[PD] Re: [GEM-dev] Codecs in Gem (again!) & ffmpeg

B. Bogart ben at ekran.org
Wed Aug 31 15:15:51 CEST 2005


Correction #2:

mpeg2enc -f 0 is mpeg1
mpeg2enc -f 1 is mpeg1 to VCD spec (huh?)

I'm trying -f 3 (mpeg2) now...

b.

B. Bogart wrote:
> Correction:
>
> I just created a TOC from a mpeg2enc -f 1 file and I CAN seek backwards!
>
> I was confused because a -f 0 mpeg (mpeg1) did not allow this, and
> "file" reports my -f 1 (mpeg2) stream as a v1 stream... blech.
>
> Anyhow I need to tweak the quality a bit and we're in business.
>
> For the archives this is the command I used to create the mpeg2 from
> frames using mjpegtools:
>
> jpeg2yuv -f 25 -I p -v 0 -j ./test/link/%d.jpg | mpeg2enc -o test3.mpg
> -g 1 -G 1 -f 1 && mpeg3toc test3.mpg test3.toc
>
> which looks pretty ugly for the record, even being 600MB (small)
>
> Thanks all.
>
> B>
>
> B. Bogart wrote:
>
>> Hi all,
>>
>> Thanks for the wealth of info.
>>
>> I made a couple mpegs and TOCs and it works fine. I decided to create
>> GOPs of only a single frame to force the lack of interframe compression.
>>
>> So it *almost* works in Gem, almost eh...
>>
>> Well I can seek only forwards, not backwards.
>>
>> Lets call the current frame Fc and the previous frame Fp:
>>
>> if Fp < Fc
>>   seeking works fine
>> if Fp > Fc
>>   the mpeg immediatly jumps to the first frame and sticks on the first
>>   frame for as long as this condition is true.
>>
>> Maybe I need a 2 frame GOP to get bidirectionality?
>>
>> I've been unable to seek any mpeg at all without using the mpeg3toc
>> thing, Including my single frame GOP, they just scrub forward as you
>> twiddle the frame number.
>>
>> We'll I'm close anyhow. off to look at lvs-studio...
>>
>> Thanks again,
>>
>> B.
>>
>>
>>
>> chris clepper wrote:
>>
>>> On Aug 30, 2005, at 10:05 PM, B. Bogart wrote:
>>>
>>>> Thanks for the suggestions.
>>>>
>>>> I'm not sure about using a mpeg for the following reasons:
>>>>
>>>> #1. was does the interframe compression thing work, in other words how
>>>> does it compare when seeking to a quicktime with photo-jpg codec?
>>>
>>>
>>>
>>>
>>> You want to avoid interframe compression at all costs.  In order to
>>> grab a random frame in an interframe codec the nearest previous
>>> keyframe must be found and all interframes between it and the  requested
>>> frame are decoded.  You could get lucky and hit a keyframe  or be very
>>> unlucky and hit the frame right before one.
>>>
>>> Interframe  compression techniques are used almost exclusively for
>>> final content distribution (web, DVD, ATSC).  Acquisition and
>>> production codec never use this with the exception of HDV (which is
>>> MPEG2).
>>>
>>>> Aside: What id the different between mjpeg and photojpeg as a video
>>>> codec?
>>>
>>>
>>>
>>>
>>> mjpeg is motion-jpeg which is for interlaced two field video.  Each
>>> field (50 or 60 per second) is a compressed with jpeg techniques.
>>>
>>>> ffmpeg reports my sequence of jpegs as "mjpeg"...
>>>
>>>
>>>
>>>
>>> It is mistaken if your sequence is single progressive frames, which  it
>>> almost certainly is.
>>>
>>> cgc
>>>
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