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• How do video encoding standards(like h.264) then serialize motion prediction ?

  12 août 2019, par Nephilim

  Motion prediction brute force algorithms, in a nutshell work like this(if I'm not mistaken) :

  



  

  1. Search every possible macroblock in the search window

  


  2. Compare each of them with the reference macroblock

  


  3. Take the one that is the most similar and encode the DIFFERENCE between the frames instead of the actual frame.

  


  



  Now this in theory makes sense to me. But when it gets to the actual serializing I'm lost. We've found the most similar block. We know where it is, and from that we can calculate the distance vector of it. Let's say it's about 64 pixels to the right.

  



  Basically, when serializing this block, we do :

  



  

  • Ignore everything but luminosity(encode only Y, i think i saw this somewhere ?), take note of the difference between it and the reference block

  


  • Encode the motion, a distance vector

  


  • Encode the MSE, so we can reconstruct it

  


  



  Is the output of this a simple 2D array of luminosity values, with an appended/prepended MSE value and distance vector ? Where is the compression in this ? We got to take out the UV component ? There seem to be many resources that take on the surface level of video encoders, but it's very hard to find actual in-depth explanations of modern video encoders. Feel free to correct me on my above statements.

  


• avcodec/pngdec : support decoding sRGB chunks

  17 janvier 2023, par Leo Izen

  avcodec/pngdec : support decoding sRGB chunks
  If an sRGB chunk is present in the PNG file, this commit will cause the
  
  png decoder to ignore the cHRM and gAMA chunks and tag the resulting AVFrames
  
  with BT.709 primaries, and ISO/IEC 61966-2-1 transfer. If these tags are
  
  present in the AVFrame, pngenc.c already writes this chunk, so no change was
  
  needed on the encode-side.
  The PNG spec does not define what happens if sRGB and iCCP are present at
  
  the same time, it just recommends that this not happen. As of this patch,
  
  the decoder will have the ICC profile take precedence, and it will not tag
  
  the pixel data as sRGB.
  Signed-off-by : Leo Izen <leo.izen@gmail.com>
  

  • [DH] libavcodec/pngdec.c

• quickdraw : Switch to greedy parsing

  18 avril 2015, par Vittorio Giovara

  quickdraw : Switch to greedy parsing
  Quickdraw packs data as a series of codes that the application is supposed
  
  to handle, but it does not define any order in which they might appear.
  
  Since it’s unfeasible to support *all* opcodes defined by the spec,
  
  only handle well-known blocks containing video data and ignore any unknown
  
  or unsupported ones.
  Move palette loading and rle decoding to separate functions to resue them
  
  in other blocks and drop format initialization in init since it can
  
  support more formats than pal8.
  Validate width and height.
  

  • [DBH] libavcodec/qdrw.c