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• Gallery of VP8 Encoding Naivete

  15 octobre 2010, par Multimedia Mike — VP8

  I’ve been toiling away as a multimedia technology generalist for so long that it’s easy for me to forget that not everyone is as versed in the minutiae of the domain as I am. But I recently experienced what it’s like to be such an outsider when I posted about my toy VP8 encoder, expressing that it’s one of the hardest things I have ever tried to do. I heard of from number of people who do have extensive experience in video encoding, particularly with the H.264 and VP8 codecs. Their reactions were predictable : What’s so hard ? Look, you might be a little too immersed in the area to really understand a relative beginner’s perspective.

  And to all the people who suggested that I should get the encoder into FFmpeg ASAP : Are you crazy ?! Did you see what the first pass of the encoder produced ? Do you have lower standards than even I do ?

  
  
  

  Not Giving Up
  I worked a little more on the toy encoder. Remember that the above image is what I’m hoping to encode somewhat faithfully for this experiment. In my first pass, I attempted vertical prediction for all planes. For my next pass, I forced the chroma planes to mid-level (which results in a greyscale image) and played with the 16×16 luma prediction modes. When implementing an extremely naive algorithm to decide which 16×16 prediction mode would be the best for a particular block, this is what the program produced :

  
  
  

  For fun, here is what the image encodes to when forcing various prediction modes :
  

  I think the DC-only prediction mode actually looks a little better than the image that the naive algorithm produced :

  
  
  

  Vertical 16×16 prediction, similar to the image from the last post (just in black and white) :
  

  
  
  

  Horizontal 16×16 prediction :
  

  
  
  

  This is the 16×16 prediction mode unique to VP8, the TrueMotion mode (based on On2/Duck’s very first video codec) :
  

  
  
  

  Wow, these encodings really bring down the cheerful tone of the original image.

  Next Steps
  I have little reason to believe that I am encoding and subsequently reconstructing the image correctly (i.e., error is likely propagating through the entire encoding). If I have time, the next step is to validate my reconstruction against the encoder. Then I need to get the entropy considerations correct so that I actually get some compression out of this format.

• Convert FLV video with alpha channel to PNGs with transparency

  8 octobre 2012, par AZAR

  I have some FLV videos with alpha channels, and I want to convert each of them to PNG images using ffmpeg but keep the transparency.

  So far, I've tried this :

  ffmpeg -i input.flv -an -y %d.png

  But this outputs the PNG files with black background.

  Is there any way to do this ?

  Alternate acceptable solution : If I can output the images and give the alpha channel a certain color of my choice. I can then remove it later via imagemagick and convert that color to transparency.

• Which FFmpeg codec should be used for video streams with single byte pixel format ?

  2 décembre 2011, par Gearoid Murphy

  I've got a black and white video stream coming off a Firewire astronomy camera, I'd like to use FFmpeg to compress the video stream but it will not accept single byte pixel formats for the MPEG1VIDEO codecs. I've been trying random codecs for the last hour without much success, could anyone give me some sage advise on how to achieve my goal ? :) thx