title: ffmpeg chromakey filter: blend parameter does not what it should
body: <p>I want to remove greenscreen background and am not completely satisfied with what I have achieved because I still have green borders (especially on semi-transparent areas like hair, when I move my head).</p>
  
  <p>The documentation for the blend-parameter of chromakey filter says </p>
  
  <blockquote>
    <p>blend</p>
    
    <p>Blend percentage.</p>
    
    <p>0.0 makes pixels either fully transparent, or not transparent at all.</p>
    
    <p>Higher values result in semi-transparent pixels, with a higher transparency the more similar the pixels color is to the key color.</p>
  </blockquote>
  
  <p>So I reckoned, I could use this to minimise greenscreen bleeding (is this the term?) when removing the background with a command like this:</p>
  
  <pre><code>ffmpeg -i DSCN0015.MOV -vf "[in] hqdn3d=4:4:8:8 [dn]; [dn] scale=iw*3:-1 [sc]; [sc] chromakey=0x005d0b:0.125:0.0 [out]" -r 24 -an -c:v ffvhuff 4.mov
  </code></pre>
  
  <p>But when I use anything else than 0.0 for blend, it seems to determine some kind of MINIMUM transparency and the entire frame is affected.</p>
  
  <p>Here are some pics to visualize: the first is the raw material from the camera. the 2nd shows what I get with blend=0.0 and the last one shows the problem: blend=0.5, but the whole frame is almost completely transparent.</p>
  
  <p>[<img src="https://i.stack.imgur.com/K0MJy.png" alt="Raw material as from camera]"></p>
  
  <p>[<img src="https://i.stack.imgur.com/00XBf.png" alt="blend=0.0]"></p>
  
  <p>[<img src="https://i.stack.imgur.com/PoLl2.png" alt="blend=0.5]"></p>