[FFmpeg-trac] #1607(avformat:new): av_seek_frame() without AVSEEK_FLAG_ANY on mpg files returns success without seeking to a keyframe

[FFmpeg]

#1607: av_seek_frame() without AVSEEK_FLAG_ANY on mpg files returns success
without seeking to a keyframe
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             Reporter:  mbradshaw   |                    Owner:
                 Type:  defect      |                   Status:  new
             Priority:  normal      |                Component:  avformat
              Version:  git-master  |               Resolution:
             Keywords:              |               Blocked By:
             Blocking:              |  Reproduced by developer:  0
Analyzed by developer:  0           |
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Comment (by mbradshaw):

 Replying to [comment:10 cehoyos]:
 > Replying to [comment:5 mbradshaw]:
 > > The problem isn't that you get a junk frame out (the decoder makes
 sure you don't).
 >
 > I wonder what this sentence means: There are many samples that return
 gray frames for above command line (including some sample that you pointed
 to).
 Really? I don't get any gray frames with those commands. I get good
 looking frames out, but that's because the decoder swallows the non-
 keyframes after the seek and ffmpeg keeps reading until it gets a keyframe
 packet and the decoder finally returns a full frame. I get:
 {{{
 ./ffmpeg -ss 1 -i ~/mpeg2video_seek_to_non_keyframe.mpg ~/out.png
 ffmpeg version N-43294-g8993c25 Copyright (c) 2000-2012 the FFmpeg
 developers
   built on Aug  7 2012 12:02:23 with gcc 4.2.1 (GCC) (Apple Inc. build
 5666) (dot 3)
   configuration:
   libavutil      51. 66.101 / 51. 66.101
   libavcodec     54. 50.100 / 54. 50.100
   libavformat    54. 22.101 / 54. 22.101
   libavdevice    54.  2.100 / 54.  2.100
   libavfilter     3.  5.102 /  3.  5.102
   libswscale      2.  1.101 /  2.  1.101
   libswresample   0. 15.100 /  0. 15.100
 [mpeg @ 0x10180ec00] max_analyze_duration 5000000 reached at 5024000
 Input #0, mpeg, from '/Users/mjbshaw/mpeg2video_seek_to_non_keyframe.mpg':
   Duration: 00:00:09.85, start: 1.320000, bitrate: 4998 kb/s
     Stream #0:0[0x1e0]: Video: mpeg2video (Main), yuv420p, 720x576 [SAR
 16:15 DAR 4:3], 8000 kb/s, 25 fps, 25 tbr, 90k tbn, 50 tbc
     Stream #0:1[0x80]: Audio: ac3, 48000 Hz, stereo, s16, 192 kb/s
 Output #0, image2, to '/Users/mjbshaw/out.png':
   Metadata:
     encoder         : Lavf54.22.101
     Stream #0:0: Video: png, rgb24, 720x576 [SAR 16:15 DAR 4:3], q=2-31,
 200 kb/s, 90k tbn, 25 tbc
 Stream mapping:
   Stream #0:0 -> #0:0 (mpeg2video -> png)
 Press [q] to stop, [?] for help
 [mpeg2video @ 0x10180f200] warning: first frame is no keyframe
 [image2 @ 0x10180f800] Could not get frame filename number 2 from pattern
 '/Users/mjbshaw/out.png'
 av_interleaved_write_frame(): Invalid argument
 }}}

 The output image looks okay though. The key here is the same thing Don
 points out (the `[mpeg2video @ 0x10180f200] warning: first frame is no
 keyframe` part). If you're able to get gray frames out with that command
 then something's different between our ffmpegs...

 @Don: I don't think (someone please correct me if I'm wrong) that
 specialized code can really be written for the mpeg's AVInputFormat seek
 function because of the structure of mpeg's. I think it would just be
 better to fix `ff_seek_frame_binary()`, because if custom code was written
 for mpeg's it would end up being so similar to `ff_seek_frame_binary()`.
 IIUC, mpeg's are impossible to properly seek by keyframes reliably, and
 the best solution would be to do a binary search for a frame, and then
 after finding it, sequentially reading (either forwards or backwards) to
 the next keyframe.

-- 
Ticket URL: <https://ffmpeg.org/trac/ffmpeg/ticket/1607#comment:12>
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