Constant Quality in H.264

[kaz219]

ok, I propose here that people that have tried this setting let know what is the best qmin in there opinion.

Note: the lower the qmin, the higher the quality.

For what I saw, qmin=22 is too less. I get bitrate of around 1700 very often, even if the video is only 720x304 pixels.

qmin=30 is definitively too much, I get 300kbps for a 720x448 video.

qmin=26 seems ok, but I'm not yet finished with the encoding. And I have to test qmin=27 and qmin=25 also. Of course it will depend of the video, but still it could be nice to know what people are using...

cheers,
kaz

[major]

File size is not relevant to measure quality, you should check the PSNR (signal/noise) value reported at the end of the encoding (the higher the better).

[kaz219]

So it seems that qmin=26 gives me here a global PSNR of 40, which a bitrate of 800. Which is not bad at all.

I didn't talk about size. I just talk about bitrate. I just said that an average of 1700kbps for a 720x304 video is too much. That's why I'd like to have some values here, in order to compare.

BTW, is there a direct relation between the PSNR and the qmin?

[major]

Indeed, if you raise the qmin (which is basically the compression factor) the PSNR will go down.

[kaz219]

ok, that I understood, but what I want to know is:
If I choose qmin=26, will I always have PSNR=40? In other words, is there a function that gives PSNR, given qmin?

Thanx for the answers!

[junkmalle]

The point of constant quality mode is to encode the movie with the specified picture quality. The encoder will use whatever bitrate is needed to achieve the desired quaility at each frame. If the video is dark, has little detail, and little motion it will require few bits. If it is bright, has lots of detail, and lots of motion it will take lots of bits. If you encode 10 different movies at the same quantizer setting you will get 10 different average bitrates.

[kaz219]

yes I know. I want to know if I will get 10 different *PSNR*, not bitrate. Or if I will always have the same PSNR, which means a given qmin is equivalent to asking for a certain PSNR.

[major]

If you encode at constant quantizer, the PSNR should still vary depending on the movie complexity. Also I-frames have a greater PSNR than P-frames, etc.