[FFmpeg-devel] Strange qmax behaviour with mpeg2

Reimar Döffinger Reimar.Doeffinger at gmx.de
Tue Sep 11 13:44:55 CEST 2012

On 11 Sep 2012, at 13:02, Joseph Artsimovich <joseph at mirriad.com> wrote:
> When encoding IMX 30 content (I-frame only mpeg2 at fixed bitrate) I noticed ffmpeg produces lower quality output than other encoders. Digging deeper, I found strange behaviour of qmax parameter with mpeg2video encoder that I can only explain away as a bug.
> Before explaining further, here is my command line:
> ffmpeg -i input.mov -vtag mx3p -vcodec mpeg2video -r 25 -pix_fmt yuv422p -minrate 30000k -maxrate 30000k -b:v 30000k -intra -flags +ildct+low_delay -dc 10 -intra_vlc 1 -non_linear_quant 1 -ps 1 -qmin 1 -qmax 8 -top 1 -bufsize 1200000 -rc_init_occupancy 1200000 -rc_buf_aggressivity 0.25 -an out.mov
> The value of qmax=8 was taken from http://www.itbroadcastanddigitalcinema.com/ffmpeg_howto.html#Encoding_D10
> This value guarantees the encoded frame size won't exceed 150000 bytes, which is the maximum for IMX 30. If I set it even higher to 12 (the maximum -non_linear_quant would allow me), I get a file with identical size but lower quality. That seems very wrong. I mean if it fits bitrate restrictions at qmax=8, it shouldn't degrate quality just because you allow it to.
> Thoughts?

For one-pass bitrate-based encoding the encoder almost certainly will make mistakes.
With larger qmax the mistakes can be bigger.
While it is likely the encoder could be improved a lot (e.g. by adding rc-lookahead) to work better with 1-pass, unless you can reproduce it in a 2-pass encode I don't think it is particularly unexpected.
Btw. you did not define "lower quality", so the encode might just disagree with you on which is better, in which case 2-pass would probably make it worse.

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