[x264-devel] Default value for the --scenecut option causes heavy undesirable flood of I slices in active scenes

[Vladimir Chernyshov]

Please don't misunderstand me, I am not asking for help. I am suggesting 
a change in default options which can help an average user to get a 
smaller bitrate without undertaking lengthy tests.

Please allow me to disagree with your assertion of the encoder's 
intelligence. The qcomp mechanism fails miserably in the scenes, where 
part of the scene is highly active, and the other part (which can well 
be the focus of viewer's attention) is essentially still. The qcomp will 
consider the frame active as a whole and will kill the overall quality. 
The qcomp thing also reduces sharpness in highly active scenes, and 
although a viewer cannot distinguish the lost detail (the presumption of 
the "constant quality" model), it can easily notice the loss of overall 
sharpness and texture.

This is why I used pure CQP mode for the video.

However this is beyond the point of our discussion. Just for the sake of 
the argument, I also repeated the test with and without scenecut 0 using 
the default value of qcomp:

--bframes 2 --b-adapt 0 --crf 18 --ref 3 --weightb \
--keyint 250 --no-cabac --progress -o out1.264 -v raw-d2.1.y4m 


x264 [info]: slice I:18    Avg QP:19.81  size: 49557  PSNR Mean Y:44.56 
U:48.31 V:47.56 Avg:45.41 Global:45.31
x264 [info]: slice P:227   Avg QP:21.93  size: 37747  PSNR Mean Y:43.12 
U:47.24 V:46.33 Avg:44.03 Global:43.98
x264 [info]: slice B:145   Avg QP:24.07  size: 29972  PSNR Mean Y:40.47 
U:44.86 V:43.98 Avg:41.43 Global:41.37
x264 [info]: consecutive B-frames: 36.8% 18.8% 44.4%
x264 [info]: mb I  I16..4:  1.5%  0.0% 98.5%
x264 [info]: mb P  I16..4:  2.1%  0.0% 80.6%  P16..4:  7.0%  8.3%  2.0% 
  0.0%  0.0%    skip: 0.0%
x264 [info]: mb B  I16..4: 22.6%  0.0%  0.0%  B16..8: 28.5% 24.2% 18.7% 
  direct: 6.0%  skip: 0.0%  L0:46.3% L1:38.4% BI:15.4%
x264 [info]: ref P L0  71.0% 17.2% 11.8%
x264 [info]: ref B L0  72.9% 27.1%
x264 [info]: SSIM Mean Y:0.9683855
x264 [info]: PSNR Mean Y:42.201 U:46.405 V:45.516 Avg:43.127 
Global:42.859 kb/s:14160.45

-----------------------------------------------------------------------
--bframes 2 --b-adapt 0 --crf 18 --scenecut 0 --ref 3 --weightb \ 

--keyint 250 --no-cabac --progress -o out1.264 -v raw-d2.1.y4m 


x264 [info]: slice I:2     Avg QP:17.61  size: 62501  PSNR Mean Y:46.19 
U:49.50 V:48.89 Avg:46.98 Global:46.63
x264 [info]: slice P:130   Avg QP:21.90  size: 38137  PSNR Mean Y:43.24 
U:47.37 V:46.48 Avg:44.15 Global:44.10
x264 [info]: slice B:258   Avg QP:24.18  size: 30687  PSNR Mean Y:40.36 
U:44.75 V:43.91 Avg:41.32 Global:41.25
x264 [info]: consecutive B-frames:  0.3%  0.0% 99.7%
x264 [info]: mb I  I16..4:  1.1%  0.0% 98.9%
x264 [info]: mb P  I16..4:  2.4%  0.0% 87.2%  P16..4:  4.5%  5.0%  0.9% 
  0.0%  0.0%    skip: 0.0%
x264 [info]: mb B  I16..4: 23.7%  0.0%  0.0%  B16..8: 27.5% 24.9% 18.7% 
  direct: 5.3%  skip: 0.0%  L0:48.4% L1:38.3% BI:13.3%
x264 [info]: ref P L0  57.7% 22.3% 20.1%
x264 [info]: ref B L0  70.4% 29.6%
x264 [info]: SSIM Mean Y:0.9627631
x264 [info]: PSNR Mean Y:41.349 U:45.646 V:44.793 Avg:42.294 
Global:42.027 kb/s:13333.52

encoded 390 frames, 3.70 fps, 13334.15 kb/s

As you can see, the scenecut=0 mode still gives the best bitrate by 
properly utilizing B-frames.

regards,
Vladimir


Jason Garrett-Glaser wrote:
>> Thus with plenty of I- and P- frames the average quality is boosted,
>> which is against the idea of CQP VBR mode, and not desirable in
>> highly-active scenes.
> 
> The entire point of constant-quality VBR mode is to reduce bitrate
> spent in complex scenes... which you have proceeded to disable by
> setting qcomp to 1.
> 
> I am not going to bother helping people who explicitly override
> intelligent encoder decisions with misinformed ones of their own and
> then complain when they get suboptimal results.
> 
> Dark Shikari
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