[Libav-user] Seeking causes the demuxer to skip some packets (after the seek)... why? Bug?

[Steve Hart]

On 25 October 2012 21:10, Michael Bradshaw <mbradshaw at sorensonmedia.com>wrote:

> On Thu, Oct 25, 2012 at 2:02 PM, Michael Bradshaw <
> mbradshaw at sorensonmedia.com> wrote:
> > I've also included
>
> Sorry, I meant to say that should you be interested, the source video file
> can be found at:
> https://docs.google.com/uc?export=download&id=0BxWx_dIBnyRoSnMyTFNGOHR1c2c
>
> The source I'm using, should you also be interested, is (remove the
> 'extern "C"' part to compile as C):
>
> #include <stdio.h>
>
> extern "C"
> {
> #include <libavcodec/avcodec.h>
> #include <libavformat/avformat.h>
> #include <libswresample/swresample.h>
> }
>
> int main(int argc, char** argv)
> {
>     AVFrame* frame;
>     AVFormatContext* formatContext;
>     AVStream* videoStream;
>     AVCodecContext* codecContext;
>     AVPacket packet;
>     int frameFinished;
>     int result;
>
>     av_register_all();
>
>     frame = avcodec_alloc_frame();
>     if (!frame)
>     {
>         printf("Error allocating the frame\n");
>         return 1;
>     }
>
>     formatContext = NULL;
>     if (avformat_open_input(&formatContext,
> "C:/Users/mbradshaw/Desktop/b/c/SIMPSONS_D2-CallOfTheSimpsons.m4v", NULL,
> NULL) != 0)
>     {
>         av_free(frame);
>         printf("Error opening the file\n");
>         return 1;
>     }
>
>     if (avformat_find_stream_info(formatContext, NULL) < 0)
>     {
>         av_free(frame);
>         avformat_close_input(&formatContext);
>         printf("Error finding the stream info\n");
>         return 1;
>     }
>
>     videoStream = NULL;
>     for (unsigned int i = 0; i < formatContext->nb_streams; ++i)
>     {
>         if (formatContext->streams[i]->codec->codec_type ==
> AVMEDIA_TYPE_VIDEO)
>         {
>             videoStream = formatContext->streams[i];
>             break;
>         }
>     }
>
>     if (videoStream == NULL)
>     {
>         av_free(frame);
>         avformat_close_input(&formatContext);
>         printf("Could not find any video stream in the file\n");
>         return 1;
>     }
>
>     codecContext = videoStream->codec;
>     codecContext->codec = avcodec_find_decoder(codecContext->codec_id);
>     if (codecContext->codec == NULL)
>     {
>         av_free(frame);
>         avformat_close_input(&formatContext);
>         printf("Couldn't find a proper decoder\n");
>         return 1;
>     }
>     else if (avcodec_open2(codecContext, codecContext->codec, NULL) != 0)
>     {
>         av_free(frame);
>         avformat_close_input(&formatContext);
>         printf("Couldn't open the context with the decoder\n");
>         return 1;
>     }
>
>     av_init_packet(&packet);
>     while (av_read_frame(formatContext, &packet) == 0)
>     {
>         if (packet.stream_index == videoStream->index)
>         {
>             printf("packet.dts = %d\n", packet.dts);
>
>             frameFinished = 0;
>             result = avcodec_decode_video2(codecContext, frame,
> &frameFinished, &packet);
>
>             if (result >= 0 && frameFinished)
>             {
>                 break;
>                 av_free_packet(&packet);
>             }
>         }
>
>         av_free_packet(&packet);
>     }
>
>     avcodec_flush_buffers(codecContext);
>     printf("seek result: %d\n",
>            av_seek_frame(formatContext, 1, -2000000000,
> AVSEEK_FLAG_BACKWARD | AVSEEK_FLAG_ANY));
>
>     while (av_read_frame(formatContext, &packet) == 0)
>     {
>         if (packet.stream_index == videoStream->index)
>         {
>             printf("packet.dts = %d\n", packet.dts);
>
>             frameFinished = 0;
>             result = avcodec_decode_video2(codecContext, frame,
> &frameFinished, &packet);
>
>             if (result >= 0 && frameFinished)
>             {
>                 break;
>                 av_free_packet(&packet);
>             }
>         }
>
>         av_free_packet(&packet);
>     }
>
>     av_free(frame);
>     avcodec_close(codecContext);
>     avformat_close_input(&formatContext);
>     return 0;
> }
> _______________________________________________
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>
>
I too have seen this behaviour. If the timestamp is negative the seek
ignores
it and always starts at first 0/positive timestamp.
This happens regardless of whether I use -1 or 1
I cannot check audio seeking in my case since I have no audio streams.

I would be very interested to know why this is.
Steve
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