[FFmpeg-trac] #3989(undetermined:new): "Overwrite?" dialog causes frozen video when capturing desktop video

[FFmpeg]

#3989: "Overwrite?" dialog causes frozen video when capturing desktop video
-------------------------------------+-------------------------------------
             Reporter:  oconnor663   |                    Owner:
                 Type:  defect       |                   Status:  new
             Priority:  normal       |                Component:
              Version:  git-master   |  undetermined
             Keywords:  x11grab      |               Resolution:
             Blocking:               |               Blocked By:
Analyzed by developer:  0            |  Reproduced by developer:  0
-------------------------------------+-------------------------------------

Comment (by oconnor663):

 I can't speak to the expectations of an expert, but as an ordinary user,
 the experience feels broken. Let's say I fire up glxgears, start an ffmpeg
 recording of my desktop, wait at the prompt for 2 seconds, and then record
 for 2 seconds. (Like in my example video attached.) I would expect one of
 two behaviors:

 1) I get 4 seconds of video of glxgears moving, because ffmpeg started
 recording immediately.
 2) I get 2 seconds of video of glxgears moving, because ffmpeg only
 started recording after I agreed to overwrite the existing file.

 But what actually happens is:

 3) I get 4 seconds of video, but for the first 2 seconds the picture is
 frozen.

 I can't use that video. I must've done something wrong. I spend a lot of
 time fiddling with all the flags I'm using. The problem seems to come and
 go when I change container types (but actually, it goes away whenever I
 used a new filename).

 The workaround of course is to delete the preexisting video file before
 re-recording it, so that I don't get prompted. What sucks is that it's not
 at all obvious *why* it's happening, at least to a beginner like me.

 Console output from my git master build:
 {{{
 # The first run, no prompt:
 $ /var/tmp/ffmpeg/ffmpeg -f x11grab -i $DISPLAY test.mkv
 ffmpeg version N-66543-g1441641 Copyright (c) 2000-2014 the FFmpeg
 developers
   built on Sep 29 2014 13:46:22 with gcc 4.9.1 (GCC) 20140903 (prerelease)
   configuration: --enable-x11grab --enable-gpl
   libavutil      54.  7.101 / 54.  7.101
   libavcodec     56.  1.101 / 56.  1.101
   libavformat    56.  7.101 / 56.  7.101
   libavdevice    56.  1.100 / 56.  1.100
   libavfilter     5.  1.102 /  5.  1.102
   libswscale      3.  1.100 /  3.  1.100
   libswresample   1.  1.100 /  1.  1.100
   libpostproc    53.  1.100 / 53.  1.100
 [x11grab @ 0x2f90c20] device: :0 -> display: :0 x: 0 y: 0 width: 640
 height: 480
 [x11grab @ 0x2f90c20] shared memory extension found
 Input #0, x11grab, from ':0':
   Duration: N/A, start: 1412025598.375747, bitrate: 294617 kb/s
     Stream #0:0: Video: rawvideo (BGR[0] / 0x524742), bgr0, 640x480,
 294617 kb/s, 29.97 tbr, 1000k tbn, 29.97 tbc
 Output #0, matroska, to 'test.mkv':
   Metadata:
     encoder         : Lavf56.7.101
     Stream #0:0: Video: mpeg4 (FMP4 / 0x34504D46), yuv420p, 640x480,
 q=2-31, 200 kb/s, 29.97 fps, 1k tbn, 29.97 tbc
     Metadata:
       encoder         : Lavc56.1.101 mpeg4
 Stream mapping:
   Stream #0:0 -> #0:0 (rawvideo (native) -> mpeg4 (native))
 Press [q] to stop, [?] for help
 frame=   70 fps= 30 q=2.0 Lsize=     263kB time=00:00:02.33 bitrate=
 923.6kbits/s
 video:262kB audio:0kB subtitle:0kB other streams:0kB global headers:0kB
 muxing overhead: 0.498994%
 Received signal 2: terminating.

 # The second run, with the overwrite prompt:
 $ /var/tmp/ffmpeg/ffmpeg -f x11grab -i $DISPLAY test.mkv
 ffmpeg version N-66543-g1441641 Copyright (c) 2000-2014 the FFmpeg
 developers
   built on Sep 29 2014 13:46:22 with gcc 4.9.1 (GCC) 20140903 (prerelease)
   configuration: --enable-x11grab --enable-gpl
   libavutil      54.  7.101 / 54.  7.101
   libavcodec     56.  1.101 / 56.  1.101
   libavformat    56.  7.101 / 56.  7.101
   libavdevice    56.  1.100 / 56.  1.100
   libavfilter     5.  1.102 /  5.  1.102
   libswscale      3.  1.100 /  3.  1.100
   libswresample   1.  1.100 /  1.  1.100
   libpostproc    53.  1.100 / 53.  1.100
 [x11grab @ 0x36c9c20] device: :0 -> display: :0 x: 0 y: 0 width: 640
 height: 480
 [x11grab @ 0x36c9c20] shared memory extension found
 Input #0, x11grab, from ':0':
   Duration: N/A, start: 1412025603.853949, bitrate: 294617 kb/s
     Stream #0:0: Video: rawvideo (BGR[0] / 0x524742), bgr0, 640x480,
 294617 kb/s, 29.97 tbr, 1000k tbn, 29.97 tbc
 File 'test.mkv' already exists. Overwrite ? [y/N] y
 Output #0, matroska, to 'test.mkv':
   Metadata:
     encoder         : Lavf56.7.101
     Stream #0:0: Video: mpeg4 (FMP4 / 0x34504D46), yuv420p, 640x480,
 q=2-31, 200 kb/s, 29.97 fps, 1k tbn, 29.97 tbc
     Metadata:
       encoder         : Lavc56.1.101 mpeg4
 Stream mapping:
   Stream #0:0 -> #0:0 (rawvideo (native) -> mpeg4 (native))
 Press [q] to stop, [?] for help
 frame=   19 fps=0.0 q=2.0 size=     103kB time=00:00:02.66 bitrate=
 315.4kbits/s dupframe=   34 fps= 34 q=2.0 size=     167kB time=00:00:03.16
 bitrate= 432.3kbits/s dupframe=   49 fps= 32 q=1.6 size=     265kB
 time=00:00:03.67 bitrate= 592.5kbits/s dupframe=   65 fps= 32 q=2.0 size=
 320kB time=00:00:04.20 bitrate= 623.1kbits/s dupframe=   67 fps= 31 q=2.4
 Lsize=     326kB time=00:00:04.27 bitrate= 625.2kbits/s dup=0 drop=28
 video:325kB audio:0kB subtitle:0kB other streams:0kB global headers:0kB
 muxing overhead: 0.396367%
 Received signal 2: terminating.
 }}}

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Ticket URL: <https://trac.ffmpeg.org/ticket/3989#comment:2>
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