[Libav-user] Libavformat - Guranteed seeking before keyframe

[ilkercan Kaya]

Hi,

I did a workaround as the following:

I seek and read a packet frame, if packet has pts higher than my target I
seek 10ms behind the original target and check again, I keep this until I
get a pts smaller than my target pts.

This is a dirty workaround and so far it works fine but I am open to any
other suggestions from the authors or anyone that wants to help, thank you!

Kind regards.


On Sun, Mar 7, 2021, 17:10 ilkercan Kaya <canilkerkaya at gmail.com> wrote:

> Hi James,
>
> Thank you for your help!
>
> I tried your code and sadly the issue still persists. I am trying to seek
> an ogg audio file. Maybe my code is faulty even though I debugged and the
> first frame's timestamp is greater than my target ts.
>
> On Sat, Mar 6, 2021 at 11:42 PM James Crisafulli <james2048 at gmail.com>
> wrote:
>
>>
>> On Sat, 6 Mar 2021 at 00:52, ilkercan Kaya <canilkerkaya at gmail.com>
>> wrote:
>>
>>> Hi Everyone,
>>>
>>> Anyone kindly wanting to help me on this matter? I am really stuck. In
>>> case of my question being vague, I can reword it. Please let me know.
>>>
>>> Kind regards.
>>>
>>>
>>> On Wed, Mar 3, 2021, 19:31 ilkercan Kaya <canilkerkaya at gmail.com> wrote:
>>>
>>>> In case this got lost, I'm kindly replying to this thread.
>>>>
>>>> On Sun, Feb 21, 2021, 04:20 ilkercan Kaya <canilkerkaya at gmail.com>
>>>> wrote:
>>>>
>>>>> Hello Everyone,
>>>>>
>>>>> I am trying to seek my audio ogg file with the following:
>>>>>
>>>>> int64_t targetPts = av_rescale_q(seekTimeMicro, AV_TIME_BASE_Q, streamTimeBase);
>>>>>
>>>>> av_seek_frame(avFormatContext, streamInfoIndex, targetPts, AVSEEK_FLAG_BACKWARD);
>>>>>
>>>>>
>>>>> This works all fine. Most of the time AVSEEK_FLAG_BACKWARD makes sure
>>>>> that the stream is seeked to a time before targetPts. However, there are
>>>>> times the opposite happens, seeking a time after targetPts. I can see this
>>>>> with the first frame I request from avcodec_receive_frame() having pts
>>>>> (avFrame->pts) greater than my targetPts. I need to ensure that the first
>>>>> frame I receive is always smaller than my targetPts. (it's an overhead in
>>>>> my system, long story).
>>>>>
>>>>> I read it that here
>>>>> <https://stackoverflow.com/questions/20734814/ffmpeg-av-seek-frame-with-avseek-flag-any-causes-grey-screen>:
>>>>> "It appears that, for some reason, av_seek_frame() using
>>>>> AVSEEK_FLAG_BACKWARD returns the next keyframe when the frame that I am
>>>>> seeking is the one directly before this keyframe. Otherwise it returns the
>>>>> previous keyframe (which is what I want).". I suspect that this is the
>>>>> case. However, this post is too old so it might be outdated.
>>>>>
>>>>> Could you help me?
>>>>> Thank you!
>>>>>
>>>>>
>>>>> _______________________________________________
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>>
>>
>>
>> Hi Ilkercan,
>>
>> I wanted to share what I used to seek so that the timestamp will be
>> behind the one you ask for.
>> I used this to implement a video player where when you seek, the
>> timestamp will be behind what you're looking for, then I can loop decode
>> until the requested PTS precisely.
>> Caveat: this worked for me for video, but may not work for you for audio.
>> In my use case I discarded the audio packets.
>> I used this API which is currently discouraged (not sure when it will be
>> completed, it has been saying that comment since 2.8 or even before):
>> https://github.com/FFmpeg/FFmpeg/blob/master/libavformat/avformat.h#L2362.
>> I couldn't get the same results with av_seek_frame.
>>
>> The code:
>>
>> int64_t streamTimestamp = YOUR_STREAM_TIMESTAMP_TO_SEEK_TO;
>> int flags = 0;
>> // The idea is that we ask to get the low bound keyframe. This way we
>> always get the keyframe before our desired one.
>> // Set min_ts to 0, max to your ts, ts to your ts.
>> int ret = avformat_seek_file(formatCtx, YOUR_STREAM_INDEX, 0,
>> streamTimestamp, streamTimestamp, flags);
>>
>> Hope it is useful.
>> - James
>> _______________________________________________
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>>
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>> libav-user-request at ffmpeg.org with subject "unsubscribe".
>
>
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