[Ffmpeg-devel] qprd and frames being stuck at high quants

[Corey Hickey]

Corey Hickey wrote:
> Michael Niedermayer wrote:
>>>> there where some bugreports aout $subj a long time ago, but all the
>>>> sample videos seem to have dissapeared, if that bug is still happening
>>>> for anyone, then please upload a sample source file which causes it
>>>> (with command line options ...)
>>> No problem with high quantizers so far, but I did find a possible 
>>> small usability bug. When encoding with vqmin=1:lmin=1, qprd appears 
>>> to be nearly preventing the use of quantizer=1. At very high 
>>> requested bitrate, this makes the output bitrate and PSNR much lower.
>>>
>>> --------------------- without qprd -------------------
>>> for i in 1:turbo 2 ; do
>>>   mencoder matrix.vob -nosound -vf crop=718:356:0:60,scale=640:272 \
>>>   -ovc lavc -lavcopts \
>>>   vcodec=mpeg4:vbitrate=16000:psnr:mbd=2:vqmin=1:lmin=1:vpass=$i \
>>>   -ofps 24000/1001 -o test.avi -frames 2400
>>> done
>>>
>>> Video stream: 7794.736 kbit/s
>>> PSNR: Y:49.91, Cb:50.19, Cr:50.72, All:50.08
>>>
>>> ---------------------- with qprd ---------------------
>>> (same as above, but adding qprd to lavcopts)
>>>
>>> Video stream: 3914.993 kbit/s
>>> PSNR: Y:45.18, Cb:46.26, Cr:45.81, All:45.44
>>>
>>>
>>> When I play the output video with -lavdopts debug=1, the lowest 
>>> frame-level quantizer I see is 2. If I use debug=16, I see three 
>>> blocks with a quantizer of 1, out of the entire video.
>>>
>>> I can upload a sample video if you need, but I think it's 
>>> reproducible with any source.
>>>
>>>
>>> I know this is a rather minor issue, since encoding at such high 
>>> bitrates is rare, but I still thought it worth pointing out.
>>
>> retry with mblmin=lmin
>>
>> QP_RD activates adaptive quantization, which uses mblmin/max
> 
> Do you mean mbqmin? mblmin doesn't seem to exist, though I haven't 
> looked at the source yet.

Ok, I made a patch for mplayer, and that seems to do it.

With qprd and mblmin=1

Video stream: 7640.677 kbit/s
PSNR: Y:49.86, Cb:50.15, Cr:50.71, All:50.04

Bitrate (and PSNR) are approximately the same as without qprd. On such a 
short video, I would expect at least that much variation.

-Corey