user27008: i'm trying to overlap two video clips using FFMpeg. I'm using the following command:
ffmpeg -i a.mp4 -i b.mp4 -filter_complex "blend=all_mode='multiply'" c.mp4

I was expecting to get something like in a bitmap editor. Instead, I get a green overlay on top of the resulting video.
Could you explain that and help me fix my command-line? I'm also open to other alternative programs.
Thanks
Edit:
Here if the full log
ffmpeg version n4.2.1 Copyright (c) 2000-2019 the FFmpeg developers
  built with gcc 9.1.0 (GCC)
  configuration: --prefix=/usr --disable-debug --disable-static --disable-stripping --enable-fontconfig --enable-gmp --enable-gnutls --enable-gpl --enable-ladspa --enable-libaom --enable-libass --enable-libbluray --enable-libdav1d --enable-libdrm --enable-libfreetype --enable-libfribidi --enable-libgsm --enable-libiec61883 --enable-libjack --enable-libmodplug --enable-libmp3lame --enable-libopencore_amrnb --enable-libopencore_amrwb --enable-libopenjpeg --enable-libopus --enable-libpulse --enable-libsoxr --enable-libspeex --enable-libssh --enable-libtheora --enable-libv4l2 --enable-libvidstab --enable-libvorbis --enable-libvpx --enable-libwebp --enable-libx264 --enable-libx265 --enable-libxcb --enable-libxml2 --enable-libxvid --enable-nvdec --enable-nvenc --enable-omx --enable-shared --enable-version3
  libavutil      56. 31.100 / 56. 31.100
  libavcodec     58. 54.100 / 58. 54.100
  libavformat    58. 29.100 / 58. 29.100
  libavdevice    58.  8.100 / 58.  8.100
  libavfilter     7. 57.100 /  7. 57.100
  libswscale      5.  5.100 /  5.  5.100
  libswresample   3.  5.100 /  3.  5.100
  libpostproc    55.  5.100 / 55.  5.100
Input #0, mov,mp4,m4a,3gp,3g2,mj2, from 'contrast.mp4':
  Metadata:
    major_brand     : isom
    minor_version   : 512
    compatible_brands: isomiso2avc1mp41
    encoder         : Lavf58.29.100
  Duration: 00:00:02.67, start: 0.000000, bitrate: 1071 kb/s
    Stream #0:0(und): Video: h264 (High 4:4:4 Predictive) (avc1 / 0x31637661), yuv444p, 1920x1080, 985 kb/s, 29.97 fps, 29.97 tbr, 30k tbn, 59.94 tbc (default)
    Metadata:
      handler_name    : VideoHandler
    Stream #0:1(und): Audio: aac (LC) (mp4a / 0x6134706D), 44100 Hz, mono, fltp, 86 kb/s (default)
    Metadata:
      handler_name    : Core Media Audio
Input #1, mov,mp4,m4a,3gp,3g2,mj2, from 'contrast.mp4':
  Metadata:
    major_brand     : isom
    minor_version   : 512
    compatible_brands: isomiso2avc1mp41
    encoder         : Lavf58.29.100
  Duration: 00:00:02.67, start: 0.000000, bitrate: 1071 kb/s
    Stream #1:0(und): Video: h264 (High 4:4:4 Predictive) (avc1 / 0x31637661), yuv444p, 1920x1080, 985 kb/s, 29.97 fps, 29.97 tbr, 30k tbn, 59.94 tbc (default)
    Metadata:
      handler_name    : VideoHandler
    Stream #1:1(und): Audio: aac (LC) (mp4a / 0x6134706D), 44100 Hz, mono, fltp, 86 kb/s (default)
    Metadata:
      handler_name    : Core Media Audio
File 'combined.mp4' already exists. Overwrite ? [y/N] y
Stream mapping:
  Stream #0:0 (h264) -> blend:top (graph 0)
  Stream #1:0 (h264) -> blend:bottom (graph 0)
  blend (graph 0) -> Stream #0:0 (libx264)
  Stream #0:1 -> #0:1 (aac (native) -> aac (native))
Press [q] to stop, [?] for help
[libx264 @ 0x558fa4a84680] using cpu capabilities: MMX2 SSE2Fast SSSE3 SSE4.2 AVX FMA3 BMI2 AVX2
[libx264 @ 0x558fa4a84680] profile High 4:4:4 Predictive, level 4.0, 4:4:4, 8-bit
[libx264 @ 0x558fa4a84680] 264 - core 157 r2945 72db437 - H.264/MPEG-4 AVC codec - Copyleft 2003-2018 - http://www.videolan.org/x264.html - options: cabac=1 ref=3 deblock=1:0:0 analyse=0x1:0x111 me=hex subme=7 psy=1 psy_rd=1.00:0.00 mixed_ref=1 me_range=16 chroma_me=1 trellis=1 8x8dct=0 cqm=0 deadzone=21,11 fast_pskip=1 chroma_qp_offset=4 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 'combined.mp4':
  Metadata:
    major_brand     : isom
    minor_version   : 512
    compatible_brands: isomiso2avc1mp41
    encoder         : Lavf58.29.100
    Stream #0:0: Video: h264 (libx264) (avc1 / 0x31637661), yuv444p, 1920x1080, q=-1--1, 29.97 fps, 30k tbn, 29.97 tbc (default)
    Metadata:
      encoder         : Lavc58.54.100 libx264
    Side data:
      cpb: bitrate max/min/avg: 0/0/0 buffer size: 0 vbv_delay: -1
    Stream #0:1(und): Audio: aac (LC) (mp4a / 0x6134706D), 44100 Hz, mono, fltp, 69 kb/s (default)
    Metadata:
      handler_name    : Core Media Audio
      encoder         : Lavc58.54.100 aac
frame=   79 fps= 19 q=-1.0 Lsize=     329kB time=00:00:02.64 bitrate=1018.5kbits/s speed=0.639x    
video:297kB audio:28kB subtitle:0kB other streams:0kB global headers:0kB muxing overhead: 1.255968%
[libx264 @ 0x558fa4a84680] frame I:2     Avg QP:11.42  size:  9143
[libx264 @ 0x558fa4a84680] frame P:27    Avg QP:20.64  size:  4721
[libx264 @ 0x558fa4a84680] frame B:50    Avg QP:22.19  size:  3150
[libx264 @ 0x558fa4a84680] consecutive B-frames: 12.7%  7.6%  3.8% 75.9%
[libx264 @ 0x558fa4a84680] mb I  I16..4: 97.7%  0.0%  2.3%
[libx264 @ 0x558fa4a84680] mb P  I16..4:  9.0%  0.0%  1.0%  P16..4:  4.9%  1.1%  0.3%  0.0%  0.0%    skip:83.7%
[libx264 @ 0x558fa4a84680] mb B  I16..4:  2.1%  0.0%  0.2%  B16..8:  9.6%  1.2%  0.0%  direct: 0.6%  skip:86.2%  L0:52.2% L1:46.4% BI: 1.4%
[libx264 @ 0x558fa4a84680] coded y,u,v intra: 4.5% 0.3% 1.0% inter: 0.9% 0.1% 0.5%
[libx264 @ 0x558fa4a84680] i16 v,h,dc,p: 68% 18%  9%  4%
[libx264 @ 0x558fa4a84680] i4 v,h,dc,ddl,ddr,vr,hd,vl,hu: 32% 10% 28%  4%  7%  9%  4%  6%  1%
[libx264 @ 0x558fa4a84680] Weighted P-Frames: Y:0.0% UV:0.0%
[libx264 @ 0x558fa4a84680] ref P L0: 52.8%  6.5% 24.4% 16.3%
[libx264 @ 0x558fa4a84680] ref B L0: 70.8% 23.2%  6.0%
[libx264 @ 0x558fa4a84680] ref B L1: 88.5% 11.5%
[libx264 @ 0x558fa4a84680] kb/s:920.37
[aac @ 0x558fa4b1ea00] Qavg: 1424.024


user27008: I solved my problem thanks to @Gyan comment:
ffmpeg -i bg.mp4 -i fg.mov -filter_complex "[0:v] format=rgba [bg]; [1:v] format=rgba [fg]; [bg][fg] blend=all_mode='multiply':all_opacity=1, format=rgba"  out.mp4



27008: <p>i'm trying to overlap two video clips using FFMpeg. I'm using the following command:</p>

<pre><code>ffmpeg -i a.mp4 -i b.mp4 -filter_complex "blend=all_mode='multiply'" c.mp4
</code></pre>

<p>I was expecting to get something like in a bitmap editor. Instead, I get a green overlay on top of the resulting video.</p>

<p>Could you explain that and help me fix my command-line? I'm also open to other alternative programs.</p>

<p>Thanks</p>

<p>Edit:</p>

<p>Here if the full log</p>

<pre><code>ffmpeg version n4.2.1 Copyright (c) 2000-2019 the FFmpeg developers
  built with gcc 9.1.0 (GCC)
  configuration: --prefix=/usr --disable-debug --disable-static --disable-stripping --enable-fontconfig --enable-gmp --enable-gnutls --enable-gpl --enable-ladspa --enable-libaom --enable-libass --enable-libbluray --enable-libdav1d --enable-libdrm --enable-libfreetype --enable-libfribidi --enable-libgsm --enable-libiec61883 --enable-libjack --enable-libmodplug --enable-libmp3lame --enable-libopencore_amrnb --enable-libopencore_amrwb --enable-libopenjpeg --enable-libopus --enable-libpulse --enable-libsoxr --enable-libspeex --enable-libssh --enable-libtheora --enable-libv4l2 --enable-libvidstab --enable-libvorbis --enable-libvpx --enable-libwebp --enable-libx264 --enable-libx265 --enable-libxcb --enable-libxml2 --enable-libxvid --enable-nvdec --enable-nvenc --enable-omx --enable-shared --enable-version3
  libavutil      56. 31.100 / 56. 31.100
  libavcodec     58. 54.100 / 58. 54.100
  libavformat    58. 29.100 / 58. 29.100
  libavdevice    58.  8.100 / 58.  8.100
  libavfilter     7. 57.100 /  7. 57.100
  libswscale      5.  5.100 /  5.  5.100
  libswresample   3.  5.100 /  3.  5.100
  libpostproc    55.  5.100 / 55.  5.100
Input #0, mov,mp4,m4a,3gp,3g2,mj2, from 'contrast.mp4':
  Metadata:
    major_brand     : isom
    minor_version   : 512
    compatible_brands: isomiso2avc1mp41
    encoder         : Lavf58.29.100
  Duration: 00:00:02.67, start: 0.000000, bitrate: 1071 kb/s
    Stream #0:0(und): Video: h264 (High 4:4:4 Predictive) (avc1 / 0x31637661), yuv444p, 1920x1080, 985 kb/s, 29.97 fps, 29.97 tbr, 30k tbn, 59.94 tbc (default)
    Metadata:
      handler_name    : VideoHandler
    Stream #0:1(und): Audio: aac (LC) (mp4a / 0x6134706D), 44100 Hz, mono, fltp, 86 kb/s (default)
    Metadata:
      handler_name    : Core Media Audio
Input #1, mov,mp4,m4a,3gp,3g2,mj2, from 'contrast.mp4':
  Metadata:
    major_brand     : isom
    minor_version   : 512
    compatible_brands: isomiso2avc1mp41
    encoder         : Lavf58.29.100
  Duration: 00:00:02.67, start: 0.000000, bitrate: 1071 kb/s
    Stream #1:0(und): Video: h264 (High 4:4:4 Predictive) (avc1 / 0x31637661), yuv444p, 1920x1080, 985 kb/s, 29.97 fps, 29.97 tbr, 30k tbn, 59.94 tbc (default)
    Metadata:
      handler_name    : VideoHandler
    Stream #1:1(und): Audio: aac (LC) (mp4a / 0x6134706D), 44100 Hz, mono, fltp, 86 kb/s (default)
    Metadata:
      handler_name    : Core Media Audio
File 'combined.mp4' already exists. Overwrite ? [y/N] y
Stream mapping:
  Stream #0:0 (h264) -&gt; blend:top (graph 0)
  Stream #1:0 (h264) -&gt; blend:bottom (graph 0)
  blend (graph 0) -&gt; Stream #0:0 (libx264)
  Stream #0:1 -&gt; #0:1 (aac (native) -&gt; aac (native))
Press [q] to stop, [?] for help
[libx264 @ 0x558fa4a84680] using cpu capabilities: MMX2 SSE2Fast SSSE3 SSE4.2 AVX FMA3 BMI2 AVX2
[libx264 @ 0x558fa4a84680] profile High 4:4:4 Predictive, level 4.0, 4:4:4, 8-bit
[libx264 @ 0x558fa4a84680] 264 - core 157 r2945 72db437 - H.264/MPEG-4 AVC codec - Copyleft 2003-2018 - http://www.videolan.org/x264.html - options: cabac=1 ref=3 deblock=1:0:0 analyse=0x1:0x111 me=hex subme=7 psy=1 psy_rd=1.00:0.00 mixed_ref=1 me_range=16 chroma_me=1 trellis=1 8x8dct=0 cqm=0 deadzone=21,11 fast_pskip=1 chroma_qp_offset=4 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 'combined.mp4':
  Metadata:
    major_brand     : isom
    minor_version   : 512
    compatible_brands: isomiso2avc1mp41
    encoder         : Lavf58.29.100
    Stream #0:0: Video: h264 (libx264) (avc1 / 0x31637661), yuv444p, 1920x1080, q=-1--1, 29.97 fps, 30k tbn, 29.97 tbc (default)
    Metadata:
      encoder         : Lavc58.54.100 libx264
    Side data:
      cpb: bitrate max/min/avg: 0/0/0 buffer size: 0 vbv_delay: -1
    Stream #0:1(und): Audio: aac (LC) (mp4a / 0x6134706D), 44100 Hz, mono, fltp, 69 kb/s (default)
    Metadata:
      handler_name    : Core Media Audio
      encoder         : Lavc58.54.100 aac
frame=   79 fps= 19 q=-1.0 Lsize=     329kB time=00:00:02.64 bitrate=1018.5kbits/s speed=0.639x    
video:297kB audio:28kB subtitle:0kB other streams:0kB global headers:0kB muxing overhead: 1.255968%
[libx264 @ 0x558fa4a84680] frame I:2     Avg QP:11.42  size:  9143
[libx264 @ 0x558fa4a84680] frame P:27    Avg QP:20.64  size:  4721
[libx264 @ 0x558fa4a84680] frame B:50    Avg QP:22.19  size:  3150
[libx264 @ 0x558fa4a84680] consecutive B-frames: 12.7%  7.6%  3.8% 75.9%
[libx264 @ 0x558fa4a84680] mb I  I16..4: 97.7%  0.0%  2.3%
[libx264 @ 0x558fa4a84680] mb P  I16..4:  9.0%  0.0%  1.0%  P16..4:  4.9%  1.1%  0.3%  0.0%  0.0%    skip:83.7%
[libx264 @ 0x558fa4a84680] mb B  I16..4:  2.1%  0.0%  0.2%  B16..8:  9.6%  1.2%  0.0%  direct: 0.6%  skip:86.2%  L0:52.2% L1:46.4% BI: 1.4%
[libx264 @ 0x558fa4a84680] coded y,u,v intra: 4.5% 0.3% 1.0% inter: 0.9% 0.1% 0.5%
[libx264 @ 0x558fa4a84680] i16 v,h,dc,p: 68% 18%  9%  4%
[libx264 @ 0x558fa4a84680] i4 v,h,dc,ddl,ddr,vr,hd,vl,hu: 32% 10% 28%  4%  7%  9%  4%  6%  1%
[libx264 @ 0x558fa4a84680] Weighted P-Frames: Y:0.0% UV:0.0%
[libx264 @ 0x558fa4a84680] ref P L0: 52.8%  6.5% 24.4% 16.3%
[libx264 @ 0x558fa4a84680] ref B L0: 70.8% 23.2%  6.0%
[libx264 @ 0x558fa4a84680] ref B L1: 88.5% 11.5%
[libx264 @ 0x558fa4a84680] kb/s:920.37
[aac @ 0x558fa4b1ea00] Qavg: 1424.024
</code></pre>
 27008: <p>I solved my problem thanks to @Gyan comment:</p>

<pre><code>ffmpeg -i bg.mp4 -i fg.mov -filter_complex "[0:v] format=rgba [bg]; [1:v] format=rgba [fg]; [bg][fg] blend=all_mode='multiply':all_opacity=1, format=rgba"  out.mp4

</code></pre>
