[FFmpeg-devel] [PATCH] Fix ratecontrol bug when time_base.num > 1

[Joseph Artsimovich]

On 16/11/2012 16:11, Michael Niedermayer wrote:
> On Fri, Nov 16, 2012 at 04:01:26PM +0000, Joseph Artsimovich wrote:
>> On 16/11/2012 15:28, Kieran Kunhya wrote:
>>>> I dont fully understand the problem, so its hard to suggest how to
>>>> fix it.
>>> The problem is his application doesn't like the timebase as a reduced
>>> fraction, but would rather
>>> have it as a reducible fraction.
>>>
>> Not quite. That's only part of the problem. The following line of
>> code from ratecontrol.c:
>>
>> wanted_bits = (uint64_t)(s->bit_rate*(double)dts_pic->f.pts / fps);
>>
>> won't work correctly for any time_base where numerator is greater
>> than 1, as it doesn't consider the scale of dts_pic. For instance,
>> it will fail on your typical NTSC timing of 1001/30000. Now, the
>> obvious fix is to include s->avctx->time_base.num into the
>> denominator of the above line of code. That would fix the NTSC case,
>> but not my case, which was 24/600, as s->avctx->time_base gets
>> automatically reduced to 1/25. Therefore, I went with s->dts_delta
>> instead of s->avctx->time_base.num. However, Michael explained it's
>> not suited for that purpose.
> fps is calculated as 1.0 / av_q2d(avctx->time_base) / FFMAX(avctx->ticks_per_frame, 1);
> that is supposed to take the numerator already into account.
>
> also 24/600 or 1/25 the timestamps should be 1,2,3,4,...
Note that we are talking about user-provided timestamps. I use 
libavcodec indirectly through libquicktime, which in my case sets:
avctx->time_base.num = 24;
avctx>time_base.denum = 600;
avframe->pts = 0, 24, 48, ...

 From libquicktime's point of view, it's perfectly reasonable.

Also, I believe it would still fail for NTSC, because pts will go like 
0, 1001, 2002, ...
Now look at the line of code again:

wanted_bits = (uint64_t)(s->bit_rate*(double)dts_pic->f.pts / fps);

It pretty much expects pts to go 0, 1, 2, ...
High pts values are not compensated by anything.

-- 
Joseph Artsimovich
Senior C++ Applications Developer
MirriAd Ltd