title: using ffmpeg to add overlay with opacity
body: <p>I have a problem with ffmpeg,I try to add a png files over a video i found how to add just I want this png file have some opacity 
  I tryed this line</p>
  
  <pre><code>ffmpeg -n -i video.mp4 -i logo.png -filter_complex "setsar=sar=1;blend=all_mode='overlay':all_opacity=0.7" -movflags +faststart tmb/video.mp4
  </code></pre>
  
  <p>with this line ffmpeg return this error</p>
  
  <pre><code>Cannot find a matching stream for unlabeled input pad 1 on filter Parsed_blend_1
  </code></pre>
  
  <p>and there the complet log</p>
  
  <pre><code>ffmpeg version 2.7.5-0ubuntu0.15.10.1 Copyright (c) 2000-2016 the FFmpeg developers
  built with gcc 5.2.1 (Ubuntu 5.2.1-22ubuntu2) 20151010
    configuration: --prefix=/usr --extra-version=0ubuntu0.15.10.1 --build-suffix=-ffmpeg --toolchain=hardened --libdir=/usr/lib/x86_64-linux-gnu --incdir=/usr/include/x86_64-linux-gnu --enable-gpl --enable-shared --disable-stripping --enable-avresample --enable-avisynth --enable-frei0r --enable-gnutls --enable-ladspa --enable-libass --enable-libbluray --enable-libbs2b --enable-libcaca --enable-libcdio --enable-libflite --enable-libfontconfig --enable-libfreetype --enable-libfribidi --enable-libgme --enable-libgsm --enable-libmodplug --enable-libmp3lame --enable-libopenjpeg --enable-openal --enable-libopus --enable-libpulse --enable-librtmp --enable-libschroedinger --enable-libshine --enable-libspeex --enable-libtheora --enable-libtwolame --enable-libvorbis --enable-libvpx --enable-libwavpack --enable-libwebp --enable-libxvid --enable-libzvbi --enable-opengl --enable-x11grab --enable-libdc1394 --enable-libiec61883 --enable-libzmq --enable-libssh --enable-libsoxr --enable-libx264 --enable-libopencv --enable-libx265
    libavutil      54. 27.100 / 54. 27.100
    libavcodec     56. 41.100 / 56. 41.100
    libavformat    56. 36.100 / 56. 36.100
    libavdevice    56.  4.100 / 56.  4.100
    libavfilter     5. 16.101 /  5. 16.101
    libavresample   2.  1.  0 /  2.  1.  0
    libswscale      3.  1.101 /  3.  1.101
    libswresample   1.  2.100 /  1.  2.100
    libpostproc    53.  3.100 / 53.  3.100
  Input #0, mov,mp4,m4a,3gp,3g2,mj2, from 'video.mp4':
    Metadata:
      major_brand     : isom
      minor_version   : 512
      compatible_brands: isomiso2avc1mp41
      encoder         : Lavf54.59.106
    Duration: 00:06:17.68, start: 0.023220, bitrate: 1660 kb/s
      Stream #0:0(und): Video: h264 (High) (avc1 / 0x31637661), yuv420p, 1080x720 [SAR 8:9 DAR 4:3], 1527 kb/s, 25 fps, 25 tbr, 12800 tbn, 50 tbc (default)
      Metadata:
        handler_name    : VideoHandler
      Stream #0:1(und): Audio: aac (LC) (mp4a / 0x6134706D), 44100 Hz, stereo, fltp, 127 kb/s (default)
      Metadata:
        handler_name    : SoundHandler
  Input #1, png_pipe, from 'logo.png':
    Duration: N/A, bitrate: N/A
      Stream #1:0: Video: png, rgba(pc), 1080x720 [SAR 3780:3780 DAR 3:2], 25 tbr, 25 tbn, 25 tbc
  Cannot find a matching stream for unlabeled input pad 1 on filter Parsed_blend_1
  </code></pre>
  
  <p>I'm a little noob with ffmpeg and I tryed to find a solution with google and don't find.</p>
  
  <p>Without setsar=sar=1 I got the error about two different sar</p>
  
  <pre><code>[Parsed_blend_0 @ 0x14bb0e0] First input link top parameters (size 1080x720, SAR 8:9) do not match the corresponding second input link bottom parameters (1080x720, SAR 1:1)
  [Parsed_blend_0 @ 0x14bb0e0] Failed to configure output pad on Parsed_blend_0
  </code></pre>
  
  <p>Thanks for your help</p>