[Libav-user] Preview video and start recording with exact same stream settings

[Strahinja Radman]

That is not entirely true, frames can be dependent on past and/or forward
frames, depending on a frame type, P or B. So you can just simply write the
key frame
to the file. You need to save the last GOP (group of frames between two I
frames). Here is the link to SO where you can read on how to extract frame
type
from the packet
https://stackoverflow.com/questions/26558271/how-to-detect-i-p-b-frame-from-h264-rtp-packet

On Wed, Jan 1, 2020 at 12:57 PM Audric Ackermann <audric.ackermann at gmail.com>
wrote:

> OK thanks, that's a good idea.
> I don't know much but I can basically write the last keyframe to the file
> and then write the last packets received after pressing record? I mean,
> between two key frames, all frames are dependent on the key frame only?
>
> How can I check if the keyframe I got is a keyframe?
>
> Audric
>
> Le mer. 1 janv. 2020 à 22:43, Strahinja Radman <dr.strashni at gmail.com> a
> écrit :
>
>> Well, you can do this. Keep last N number of packets in a buffer, where N
>> is guaranteed to contain keyframe packet. When you encounter new keyframe
>> packet
>> you can free the packets that are not needed anymore. When someone starts
>> to record, you can mux from the last known key frame packet.
>>
>> On Wed, Jan 1, 2020 at 11:10 AM Audric Ackermann <
>> audric.ackermann at gmail.com> wrote:
>>
>>> But if I want to avoid missing the first few frames  which are not a key
>>> frame, I have to reencode the data, so the first frame recorded is a key
>>> frame, right?
>>>
>>> Le mer. 1 janv. 2020 à 20:49, Strahinja Radman <dr.strashni at gmail.com>
>>> a écrit :
>>>
>>>> If you only want to copy the input, there is no need to transcode. You
>>>> can simply duplicate the input packets, one packet goes to the decoder so
>>>> it can be shown, the other
>>>> goes to the muxer for the recorder when someone presses the record
>>>> button. You can save to whatever format you want but input may not be
>>>> playable from the start until
>>>> it reaches the key frame. Deep packet copy can be achieved with the
>>>> following code:
>>>>
>>>> AVPacket packet;
>>>> av_init_packet(&packet);
>>>> av_packet_ref(packet, src);
>>>> av_packet_make_writable(&packet);
>>>>
>>>> You can based on the decoded frames, mark a key frame packet and start
>>>> saving from there.
>>>>
>>>> On Wed, Jan 1, 2020 at 10:06 AM Audric Ackermann <
>>>> audric.ackermann at gmail.com> wrote:
>>>>
>>>>> Ok thank you.
>>>>> As I do not control the input parameters I will go for the transcode
>>>>> option. Is there an easy way to duplicate all required decoder parameters
>>>>> to the encoder?
>>>>>
>>>>> Like doing this, but with all important params in one step:
>>>>>
>>>>> Thanks
>>>>>
>>>>> outst->codec->width = this->codec_ctx->width;
>>>>> outst->codec->height = this->codec_ctx->height;
>>>>> outst->codec->pix_fmt = this->codec_ctx->pix_fmt;
>>>>> outst->codec->time_base = this->codec_ctx->time_base;
>>>>> _______________________________________________
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>>>>>
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>>>>
>>>>
>>>>
>>>> --
>>>>
>>>> Regards
>>>> Strahinja Radman
>>>> _______________________________________________
>>>> Libav-user mailing list
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>>>>
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>>>> libav-user-request at ffmpeg.org with subject "unsubscribe".
>>>
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>>
>>
>>
>> --
>>
>> Regards
>> Strahinja Radman
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>
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-- 

Regards
Strahinja Radman
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