't I simply always specify "dejudder"?


Is anyone interested in pictures that visually display what telecine &
pulldown are and what what they do? I have such pictures. They're very
helpful and work much better than words.

Is anyone interested in me acting as a critical editor for ffmpeg
documentation? I would function like a book editor by suggesting textual
changes to improve English language usage and meaning. Of course, all
suggestions would be subject to approval by the original authors. This
is a serious offer. My qualifications are as a documentarian and as an
electronic hardware engineer (retired). Simply put, I'd be another pair
of eyes, eyes that care.

Regards,
Mark.
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Re: Dejudder

Carl Eugen Hoyos-2


> Am 07.02.2020 um 01:15 schrieb Mark Filipak <[hidden email]>:
>
> This is a most important filter.

It is a useful filter but only in relatively rare cases as explained by the documentation.

> I thoroughly understand judder caused by hard-telecine.

But this filter is not (at least not primarily) related to judder caused by the telecine process but to remove judder caused by the pullup filter.

> I understand pulldown. I simply seek clarification of the dejudder filter's behavior.
>
> Re: https://ffmpeg.org/ffmpeg-filters.html#dejudder
>
> "If the original source was partially telecined content then the output of pullup,dejudder will have a variable frame rate."

This is not an ideal quote: The sentence before this one contains a link that significantly helps understanding this sentence.
(I know you cannot quote the link but the fact that the sentence before is missing here makes me wonder if you have completely missed it because you have already heard the terms dejudder and pullup before.)

> What does the author mean by "pullup,dejudder"?

A sequence of two filters in a filter chain as in the following command line:
$ ffmpeg -i input -vf pullup,dejudder output...

Again, the formatting supports this reading, I understand that by itself this interpretation isn’t completely obvious but we don’t want to change the whole documentation to explain the formatting every time it appears.

> That is: Why did the author jamb the words together separated by a comma? What is the author trying to 'say'?

That the described filter (dejudder) is (mostly or only) useful if the pullup filter was used.

> I suspect that "of pullup,dejudder" would best be left out of the text, but I can't know because the current text somewhat mystifies me due to errors in language usage.



> "May change the recorded frame rate of the container."
>
> Under what specific circumstances?

Two possible circumstances are mentioned by the documentation: When the original content (before the telecine process) was 24fps (actually 24000/1001fps, this looks like documentation bug) or 25fps.

You should imo only specify this filter in your inverse telecine process once you verified it is needed to smooth-iron the timestamps.

> And (with tongue in cheek) how is the "recorded frame rate of the container" changed when the container is on a DVD-ROM? Should the sentence read: "Will change the frame rate of the output"?

This is a filter description trying to help general understanding: The filter doesn’t know about DVDs not even mpeg program streams, but the reader might.

The filter may also be helpful if the input was badly inverse telecined and contains uneven timestamps.

Carl Eugen
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