[PD] Re: [GEM-dev] Codecs in Gem (again!) & ffmpeg
ben at ekran.org
Wed Aug 31 15:01:27 CEST 2005
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)
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,
> 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
>>> Aside: What id the different between mjpeg and photojpeg as a video
>> 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.
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