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

[Corey Hickey]

Michael Niedermayer wrote:
> Hi
> 
> 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.

-Corey