[FFmpeg-devel] Strange qmax behaviour with mpeg2
joseph at mirriad.com
Tue Sep 11 13:02:14 CEST 2012
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
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.
Senior C++ Applications Developer
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