[x264-devel] VBV Encoding Questions

[Jason Garrett-Glaser]

On Fri, Jul 15, 2011 at 1:54 AM, Horacio Sanson <hsanson at gmail.com> wrote:
>
> I have been reading around about the VBV econding in x264 but still I have
> troubles digesting how it works:
>
> If I set --vbv-maxrate/--vbv-bufsize in combination with --crf then x264 will
> vary the crf value to adjust to vbv settings (correct?).

More specifically, it'll constrain the resulting bitrate to obey the VBV.

> In the case I set --vbv-maxrate/--vbv-bufsize in combination with --bitrate
> (CBR encode) will x264 also change the bitrate setting to adjust to the vbv
> ones? If so then is it better to keep bitrate equal to vbv-maxrate? Or are
> there any situations were different bitrate and vbv-maxrate values are best?

Suppose you wanted to encode a Blu-ray and only had space for a 20mbit
video track.  However, the VBV on Blu-ray is 40mbit.  This would be a
case where bitrate != VBV-maxrate would be useful.  Not every use-case
demands CBR.

> Aren't H.264 levels defined to limit exactly the same things that vbv-maxrate
> and vbv-bufsize do? If I set --level 31 in x264 will it ignore the vbv options
> or do the vbv options take precedence?

It will ignore the VBV options, but it'll warn you if your VBV isn't compatible.

Jason