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

Marton Balint cus at passwd.hu
Sat Jan 2 14:54:47 EET 2021



On Thu, 31 Dec 2020, "zhilizhao(赵志立)" wrote:

>
>
>> 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.

If the question is what the user would expect, then the answer is probably 
simple rounding for both min_ts and max_ts. That should also solve your 
error of ts becoming out of the [min_ts,max_ts] interval.

However in your example the time base of the stream is more precise than 
AV_TIME_BASE_Q, and that can cause unexpected behaviour regardless of the 
rouding you use. If start_time is not represented accurately, a 
seek attempt to start_time may well cause a seek after, or before the 
actual start of the stream, because the error is at most AV_TIME_BASE_Q, 
not AVStream->time_base.

Regards,
Marton

> 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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