[x264-devel] Re: clarification on rate control

[Loren Merritt]

On Wed, 5 Apr 2006, Jeff Clagg wrote:
> On Wed, Apr 05, 2006 at 12:42:15PM -0700, Daniel Rogers wrote:
>
>> Q5: how does rate_tolerance affect the variability of the bitrate?  (e.g
>> is it a percentange variability, or what?)

rate_tolerance is useful only in 1pass ABR encodes, with either no 
vbv_max_bitrate, or a max rate significantly higher than the average rate.

For reasonably long videos (10 ~ 100 minutes), and small values of 
ratetol (<10), 1 unit of ratetol is +/- 1% filesize. For short videos, 
the variation is higher. For large values of ratetol, the variation 
slows asymptotically to some content-dependent maximum at 
ratetol=infinity, which may be around +/- 20% for an average movie.

>> Q7: My guess here is that the max 1 second long bit rate in the stream
>> determines the bitrate of the entire stream. If so, does this mean that
>> a vbv_buffer_size equal to one second at my desired max bitrate will
>> suit me?  How does picking a larger or smaller vbv_buffer_size affect
>> the final bitrate of the stream.  Also, as an important clarification,
>> what are the units of vbv_buffer_size (ie bits or bytes)?
>
> x264.h's i_vbv_buffer_size is in bits. the x264 executable takes
> arguments that are bits*1000 (kbits).

both are kbits.

--Loren Merritt

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