[x264-devel] x264 rate control

[Jason Garrett-Glaser]

On Wed, Jan 30, 2013 at 2:21 AM, Jason Garrett-Glaser <jason at x264.com> wrote:
> On Wed, Jan 30, 2013 at 2:00 AM, eloise vidal <eloise.vidal at gmail.com> wrote:
>> Thank you for this quick response,
>>
>> I don't indicate the number of thread, but I checked the value of the
>> parameter "i_thread_frames", it is automatically set to 24.
>>
>> So I made two tests (x264 --input-res 1920x1080 --fps 25 --bitrate 8000
>> --vbv-maxrate 8000 ...) :
>>
>> 1. I increase the buffer size (initially 3M=9~10frames) to 8M (25 frames)
>> vbv_bufsize = 3M --> achieved bitrate =  6125.30
>> vbv_bufsize = 8M --> achieved bitrate =  6904.92.
>>
>> 2. I decrease the number of thread (initially 24) to 8, with bufsize 3M :
>> i_thread_frames = 24 --> achieved bitrate =  6125.30, fps = 22.04
>> i_thread_frames = 8 --> achieved bitrate =  7166.56, fps = 10.99
>> --> Actually, if I decrease the number of threads, the achieved bitrate is
>> better but of course, the number of frames coded per second, decreases and
>> that is not a solution for a real time application.
>
> Sure it is; x264 can still easily achieve 1080p60 in realtime with 8
> threads or less.  You just might have to use a slightly faster preset
> if you're using extremal VBV settings that force you to use fewer
> threads.

On this note, to others on the mailing list, thoughts on a patch
that'd limit --threads auto in the case of frame threads to (bufsize /
maxrate)*(fps-num / fps-den), so that people don't accidentally do
this to themselves anymore?

Jason