CODEWITHSUNDEEP
libavcodeclibavlibavformat
B.Gen.Jack.O.Neill
B.Gen.Jack.O.Neill

Reputation: 8397

Avformat cannot seek to beginning of file

I need to quickly seek thru H.264 encoded video stream in MP4 container. I am using libav to decode frames, so I stumbled upon avformat_seek_file() method.

My problem is, assuming H.264 stream begins with keyframe, when I seek to timestamp 0 (regardless of time_base), I should be at the beggining of the stream. But Im not. I usually get few seconds into video. Also, if I seek to, for example 10 seconds, I usually get around 12 or so. Is it possible for keyframes to be so "rare"? It seems that AVSEEK_FLAG_ANY has no impact on seek result. Tested on multiple FullHD H.264 MP4 videos.

Code:

unsigned long seekTo = 0;
//Doesen´t actually matter for 0 since it will be also 0
seekTo = av_rescale_q(seekTo, AVRational{1, AV_TIME_BASE}, pFormatCtx->streams[videoStream]->time_base);
int result = avformat_seek_file(pFormatCtx, videoStream, INT_FAST64_MIN, seekTo, seekTo, AVSEEK_FLAG_ANY);
avcodec_flush_buffers(pCodecCtx);

Upvotes: 4

Views: 3839

Answers (2)

Jean-Michaël Celerier
Jean-Michaël Celerier

Reputation: 7906

My problem is, assuming H.264 stream begins with keyframe, when I seek to timestamp 0 (regardless of time_base), I should be at the beggining of the stream

Note that some files can have their first keyframe at a negative DTS, e.g. you need to seek to timestamp -1 or something like this.

You can set the flag inside AVFMT_SEEK_TO_PTS into AVInputFormat::flags before opening the AVFormatContext to use PTS which will be 0-based.

Upvotes: 0

Jack
Jack

Reputation: 595

Try using av_seek_frame instead. Read here for some gotchas about using that and seeking around.

Upvotes: 0

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