> For MPEG2 what I did was do a seek to frame.. Then do a tight loop of > reading and decoding until I got a picture.
I also do it like that. With MPEG2 streams it works. > me to the closet picture. Frame accuracy was hard for me because the files I > was dealing with had re-ordered b-frames (so some bframes were behind the > iframe). So we settled for the closest picture we could decode. We didn't I think this is the point. It seems, that seeking functions disregards re-ordered b-frames. > One other thing... Make sure when seeking that you also flush your buffers. > Any residual data in you video decoder buffer will be considered when > decoding and will get you artifacts. Right. I'm calling avcodec_flush_buffers prior to decoding. Are there other functions which I should call? Now I probably have a proof of fact that frame accurate seeking on H.264 video (transport) streams is apparently not possible with ffmpeg until now: I tried out VLC media player V 1.0.0 and noticed that VLC also has problems with frame accurate seeking. When I open H.264 mts files and scroll the slider most frames are not decoded properly and show artifacts. Under assumption that VLC uses ffmpeg to decode H.264 I can assume that frame accurate seeking on H.264 doesn't work with ffmpeg. Can anybody confirm that? -----Ursprüngliche Nachricht----- Von: [email protected] [mailto:[email protected]] Im Auftrag von Scott Andrew Gesendet: Dienstag, 7. Juli 2009 20:00 An: Libav* user questions and discussions Betreff: Re: [libav-user] Frame accurate seeking on H.264 video streams For MPEG2 what I did was do a seek to frame.. Then do a tight loop of reading and decoding until I got a picture. Storing PTS when I finally got my picture so my next seek didn't go beyond. I basically seeked to the closest picture. Not necessarily frame accurate. Again this seek only gets me to the closet picture. Frame accuracy was hard for me because the files I was dealing with had re-ordered b-frames (so some bframes were behind the iframe). So we settled for the closest picture we could decode. We didn't need to be frame accurate. One other thing... Make sure when seeking that you also flush your buffers. Any residual data in you video decoder buffer will be considered when decoding and will get you artifacts. On 7/7/09 10:50 AM, "Mediasystems GmbH" <[email protected]> wrote: > Hi. > > Thanks for your recommendations. I tried it with avformat_seek_file, but > couldn't find any difference to av_seek_frame. With avformat_seek_file I'm > also getting artifacts on decoding H.264 video streams. How I have to call > this function to seek to the right I-frame and to decode frames without any > artifacts? Is there perhaps a sample application with which I could test it? > > What do you mean with sequenced searching method? > > > Thanks. > > > -----Ursprüngliche Nachricht----- > Von: [email protected] > [mailto:[email protected]] Im Auftrag von ejlim > Gesendet: Mittwoch, 1. Juli 2009 03:53 > An: 'Libav* user questions and discussions' > Betreff: Re: [libav-user] Frame accurate seeking on H.264 video streams > > Hi. > > I recommend avformat_seek_file() in avformat/util.c, because this function > use new semantics of various demuxer. > And u will reference av_seek_frame_generic(). > > Internally, AVSEEK_FLAG_BACKWARD flag use > AVFormatContext->streams[idxVideo]->nb_index_entries. > BTW, some container have not this indexes. If this case, u can use > AVSEEK_FLAG_BACKWARD and sequenced searching method. > > GL~~ > > -----Original Message----- > From: [email protected] > [mailto:[email protected]] On Behalf Of Mediasystems GmbH > Sent: Wednesday, July 01, 2009 2:57 AM > To: 'Libav* user questions and discussions' > Subject: Re: [libav-user] Frame accurate seeking on H.264 video streams > > Right, exactly that way I tried it. However, I assumed that av_seek_frame > with flag AVSEEK_FLAG_BACKWARD set should seek to the previous I-frame. Am I > wrong here? If av_seek_frame doesn't work like I thought, is there another > libav function to achieve this? > > > On Jun 26, 2009, at 10:57 AM, Mediasystems GmbH wrote: > >> Hello, >> >> I use ffmpeg for about one year with MPEG-2 streams. Everything >> works fine >> so far. Now I want to use H.264 coded videos as well. I'm trying to >> do a >> frame accurate random seeking on H.264 video streams. But I cannot >> accomplish to decode all single frames without any artefacts after >> calling >> av_seek_frame. If I go through the stream from the beginning, it >> works. Is >> it generally possible to do frame accurate seeking on H.264 video >> streams >> and how? > > I think for this you need to seek to an I-frame, as H.264 is a > progressive encoder, and some of the frames only contain the changes > from the last frame to the current one. Once you have the previous I- > frame, you can then keep grabbing and decoding frames until you reach > the desired frame. This is how I was able to implement frame seeking > in one of my simple projects from a while ago. > >> >> Code snippet: >> >> ret = av_seek_frame(pFormatCtx, videoStreamIndex, timestamp, >> AVSEEK_FLAG_BACKWARD); avcodec_flush_buffers(pCodecCtx); >> >> ... >> while (av_read_frame(pFormatCtx, pPacket) >= 0) { ... >> bytesDecoded = avcodec_decode_video(pCodecCtx, pFrame, ref >> frameFinished, packet.data, packet.size); ... >> >> >> Any help would be appreciated! >> Many thanks. > _______________________________________________ > libav-user mailing list > [email protected] > https://lists.mplayerhq.hu/mailman/listinfo/libav-user > > > _______________________________________________ > libav-user mailing list > [email protected] > https://lists.mplayerhq.hu/mailman/listinfo/libav-user > > _______________________________________________ > libav-user mailing list > [email protected] > https://lists.mplayerhq.hu/mailman/listinfo/libav-user > > > _______________________________________________ > libav-user mailing list > [email protected] > https://lists.mplayerhq.hu/mailman/listinfo/libav-user _______________________________________________ libav-user mailing list [email protected] https://lists.mplayerhq.hu/mailman/listinfo/libav-user _______________________________________________ libav-user mailing list [email protected] https://lists.mplayerhq.hu/mailman/listinfo/libav-user
