libx264 & ffmpeg (0.11.x) understanding quality options -q:v and -b:v

[feelart]

Hello,

I have a source video mpeg4 9000kb/s

I try different convertion with ffmpeg 0.11.x (build of 7 sep 2012), but I wonder if there is a bug or for libx264 ffmpeg ignores the options.

Indeed, with this command

ffmpeg -i "2012-03 01Praha.MP4" -acodec copy -vcodec wmv2 -q:v 8 out.avi

The dos shell outputs q=8, also if I use video codec h263p.

Now, the same command, but with video codec libx264, viz

ffmpeg -i "2012-03 01Praha.MP4" -acodec copy -vcodec libx264 -q:v 8 out.avi

The shell outputs q=29

So does ffmpeg discard the quality option with video codec libx264 or libschroedinger ?

Well, there is a workaround with -b:v options, but I just want to understand.

ffmpeg -i "2012-03 01Praha.MP4" -acodec copy -vcodec libx264 -b:v 823K outVB.avi

P.S.

ffmpeg -h encoder=libx264

does not give a clue for that

[bat999]

So does ffmpeg discard the quality option with video codec libx264 ?

Hi
q:v is not used with FFmpeg x264.
crf is used instead.
Range 0-69
crf 0 is lossless.
crf 69 is worst.

Also use a preset...
ultrafast
superfast
veryfast
faster
fast
medium
slow
slower
veryslow

(veryslow makes smallest file size, ultrafast makes largest file size)

Not good to use avi with x264.
mp4 or mkv is preferred.

ffmpeg -i "2012-03 01Praha.MP4" -c:v libx264 -crf 23 -preset medium -c:a copy output.mp4

Use "-t 60" to make a 60 second sample.
Then change "crf" and "preset" values to suit.

ffmpeg -i "2012-03 01Praha.MP4" -t 60 -c:v libx264 -crf 23 -preset medium -c:a copy output.mp4

There are some different ways to use FFmpeg x264, more information is here ---> http://ffmpeg.org/ffmpeg.html#libx264

[Vidd]

The lower the CRF, the higher the bitrate, higher quality. The higher the CRF, the lower the bitrate, lower quality. The presets specify encoding speed and to some extent quality, not file size. CRF / bitrate value used determines file size.

[feelart]

RE: http://ffmpeg.org/trac/ffmpeg/wiki/x264EncodingGuide

Thanks I made some test and I wonder the difference between -b:v and -cfr
It seems that there are equal at equivalent bit rate & same preset.

I tried the following on my Core5 2450M (2.5GHz) with the 64bits ffmpeg version:

ffmpeg -i "2012-03 01Praha.MP4" -c:v libx264 -crf 30 -preset veryslow -c:a copy ox264_veryslow30.mp4
encoding time 15min30 (492kps)

ffmpeg -i "2012-03 01Praha.MP4" -c:v libx264 -crf 30 -preset ultrafast -c:a copy ox264_ultrafast30.mp4
encoding time 30 sec (1290 kbps)

ffmpeg -i "2012-03 01Praha.MP4" -c:v libx264 -b:v 1290k -c:a copy ox264_bv1290.mp4
encoding time 3min56

ffmpeg -i "2012-03 01Praha.MP4" -c:v libx264 -b:v 1290k -preset ultrafast -c:a copy ox264_bv1290ultrafast.mp4
encoding time 30sec

ox264_bv1290ultrafast and ox264_ultrafast30 are about the same size and visualy the same, while ox264_bv1290 is also about the same size, but visually better.

Source : 243Mb, HD Ready, mpeg4 9000kb/s, 3min46
Output file size:
38.754.204 ox264_bv1290.mp4
38.830.130 ox264_bv1290ultrafast.mp4
38.420.954 ox264_ultrafast30.mp4
15.925.175 ox264_veryslow30.mp4

P.S. Regarding preset, no preset -> medium preset, indeed,

ffmpeg -i "2012-03 01Praha.MP4" -c:v libx264 -b:v 1290k -c:a copy ox264_bv1290.mp4

is MD5 equivalent to

ffmpeg -i "2012-03 01Praha.MP4" -c:v libx264 -b:v 1290k -preset medium -c:a copy ox264_bv1290Medium.mp4