On Tue, 2010-06-08 at 07:52 -0400, robert...@aol.com wrote: > >On Mon, 2010-06-07 at 19:55 -0400, robert...@aol.com wrote: > >> I am working on integrating a new video encoder. One issue I am > having > >> is determining the frame rate of the input video stream. The is > >> available in the AVFormatContext but not in the AVCodecContext. The > >> AVCodecContext does contain a time_base value but this does not seem > to > >> translate properly into a frame rate in all cases. There are some > >> video streams where the time_base is 50 / 2997 which would come to > >> 59.94 fps. However, the AVFormatContext indicates the frame rate is > >> 29.92 fps. I believe the actual frame rate is more likely the 29.92 > >> value, however, this is not available at the libavcodec scope. Is > >> there any undocumented process for determining the actual frame rate > at > >> the libavcodec scope?? I also tried using the pts value in the > AVFrame > >> structure from the decoded frame. This turned out not to be > provided > >> by the decoder and is always 0. > >> > >> One example . . . > >> << > >> Duration: 00:02:42.36, start: 0.000000, bitrate: 820 kb/s > >> Stream #0.0: Video: h264, yuv420p, 640x352 [PAR 1:1 DAR 20:11], > 820 > >> kb/s, 58.82 fps, 29.92 tbr, 1k tbn, 59.94 tbc > >> Stream #0.1: Audio: aac, 22050 Hz, stereo, s16 > >> Output #0, flv, to '': > >> Stream #0.0: Video: libx264, yuv420p, 640x352, q=2-31, 650 > kb/s, > >> 58.82 fps, 29.92 tbr, 1k tbn, 59.94 tbc > >> Stream #0.1: Audio: aac, 22050 Hz, stereo, s16 > >> >> > >> > >> Here's a an even worse case . . . > >> > >> << > >> Duration: 00:04:45.05, start: 0.000000, bitrate: 195 kb/s > >> Stream #0.0, 21, 1/1000: Video: h264, yuv420p, 320x214 [PAR 1:1 > DAR > >> 160:107], 1/2000, 195 kb/s, 29.92 tbr, 1k tbn, 2k tbc > >> Stream #0.1, 15, 1/1000: Audio: aac, 22050 Hz, stereo, s16 > >> Output #0, flv, to '': > >> Stream #0.0, 0, 1/1000: Video: libx264, yuv420p, 320x214, > 1/2000, > >> q=2-31, 650 kb/s, 29.92 tbr, 1k tbn, 2k tbc > >> Stream #0.1, 0, 1/90000: Audio: aac, 22050 Hz, stereo, s16 > >> >> > >> > > > >FLV has variable frame rate - the timestamps are always specified with > a > >1 kHz time base. It's fairly common for the "average" frame rate to > >vary. I've seen YouTube FLVs start at 3 fps, but switch to ~29.97 fps a > >few seconds later. In other words, in general lavf can't figure out the > >exact frame rate. Also, if I remember correctly the codec's time base > is > >the field rate, which might in fact be useful (divide by 2 if > >interlaced). > > One thing I can't figure out is how to use the time_base values as in > the above two examples. The first has a value (50 / 2997) which would > lead me to calculate a frame rate of 59.94 when it is really on the > order of 30 fps. The decoded frame does not indicate interlaced. In > the second case the time_base is (1/2000). How is 2000 fps a useful > value??
Btw, you can also look at r_frame_rate. It's a guess though, but it might be more useful. > > >Your best bet would be to preserve the time stamps and convert them to > >the muxer's time base when written (av_rescale_q() between the > >AVFormatContext::time_base:es). If you want constant frame rate output > >you'll have to drop and/or duplicate frames to reach the desired rate. > >If you're outputing to flv again then you don't need to worry too much > >since the output is VFR too. You will need to give the encoder a frame > >rate guess for it to perform proper rate control though. > > Ok, I am sure I can eventually figure out how to do this in my > application. The Dranger tutorial seems to have some useful details. > However, what about the ffmpeg CLI scenario? How can I pass a frame > rate for bit rate control if the decoded frame does not even provide > the encoder a pts value? Will I be able to directly convert flv to flv > using the built-in h264 decoder and my video encoder? Use reasonable frame rate, then convert the demuxed PTS values to the frame rate you chose and feed them to the encoder via coded_frame->pts. This will cause it to perform proper (sort of) rate control even if the input has variable frame rate. This will work fairly well unless you want B-frames. In that case you need to reorder your timestamps in the same order as the encoder has reordered its timestamps. Unfortunately the encoder only outputs synthetic timestamps via coded_frame->pts (after the call to avcodec_encode_video()). What you need to do is generate your own synthetic timestamps that match what lavc outputs (normal sequence starting at zero), then map those to what the actual values should be. Say you have 0, 333, 666 in. You rescale those to maybe 0, 20, 40 (60 fps) and put them in coded_frame->pts. Then you might get 0, 2, 1 out, which means you need to output 0, 666, 333 in the original 1 kHz time base. A sorted associative container like std::map will make this job easier. /Tomas
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