title: Video frame difference with FFMPEG
body: <p>I need to compute the frame differences between a source video and a compressed one.
  For now I'm using <em>OpenCV</em> with <em>Java</em>, by extracting each frame and doing a simple difference, but it's quite slow (working a 0.5 fps, meaning that a 500 frames video will take more than 15 mins) so I was thinking to move to <em>FFMPEG</em>.</p>
  
  <p><em>FFMPEG</em> feels a lot faster (everything's done under 1 minute) but it has one big issue that makes the results useless: when compressing the source file, done with <em>FFMPEG</em> too, an extra gray frame is added at the beginning and this fakes the results because different frames are compared.</p>
  
  <p>This is what I'm doing now (knowing that the extra frame messes it all):</p>
  
  <pre><code>ffmpeg -y -i src.avi -i compressed.avi -filter_complex "blend=all_mode=difference,hue=s=0" -c:v libx264 -crf 18 -c:a copy difference.avi
  </code></pre>
  
  <p>To fix the frame issue I was trying to remove the first frame by re-encoding the compressed video with this command</p>
  
  <pre><code>ffmpeg -y -ss 0.02 -i compressed.mpg -an -f mpeg2video compressed-cut.mpg"
  </code></pre>
  
  <p>(Note that <code>-ss</code> is <em>0.02</em> because it's a <em>50 fps</em> video, so I did <code>1/FPS</code> as suggested <a href="http://ffmpeg-users.933282.n4.nabble.com/delete-1st-frame-from-a-video-file-tp3001083p3001712.html" rel="nofollow">here</a>)</p>
  
  <p>But I get this response</p>
  
  <blockquote>
    <p>Output file is empty, nothing was encoded (check -ss / -t / -frames parameters if used)</p>
  </blockquote>
  
  <p>So, finally, the question is: since extracting all the frames and then compute differences with <em>OpenCV</em> is really slow, how can I use <em>FFMPEG</em> to produce a video containing the difference between two sources while keeping in mind that one of them has an extra frame at the beginning?</p>
  
  <p>EDIT: I wanted to avoid posting endless console outputs but since you asked for it, here we go.</p>
  
  <p><strong>1) Encoding</strong></p>
  
  <p><em>Input</em></p>
  
  <pre><code>ffmpeg -i "720p50_mobcal_ter.avi" -an -f mpeg2video -y "720p50_mobcal_ter.mpg"
  </code></pre>
  
  <p><em>Output</em></p>
  
  <pre><code>ffmpeg version N-76684-g1fe82ab Copyright (c) 2000-2015 the FFmpeg developers
    built with gcc 5.2.0 (GCC)
    configuration: --enable-gpl --enable-version3 --disable-w32threads --enable-avisynth --enable-bzlib --enable-fontconfig --enable-frei0r --enable-gnutls --enable-iconv --enable-libass --enable-libbluray --enable-libbs2b --enable-libcaca --enable-libdcadec --enable-libfreetype --enable-libgme --enable-libgsm --enable-libilbc --enable-libmodplug --enable-libmp3lame --enable-libopencore-amrnb --enable-libopencore-amrwb --enable-libopenjpeg --enable-libopus --enable-librtmp --enable-libschroedinger --enable-libsoxr --enable-libspeex --enable-libtheora --enable-libtwolame --enable-libvidstab --enable-libvo-aacenc --enable-libvo-amrwbenc --enable-libvorbis --enable-libvpx --enable-libwavpack --enable-libwebp --enable-libx264 --enable-libx265 --enable-libxavs --enable-libxvid --enable-libzimg --enable-lzma --enable-decklink --enable-zlib
    libavutil      55.  6.100 / 55.  6.100
    libavcodec     57. 15.100 / 57. 15.100
    libavformat    57. 14.100 / 57. 14.100
    libavdevice    57.  0.100 / 57.  0.100
    libavfilter     6. 15.100 /  6. 15.100
    libswscale      4.  0.100 /  4.  0.100
    libswresample   2.  0.101 /  2.  0.101
    libpostproc    54.  0.100 / 54.  0.100
  Input #0, avi, from '720p50_mobcal_ter.avi':
    Metadata:
      encoder         : Lavf57.14.100
    Duration: 00:00:10.08, start: 0.000000, bitrate: 552974 kb/s
      Stream #0:0: Video: rawvideo (I420 / 0x30323449), yuv420p, 1280x720, 554059 kb/s, SAR 1:1 DAR 16:9, 50 fps, 50 tbr, 50 tbn, 50 tbc
  Output #0, mpeg2video, to '720p50_mobcal_ter.mpg':
    Metadata:
      encoder         : Lavf57.14.100
      Stream #0:0: Video: mpeg2video, yuv420p, 1280x720 [SAR 1:1 DAR 16:9], q=2-31, 200 kb/s, 50 fps, 50 tbn, 50 tbc
      Metadata:
        encoder         : Lavc57.15.100 mpeg2video
  Stream mapping:
    Stream #0:0 -&gt; #0:0 (rawvideo (native) -&gt; mpeg2video (native))
  Press [q] to stop, [?] for help
  frame=   41 fps=0.0 q=31.0 size=     984kB time=00:00:00.78 bitrate=10330.5kbits/frame=   80 fps= 78 q=31.0 size=    1323kB time=00:00:01.56 bitrate=6948.1kbits/frame=  124 fps= 80 q=31.0 size=    1725kB time=00:00:02.44 bitrate=5790.0kbits/frame=  168 fps= 81 q=31.0 size=    2084kB time=00:00:03.32 bitrate=5142.8kbits/frame=  212 fps= 81 q=31.0 size=    2482kB time=00:00:04.20 bitrate=4841.4kbits/frame=  255 fps= 82 q=31.0 size=    2840kB time=00:00:05.06 bitrate=4597.2kbits/frame=  296 fps= 82 q=31.0 size=    3133kB time=00:00:05.88 bitrate=4364.5kbits/frame=  338 fps= 82 q=24.8 size=    3453kB time=00:00:06.72 bitrate=4209.2kbits/frame=  382 fps= 82 q=31.0 size=    3723kB time=00:00:07.60 bitrate=4013.4kbits/frame=  426 fps= 83 q=31.0 size=    4005kB time=00:00:08.48 bitrate=3869.1kbits/frame=  470 fps= 83 q=24.8 size=    4276kB time=00:00:09.36 bitrate=3742.5kbits/frame=  504 fps= 83 q=31.0 Lsize=    4469kB time=00:00:10.06 bitrate=3639.3kbits/s
  video:4469kB audio:0kB subtitle:0kB other streams:0kB global headers:0kB muxing overhead: 0.000000%
  </code></pre>
  
  <p><em>This adds the extra grey frame at the beginning, it just duplicates the first one</em></p>
  
  <p><strong>2) Removing first frame</strong></p>
  
  <p><em>Input</em></p>
  
  <pre><code>ffmpeg -y -i "720p50_mobcal_ter.mpg" -an -f mpeg2video -vf select=gte(n\,1) "CUT-720p50_mobcal_ter.mpg"
  </code></pre>
  
  <p><em>Output</em></p>
  
  <pre><code>ffmpeg version N-76684-g1fe82ab Copyright (c) 2000-2015 the FFmpeg developers
    built with gcc 5.2.0 (GCC)
    configuration: --enable-gpl --enable-version3 --disable-w32threads --enable-avisynth --enable-bzlib --enable-fontconfig --enable-frei0r --enable-gnutls --enable-iconv --enable-libass --enable-libbluray --enable-libbs2b --enable-libcaca --enable-libdcadec --enable-libfreetype --enable-libgme --enable-libgsm --enable-libilbc --enable-libmodplug --enable-libmp3lame --enable-libopencore-amrnb --enable-libopencore-amrwb --enable-libopenjpeg --enable-libopus --enable-librtmp --enable-libschroedinger --enable-libsoxr --enable-libspeex --enable-libtheora --enable-libtwolame --enable-libvidstab --enable-libvo-aacenc --enable-libvo-amrwbenc --enable-libvorbis --enable-libvpx --enable-libwavpack --enable-libwebp --enable-libx264 --enable-libx265 --enable-libxavs --enable-libxvid --enable-libzimg --enable-lzma --enable-decklink --enable-zlib
    libavutil      55.  6.100 / 55.  6.100
    libavcodec     57. 15.100 / 57. 15.100
    libavformat    57. 14.100 / 57. 14.100
    libavdevice    57.  0.100 / 57.  0.100
    libavfilter     6. 15.100 /  6. 15.100
    libswscale      4.  0.100 /  4.  0.100
    libswresample   2.  0.101 /  2.  0.101
    libpostproc    54.  0.100 / 54.  0.100
  Input #0, mpegvideo, from '720p50_mobcal_ter.mpg':
    Duration: N/A, bitrate: N/A
      Stream #0:0: Video: mpeg2video (Main), yuv420p(tv), 1280x720 [SAR 1:1 DAR 16:9], max. 104857 kb/s, 50 fps, 50 tbr, 1200k tbn, 100 tbc
  Output #0, mpeg2video, to 'CUT-720p50_mobcal_ter.mpg':
    Metadata:
      encoder         : Lavf57.14.100
      Stream #0:0: Video: mpeg2video, yuv420p, 1280x720 [SAR 1:1 DAR 16:9], q=2-31, 200 kb/s, 50 fps, 50 tbn, 50 tbc
      Metadata:
        encoder         : Lavc57.15.100 mpeg2video
  Stream mapping:
    Stream #0:0 -&gt; #0:0 (mpeg2video (native) -&gt; mpeg2video (native))
  Press [q] to stop, [?] for help
  frame=  255 fps=0.0 q=31.0 size=    2781kB time=00:00:05.10 bitrate=4467.3kbits/frame=  503 fps=0.0 q=31.0 Lsize=    4415kB time=00:00:10.08 bitrate=3588.5kbits/s
  video:4415kB audio:0kB subtitle:0kB other streams:0kB global headers:0kB muxing overhead: 0.000000%
  </code></pre>
  
  <p><strong>3) Frame difference</strong></p>
  
  <p><em>Input</em></p>
  
  <pre><code>ffmpeg -y -i "720p50_mobcal_ter.avi" -i "CUT-720p50_mobcal_ter.mpg" -filter_complex "blend=all_mode=difference,hue=s=0" -c:v libx264 -crf 18 -c:a copy "DIFF-720p50_mobcal_ter.mpg"
  </code></pre>
  
  <p><em>Output</em></p>
  
  <pre><code>ffmpeg version N-76684-g1fe82ab Copyright (c) 2000-2015 the FFmpeg developers
    built with gcc 5.2.0 (GCC)
    configuration: --enable-gpl --enable-version3 --disable-w32threads --enable-avisynth --enable-bzlib --enable-fontconfig --enable-frei0r --enable-gnutls --enable-iconv --enable-libass --enable-libbluray --enable-libbs2b --enable-libcaca --enable-libdcadec --enable-libfreetype --enable-libgme --enable-libgsm --enable-libilbc --enable-libmodplug --enable-libmp3lame --enable-libopencore-amrnb --enable-libopencore-amrwb --enable-libopenjpeg --enable-libopus --enable-librtmp --enable-libschroedinger --enable-libsoxr --enable-libspeex --enable-libtheora --enable-libtwolame --enable-libvidstab --enable-libvo-aacenc --enable-libvo-amrwbenc --enable-libvorbis --enable-libvpx --enable-libwavpack --enable-libwebp --enable-libx264 --enable-libx265 --enable-libxavs --enable-libxvid --enable-libzimg --enable-lzma --enable-decklink --enable-zlib
    libavutil      55.  6.100 / 55.  6.100
    libavcodec     57. 15.100 / 57. 15.100
    libavformat    57. 14.100 / 57. 14.100
    libavdevice    57.  0.100 / 57.  0.100
    libavfilter     6. 15.100 /  6. 15.100
    libswscale      4.  0.100 /  4.  0.100
    libswresample   2.  0.101 /  2.  0.101
    libpostproc    54.  0.100 / 54.  0.100
  Input #0, avi, from '720p50_mobcal_ter.avi':
    Metadata:
      encoder         : Lavf57.14.100
    Duration: 00:00:10.08, start: 0.000000, bitrate: 552974 kb/s
      Stream #0:0: Video: rawvideo (I420 / 0x30323449), yuv420p, 1280x720, 554059 kb/s, SAR 1:1 DAR 16:9, 50 fps, 50 tbr, 50 tbn, 50 tbc
  Input #1, mpegvideo, from 'CUT-720p50_mobcal_ter.mpg':
    Duration: N/A, bitrate: N/A
      Stream #1:0: Video: mpeg2video (Main), yuv420p(tv), 1280x720 [SAR 1:1 DAR 16:9], max. 104857 kb/s, 50 fps, 50 tbr, 1200k tbn, 100 tbc
  [libx264 @ 000002784dbeb980] using SAR=1/1
  [libx264 @ 000002784dbeb980] using cpu capabilities: MMX2 SSE2Fast SSSE3 SSE4.2 AVX FMA3 AVX2 LZCNT BMI2
  [libx264 @ 000002784dbeb980] profile High, level 3.2
  [mpeg @ 000002784dbeaf20] VBV buffer size not set, using default size of 130KB
  If you want the mpeg file to be compliant to some specification
  Like DVD, VCD or others, make sure you set the correct buffer size
  Output #0, mpeg, to 'D:\DOWNLOADS\TMP\Video TDI\AVI\DIFF-720p50_mobcal_ter.mpg':
    Metadata:
      encoder         : Lavf57.14.100
      Stream #0:0: Video: h264 (libx264), yuv420p, 1280x720 [SAR 1:1 DAR 16:9], q=-1--1, 50 fps, 90k tbn, 50 tbc (default)
      Metadata:
        encoder         : Lavc57.15.100 libx264
  Stream mapping:
    Stream #0:0 (rawvideo) -&gt; blend:top
    Stream #1:0 (mpeg2video) -&gt; blend:bottom
    hue -&gt; Stream #0:0 (libx264)
  Press [q] to stop, [?] for help
  frame=  504 fps= 39 q=-1.0 Lsize=   32182kB time=00:00:10.04 bitrate=26258.5kbits/s
  video:32061kB audio:0kB subtitle:0kB other streams:0kB global headers:0kB muxing overhead: 0.377054%
  [libx264 @ 000002784dbeb980] frame I:30    Avg QP:19.69  size:149974
  [libx264 @ 000002784dbeb980] frame P:299   Avg QP:23.28  size: 69423
  [libx264 @ 000002784dbeb980] frame B:175   Avg QP:24.48  size: 43280
  [libx264 @ 000002784dbeb980] consecutive B-frames: 30.6% 69.4%  0.0%  0.0%
  [libx264 @ 000002784dbeb980] mb I  I16..4: 18.3% 51.4% 30.4%
  [libx264 @ 000002784dbeb980] mb P  I16..4:  0.6%  5.6%  2.4%  P16..4: 35.9% 22.9% 15.6%  0.0%  0.0%    skip:17.0%
  [libx264 @ 000002784dbeb980] mb B  I16..4:  0.2%  0.5%  0.3%  B16..8: 49.5% 12.4%  5.6%  direct:15.5%  skip:16.1%  L0:47.8% L1:42.1% BI:10.1%
  [libx264 @ 000002784dbeb980] 8x8 transform intra:57.5% inter:38.5%
  [libx264 @ 000002784dbeb980] coded y,uvDC,uvAC intra: 90.7% 0.0% 0.0% inter: 50.3% 0.0% 0.0%
  [libx264 @ 000002784dbeb980] i16 v,h,dc,p: 32% 23% 35% 10%
  [libx264 @ 000002784dbeb980] i8 v,h,dc,ddl,ddr,vr,hd,vl,hu: 11% 11% 41%  7%  5%  6%  5%  6%  8%
  [libx264 @ 000002784dbeb980] i4 v,h,dc,ddl,ddr,vr,hd,vl,hu: 18% 14% 23%  8%  7%  7%  7%  7%  8%
  [libx264 @ 000002784dbeb980] i8c dc,h,v,p: 100%  0%  0%  0%
  [libx264 @ 000002784dbeb980] Weighted P-Frames: Y:33.8% UV:0.0%
  [libx264 @ 000002784dbeb980] ref P L0: 58.1% 16.3% 14.2%  9.4%  2.0%
  [libx264 @ 000002784dbeb980] ref B L0: 79.3% 20.7%
  [libx264 @ 000002784dbeb980] kb/s:26056.02
  </code></pre>
  
  <p>The second command made everything work while the second one in the first part didn't (the one with the <code>-ss</code> option), so I could be quite happy about it but I'm not that sure if <em>FFMPEG</em> duplicates the first frame for every video or if it's just related to the one I'm using now, so it could be better to start off with a compressed video that has the same frame count of the original one.</p>
  
  <p>So let's get to one final question: why does <em>FFMPEG</em> add a duplicated first frame at the beginning of the compressed video and how can I avoid that?</p>