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List:       mjpeg-users
Subject:    Re: [Mjpeg-users] Shifting fields...
From:       Andrea Giuliano <sarkiaponius () alice ! it>
Date:       2007-08-09 10:15:24
Message-ID: 46BAE93C.8070105 () alice ! it
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Bernhard Praschinger wrote:
> Hallo

Hello, and thanks for your kind answer.

> 
>> I'm capturing an old Video8 tape at decimation 1, as I always do. Quite 
>> often, I need to encode shifting fields, but that's no problem 
>> (-Jfields=shift with transcode. e.g.).
> You can also shift the fields with yuvcorrect -M LINE_SWITCH/ 
> BOTT_FORWARD/ TOP_FORWARD

Yes, I know, but this would shift all fields, the "bad" as the "good" 
ones, simply switching between the two class of fields. The result would 
be the same, since the parts with shifted fields are approximately the 
same many as the parts with fields in the correct order.
> 
>> But this is the first time I see that the fields are not shifted along 
>> the whole output file from lavrec: for many minutes they are shifted, 
>> but after the middle of the tape or such they appear to be shifted every 
>> now and then, so I can't shift them again to correct this effect as above.
> Did you see a lot of framedrops when you where recording ?
> That might be one answer for you problem.

I always have some lost frames, but not so many. For example, the movie 
we're talking about is a 90 minutes long video8 tape I recorded from 
2000 through 2001, so it contains 135000 frames. During the capture with 
lavrec, I just lost 160 frames (aout 0.1% of the total), I think it's 
not much.

I wouldn't say that the fields get shifted when frames are lost. In 
fact, during the edit, I saw some lost frames but no fields shifting. 
Somewhere else, I noticed fields shifting but apparently no lost frames.

> 
>> More precisely, I should applie the -Jfields=shift option only on the 
>> portions of the movie that need it, but these portions are mixed in such 
>> a way that this clearly cannot be the way.
> That would be a painful but working way.

Definitely too painful.

> 
> Is that Video 8 a recorded TV show ?
> There it would be legal that the fieldorder switches during the show.

No, as I told above, it's a movie I took with a handycam when my son was 
making his first steps. But the tape is long and old, it could be a 
mechanical problem.

> 
>> What can I do, without going crazy?
> If it is possible I would try to rerecord it again.

If you mean "capture it again", clearly I can do it, but I guess the 
problems would still be there...
> 
> If that is not possible, you can scale it down to VCD size, lose the 
> size and quality, and also "solve" the interlacing problem.
> Or you encode it as is. And play it only back on a TV. Where you don't 
> see that problem.

That's the first thing I will try. Actually, I only have to watch this 
movie, and many other, on a TV. If you're right, this is sufficient for me.

I was also thinking of another transcode filter, 32detect, that seems to 
be able to operate some correction frame-wise, but I didn't understand 
it enough yet. Also, it's not a MJPEG issue.
> 
> Sorry for offering no better solution.
> 
> auf hoffentlich bald,

Best regard and many thanks!
> 
> Berni the Chaos of Woodquarter
> 
> Email: shadowlord@utanet.at
> www: http://www.lysator.liu.se/~gz/bernhard
> 
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-- 
Andrea

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