[Kdenlive-devel] IPB frame order and possible link to my "jerking" video?

[Dan Dennedy]

On Saturday 02 June 2007 11:40:16 am Jan Uhlir wrote:

First of all, let me explain something about MLT and its history... I would 
not even expect to generate DVD compliant output directly from kdenlive/MLT 
at this time. MLT was written firstly and with priority given to 
*uncompressed* output. I have even hardly used the ffmpeg-based output 
module, and I already know of some limitations and flaws. I have on my TODO 
for MLT to revisit the avformat (ffmpeg) module. 

For starters, it does not provide for any sort interlace-based encoding as is 
typical with DVD Video output. Therefore, you should be deinterlacing until 
this is fixed. Without it, you will get a lot of compression artifacts on 
inter-field motion. Try adding progressive=1 to the kdenlive export profile.

> Avidemux exported sequence, non-jerking video:

Avidemux has placed much emphasis on encoding and output, particularly DVD 
Video.

> Frames sequnce
> I(06) B(08) B(08) P(06) B(08) B(08) P(06) B(08) B(07) P(06) B(08) B(07)
> I(06)
>
> I frame i every 12 frame, so GOP = 12 ?
> B frames present, interesting.
>
> Image size		720x 576
> Frame Rate		25.000 fps
> # of frames		332 frames
> Codec 4CC		MPEG
> Video duration		00:00:13.280
> Aspect Ratio		Pal 4:3 (16:15)
>
> Extra properties:
> Packed Bitstream	No
> Quartel Pixel		No
> GMC			No
>
> Audio:
> Codec			AC3
> Channels		STEREO
> Bitrate			32000 Bps / 256  kbps
> Frequency		48000 Hz
> Audio duration		00:00:13.472
> VBR			No
>
>
> Kdenlive rendered, jerking video, the same clip:
> I(31) P(18) P(12) P(08) P(06) P(06) P(06) P(06) P(06) P(06) P(04) P(04)

The "(31)" after the first frame (I) indicates quantisation, and 31 is the max 
quantisation. That explains the very poor quality at the beginning, but it 
might be a bug in consumer_avformat; I do not yet know.

> P(06) P(06) P(06) I (06) I frame i every 15 frame, so GOP = 15 (as was set
> in render profile) There are no B frames at all!

In the kdenlive export profile you can add b_frames=2, and it should output 
with b frames.

> Image size		720x 576
> Frame Rate		25.000 fps
> # of frames		320 frames (12 frames missing, interesting!)

Hmm.. is kdenlive adding real_time=0 to all export consumers? It should. If 
not, or to be sure, try adding real_time=0 to the export profile. Realtime 
behaviour will drop video frames when computer can not output fast enough. It 
should only be used for monitoring scenarios (playback) and turned off for 
transcoding.

> Codec 4CC		MPEG
> Video duration		00:00:12.800
> Aspect Ratio		Pal 4:3 (16:15)
>
> Extra properties:
> Packed Bitstream	No
> Quartel Pixel		No
> GMC			No
>
> Audio:
> Codec			AC3 (MP2 also tested)
> Channels		STEREO
> Bitrate			56000 Bps / 448  kbps (128, 192, 256 kbps also tested)
> Frequency		48000 Hz
> Audio duration		00:00:12.768
> VBR			No
>
> forum.doom9.org: GOP's: I, P, and B frames explained...
> http://forum.doom9.org/archive/index.php/t-19436.html
>
> This information is interesting:
>
> I noticed some confusion about B frames, so I thought I would write down a
> quick explaination of an MPEG "GOP", or, "Group Of Pictures", as explained
> to me by a professor ...
> Oh, BTW, in MPEG2 at least, a GOP order is always IPBBPBBPBBIPBBPBB etc
> etc. (pending on your GOP size), but it is always 1 I, 1 P, and 2 B's, then
> you can stack more groups of "PBB"'s in that one GOP if needed (usually up
> to 15 total frames. ...

I do not believe this is correct. It represents a most typical scenario, but 
not a requirement AFAIK. I will have to lookup whether DVD Video specifies 
usage of b frames.

> This information indicates that Avidemux processed clip (cut and copy
> operation from original stream) is very close to this IBBPBBPBBPBBI..
> order. And this works!

Well, avidemux does not directly use the ffmpeg implementation. It builds upon 
it as well as mpeg2enc. I think it is a bit of a stretch to say lack of B 
frames is the problem at this point, but it could be.

> Contrary, Kdenlive exported video is IPPPPPPPPIPPPP.. only stuff. No B
> frames at all. May this be related?  Is this MTL or Kdenlive specific? What
> avlib parameters I can use to achive desired IBP frames combination?

b_frames=2

Also, I think it would be interesting to work on getting a good quality DV AVI 
out of kdenlive and then try using it with mjpegtools mpeg2enc (you can use 
Kino as a frontend) to see what the result is. 
At this time, I would suggest then just focus on getting a high quality output 
from kdenlive and then using avidemux if you want to get good encoded output 
to MPEG-2 or MPEG-4.

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