FFmpeg: What is an acceptable value for the q= parameter?

Question

I'm trying out encoding video with ffmpeg using x264 on a NAS machine with a 2.4 GHz processor. This yields a terminal output that has a high number in the q= parameter:

frame= 236 fps=0.2 q=29.0 size= 1038kB time=00:00:08.82 bitrate= 963.4kbits/

When I encode using x265 on my standard computer, I get something that looks like this:

frame=67350 fps=0.7 q=-0.0 size= 416991kB time=00:37:28.41 bitrate=1519.3kbits/

It is my understanding that q= stands for quality, and should probably be as close to 0 as possible, while a low number close to q=31 indicates poor quality and that ffmpeg has difficulties reaching an acceptable bitrate.

Is this something specific to x264 or is it that my settings for the NAS machine are too demanding? The video sources are 1080p at 30 fps.

ffmpeg -i "$i" -c:v libx264 -preset veryslow -crf 23 -af "volume=25dB, highpass=f=200, equalizer=f=50:width_type=h:width=100:g=-15" -c:a aac -strict experimental -b:a 192k

Should I be concerned about this, and what can be done about it?

Answer 1

Your rate control mode is CRF, so there's no target bitrate to hit. And for a value of 23, QP of 29 is what I normally see used. Ultimately, the test is whether the video looks acceptable.

Answer 2

I've noticed that my HEVC h.265 readout/log in ffmpeg wasn't showing anything other than 0 for the q value during the transcode. Yeah, makes it difficult to evaluate quality during an encode.

For h264, I find that the q value is always 4-6 higher than my stated crf. If crf is 18, my q during transcode might be 24. But that might be an averaging, the verbosity of the readout isn't showing you all frames. I'm guessing we are seeing an averaging of what is held in buffer verifier.