[FFmpeg-devel] [PATCH] avformat/utils: fix seek failed

["zhilizhao(赵志立)"]

> On Dec 31, 2020, at 6:51 PM, Marton Balint <cus at passwd.hu> wrote:
> 
> 
> 
> On Thu, 31 Dec 2020, Zhao Zhili wrote:
> 
>> Rounding min_ts towards +infinity and max_ts towards -infinity can
>> make ts out of the [min_ts, max_ts] range, and then leads to seek
>> failure. For example,
> 
> I think this is intentional to adhere to the docs of the function:
> 
> "Seeking will be done so that the point from which all active streams
> can be presented successfully will be closest to ts and within min/max_ts."
> 
> If you round down min_ts, you might return a packet with a timestamp less than the original min_ts, which contradicts the documentation.
> 

Yes, it’s not comply with the documentation strictly. On the other side, does
the documentation have strong guarantee about the rounding error? Rounding
error is less of a surprise than seek failure in my opinion.

ffmpeg.c seek_to_start() call avformat_seek_file() with max_ts equal to ts.
If avformat_seek_file() should be kept as it is, then the caller should be fixed.

> Regards,
> Marton
> 
> 
>> 
>> max_ts = ts = -25057
>> time_base = (num = 1, den = 14112000)
>> 
>> After rescale, ts = -353604, and max_ts = -353605.
>> ---
>> libavformat/utils.c | 4 ++--
>> 1 file changed, 2 insertions(+), 2 deletions(-)
>> 
>> diff --git a/libavformat/utils.c b/libavformat/utils.c
>> index 503e583ad0..69a0f901b2 100644
>> --- a/libavformat/utils.c
>> +++ b/libavformat/utils.c
>> @@ -2500,10 +2500,10 @@ int avformat_seek_file(AVFormatContext *s, int stream_index, int64_t min_ts,
>>            ts = av_rescale_q(ts, AV_TIME_BASE_Q, time_base);
>>            min_ts = av_rescale_rnd(min_ts, time_base.den,
>>                                    time_base.num * (int64_t)AV_TIME_BASE,
>> -                                    AV_ROUND_UP   | AV_ROUND_PASS_MINMAX);
>> +                                    AV_ROUND_DOWN | AV_ROUND_PASS_MINMAX);
>>            max_ts = av_rescale_rnd(max_ts, time_base.den,
>>                                    time_base.num * (int64_t)AV_TIME_BASE,
>> -                                    AV_ROUND_DOWN | AV_ROUND_PASS_MINMAX);
>> +                                    AV_ROUND_UP | AV_ROUND_PASS_MINMAX);
>>            stream_index = 0;
>>        }
>> -- 
>> 2.28.0
>> 
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