title: ffmpeg filter_complex concat never completes
body: <p>I am trying to concatenate <code>mp4</code> files into a single file. I am attempting to use the <code>concat</code> filter directly for reasons independent from this question (so <code>-i list.txt</code> is not a valid solution). All video files in question are between 4 and 20 minutes in length.</p>
  <p><strong>What I am executing:</strong></p>
  <pre class="lang-sh prettyprint-override"><code>% ffmpeg -i f01.mp4 -i f02.mp4 -filter_complex &quot;[0:v][0:a][1:v][1:a]concat=n=2:v=1:a=1[v][a]&quot; -map &quot;[v]&quot; -map &quot;[a]&quot; output.mp4
  </code></pre>
  <p>This is my understanding of the form of this command from references such as <a href="https://ffmpeg.org/ffmpeg-filters.html#concat" rel="noreferrer">FFmpeg Filters Documentation: concat</a> and <a href="https://dev.to/dak425/concatenate-videos-together-using-ffmpeg-2gg1" rel="noreferrer">Concatenate Videos Together Using FFMPEG!</a>.</p>
  <p><strong>What happens:</strong></p>
  <p><code>ffmpeg</code> gives a lot of output as it checks the metadata for each stream and then begins processing. After a short duration (several seconds to a minute or so, seems to be prortional to the duration of the first video) I start seeing messages like this:</p>
  <pre><code>More than 1000 frames duplicated
  More than 10000 frames duplicated     512kB time=00:00:00.12 bitrate=32771.0kbits/s dup=33365 drop=0 speed=0.00449x
  More than 100000 frames duplicated   1280kB time=00:00:00.17 bitrate=61442.1kbits/s dup=66730 drop=0 speed=0.00272x
  </code></pre>
  <p>... and then the process never completes. If I leave my computer running for 24 hours, <code>ffmpeg</code> is still using max available CPU (200-300%). The output file is 48 bytes in length.</p>
  <p><strong>Note:</strong> the inputs are only a few minutes each, so individually re-encoding the inputs would take only a few minutes each.</p>
  <p><strong>Note:</strong> if I change the command to use <code>f01.mp4</code> for both source 0 and source 1, the command completes as expected in under 5 minutes:</p>
  <pre class="lang-sh prettyprint-override"><code>% # example that works:
  % # note, however, that this just repeats the same source twice
  % ffmpeg -i f01.mp4 -i f01.mp4 -filter_complex &quot;[0:v][0:a][1:v][1:a]concat=n=2:v=1:a=1[v][a]&quot; -map &quot;[v]&quot; -map &quot;[a]&quot; output.mp4
  </code></pre>
  <p><strong>Full output:</strong></p>
  <p>Here is the complete output of a run in case it's helpful:</p>
  <pre class="lang-sh prettyprint-override"><code>% ffmpeg -i f01.mp4 -i f02.mp4 -filter_complex &quot;[0:v][0:a][1:v][1:a]concat=n=2:v=1:a=1[v][a]&quot; -map &quot;[v]&quot; -map &quot;[a]&quot; output.mp4
  ffmpeg version 4.3 Copyright (c) 2000-2020 the FFmpeg developers
    built with Apple clang version 11.0.3 (clang-1103.0.32.62)
    configuration: --prefix=/usr/local/Cellar/ffmpeg/4.3_2 --enable-shared --enable-pthreads --enable-version3 --enable-avresample --cc=clang --host-cflags= --host-ldflags= --enable-ffplay --enable-gnutls --enable-gpl --enable-libaom --enable-libbluray --enable-libdav1d --enable-libmp3lame --enable-libopus --enable-librav1e --enable-librubberband --enable-libsnappy --enable-libsrt --enable-libtesseract --enable-libtheora --enable-libvidstab --enable-libvorbis --enable-libvpx --enable-libwebp --enable-libx264 --enable-libx265 --enable-libxvid --enable-lzma --enable-libfontconfig --enable-libfreetype --enable-frei0r --enable-libass --enable-libopencore-amrnb --enable-libopencore-amrwb --enable-libopenjpeg --enable-librtmp --enable-libspeex --enable-libsoxr --enable-videotoolbox --disable-libjack --disable-indev=jack
    libavutil      56. 51.100 / 56. 51.100
    libavcodec     58. 91.100 / 58. 91.100
    libavformat    58. 45.100 / 58. 45.100
    libavdevice    58. 10.100 / 58. 10.100
    libavfilter     7. 85.100 /  7. 85.100
    libavresample   4.  0.  0 /  4.  0.  0
    libswscale      5.  7.100 /  5.  7.100
    libswresample   3.  7.100 /  3.  7.100
    libpostproc    55.  7.100 / 55.  7.100
  Input #0, mov,mp4,m4a,3gp,3g2,mj2, from 'f01.mp4':
    Metadata:
      major_brand     : mp42
      minor_version   : 512
      compatible_brands: isomiso2avc1mp41
      creation_time   : 2020-06-04T21:34:26.000000Z
      encoder         : HandBrake 1.3.2 2020050300
    Duration: 00:04:14.66, start: 0.000000, bitrate: 525 kb/s
      Stream #0:0(und): Video: h264 (Main) (avc1 / 0x31637661), yuv420p(tv, smpte170m/smpte170m/bt709), 708x478 [SAR 8:9 DAR 944:717], 366 kb/s, 29.97 fps, 29.97 tbr, 90k tbn, 180k tbc (default)
      Metadata:
        creation_time   : 2020-06-04T21:34:26.000000Z
        handler_name    : VideoHandler
      Stream #0:1(jpn): Audio: aac (LC) (mp4a / 0x6134706D), 48000 Hz, stereo, fltp, 149 kb/s (default)
      Metadata:
        creation_time   : 2020-06-04T21:34:26.000000Z
        handler_name    : Stereo
  Input #1, mov,mp4,m4a,3gp,3g2,mj2, from 'f02.mp4':
    Metadata:
      major_brand     : mp42
      minor_version   : 512
      compatible_brands: isomiso2avc1mp41
      creation_time   : 2020-06-04T21:38:15.000000Z
      encoder         : HandBrake 1.3.2 2020050300
    Duration: 00:06:30.95, start: 0.000000, bitrate: 1328 kb/s
      Stream #1:0(und): Video: h264 (Main) (avc1 / 0x31637661), yuv420p(tv, smpte170m/smpte170m/bt709), 708x478 [SAR 8:9 DAR 944:717], 1179 kb/s, 29.97 fps, 30 tbr, 90k tbn, 180k tbc (default)
      Metadata:
        creation_time   : 2020-06-04T21:38:15.000000Z
        handler_name    : VideoHandler
      Stream #1:1(jpn): Audio: aac (LC) (mp4a / 0x6134706D), 48000 Hz, stereo, fltp, 140 kb/s (default)
      Metadata:
        creation_time   : 2020-06-04T21:38:15.000000Z
        handler_name    : Stereo
  File 'output.mp4' already exists. Overwrite? [y/N] y
  Stream mapping:
    Stream #0:0 (h264) -&gt; concat:in0:v0
    Stream #0:1 (aac) -&gt; concat:in0:a0
    Stream #1:0 (h264) -&gt; concat:in1:v0
    Stream #1:1 (aac) -&gt; concat:in1:a0
    concat:out:v0 -&gt; Stream #0:0 (libx264)
    concat:out:a0 -&gt; Stream #0:1 (aac)
  Press [q] to stop, [?] for help
  [mp4 @ 0x7ff130014000] Frame rate very high for a muxer not efficiently supporting it.
  Please consider specifying a lower framerate, a different muxer or -vsync 2
  [libx264 @ 0x7ff130021200] using SAR=8/9
  [libx264 @ 0x7ff130021200] MB rate (1350000000) &gt; level limit (16711680)
  [libx264 @ 0x7ff130021200] using cpu capabilities: MMX2 SSE2Fast SSSE3 SSE4.2 AVX FMA3 BMI2 AVX2
  [libx264 @ 0x7ff130021200] profile High, level 6.2, 4:2:0, 8-bit
  [libx264 @ 0x7ff130021200] 264 - core 160 r3011 cde9a93 - H.264/MPEG-4 AVC codec - Copyleft 2003-2020 - http://www.videolan.org/x264.html - options: cabac=1 ref=3 deblock=1:0:0 analyse=0x3:0x113 me=hex subme=7 psy=1 psy_rd=1.00:0.00 mixed_ref=1 me_range=16 chroma_me=1 trellis=1 8x8dct=1 cqm=0 deadzone=21,11 fast_pskip=1 chroma_qp_offset=-2 threads=6 lookahead_threads=1 sliced_threads=0 nr=0 decimate=1 interlaced=0 bluray_compat=0 constrained_intra=0 bframes=3 b_pyramid=2 b_adapt=1 b_bias=0 direct=1 weightb=1 open_gop=0 weightp=2 keyint=250 keyint_min=25 scenecut=40 intra_refresh=0 rc_lookahead=40 rc=crf mbtree=1 crf=23.0 qcomp=0.60 qpmin=0 qpmax=69 qpstep=4 ip_ratio=1.40 aq=1:1.00
  Output #0, mp4, to 'output.mp4':
    Metadata:
      major_brand     : mp42
      minor_version   : 512
      compatible_brands: isomiso2avc1mp41
      encoder         : Lavf58.45.100
      Stream #0:0: Video: h264 (libx264) (avc1 / 0x31637661), yuv420p(progressive), 708x478 [SAR 8:9 DAR 944:717], q=-1--1, 1000k tbn, 1000k tbc (default)
      Metadata:
        encoder         : Lavc58.91.100 libx264
      Side data:
        cpb: bitrate max/min/avg: 0/0/0 buffer size: 0 vbv_delay: N/A
      Stream #0:1: Audio: aac (LC) (mp4a / 0x6134706D), 48000 Hz, stereo, fltp, 128 kb/s (default)
      Metadata:
        encoder         : Lavc58.91.100 aac
  More than 1000 frames duplicated
  More than 10000 frames duplicated     512kB time=00:00:00.12 bitrate=32771.0kbits/s dup=33365 drop=0 speed=0.00449x
  More than 100000 frames duplicated   1280kB time=00:00:00.17 bitrate=61442.1kbits/s dup=66730 drop=0 speed=0.00272x
  frame=667333 fps=1079 q=33.0 size=   14848kB time=00:00:00.76 bitrate=158379.2kbits/s dup=667312 drop=0 speed=0.00124x
  </code></pre>