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• More Weird VP8 Encodings

  10 décembre 2010, par Multimedia Mike — VP8

  When I announced that I had transitioned my VP8 encoder’s status from "toy" to "working", Jim L. lamented the loss of humorous posts about oddly encoded images output from my encoder. Not so ! There are still plenty of features that I have yet to implement, each of which carries the possibility of bizarre images.

  For example, I dusted off my work-in-progress intra 4x4 encoding, fixed a few of the more obvious bugs, and told the encoder to encode the first block in 4x4 mode and the rest in the usual, working, debugged 16x16 mode. The results of the first pass surprised me :

  
  
  

  The reason this surprised me was that I intuitively expected one of 2 outcomes :

  • Perfect image right away since everything is correct (very unlikely but not outside the realm of possibility)
  • Total garbage with, at most, the first macroblock looking somewhat legible ; this would be due to having some of the first macroblock correct but completely desynchronizing the bitstream for the purpose of decoding the rest of the coefficients.

  I absolutely did not expect the first macroblock to look messed up but for the rest of the picture to look fine. For fun, I reversed the logic and encoded the first block as 16x16 and the rest with the experimental 4x4 mode :

  
  
  

  If you examine carefully, you will see that the color planes are correct (though faint). There just isn’t much going on in the luma plane. This made sense when I noticed the encoder was encoding a blank (undefined, actually) set of luma coefficients for 4x4 mode macroblocks due to a bug. This helps to rationalize the first image as well— the first macroblock was encoding nonsense for the first macroblock which messed up the macroblocks which immediately surrounded it. Eventually, macroblock decoding got back on track when the prediction modes weren’t relying on the errantly decoded macroblocks.

  After I fixed that bug, I let the 4x4 mode rip through the whole image. That’s when I got what I am terming the "dark and gritty reboot of Big Buck Bunny" :

  
  
  

  Fortunately, this also turned out to be traceable to a pretty obvious code bug.

  One day, this VP8 encoder might do the right thing while implementing all of the algorithm’s features. In the meantime, it’s at least entertaining to watch it make mistakes.

• issues with gas-preprocessor and ffmpeg4iphone

  4 janvier 2012, par Shoeb Amin

  I want to install gas pre-processor on my mac from
  gas pre-processor

  it is told that i have to install in usr/local/bin, But I didnt find my local folder in usr thats why I created a local/bin under usr then tried to install it from terminal...

  but It didnt work.. it says that "Unrecognized input filetype at gas-preprocessor.pl line 33."

  BTW my ultimate goal is to build ffmpeg4iphone in my xcode 4.2 and iOS5 sdk from ffmpeg4iphone

• Revision 5563874783 : Backports from the trunk : flvmeta.c (inject_metadata) : Output file is only opene

  11 octobre 2007, par Marc Noirot

  Changed Paths :
   Modify /ChangeLog

  
   Modify /amf.c

  
   Modify /flvmeta.c

  
   Modify /types.c

  
   Modify /types.h

  
  
  Backports from the trunk :
  flvmeta.c (inject_metadata) : Output file is only opened after input file reading
  succeeded.
  types.h, types.c : Transformed most conversion functions into macros, to improve
  execution speed.
  amf.c (amf_data_decode, amf_data_encode) : Fixed missing break statements.
  amf.c (amf_string_encode) : Forgot to count the string length prefix size.