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• Why are Cb and Cr planes displaced differently from lum by the displace complex filter in ffmpeg ?

  6 mai 2017, par Neb

  I have a video encoded with the yuv420p pixel format and I want to displace its pixels. I’m using ffmpeg and its new displace filter. The filter takes as inputs (the video to be displaced and) two displacement maps respectively for X and Y axis. I decided to create the displacement maps directly into ffmpeg using the nullsrc video source filter and the geq filter to specify the value of the three planes : lum, Cb, Cr. The script is the following :

  ffmpeg INPUT.mp4 -f lavfi -i nullsrc=size=${WIDTH}x${HEIGHT}:d=0.1,geq='lum=128+30*sin(2*PI*X/400):Cb=128+30*sin(2*PI*X/400):Cr='128+30*sin(2*PI*X/400)' -f lavfi -i nullsrc=size=${WIDTH}x${HEIGHT}:d=0.1,geq='lum=128+30*sin(2*PI*X/400):Cb=128+30*sin(2*PI*X/400):Cr=128+30*sin(2*PI*X/400)' -lavfi '[0][1][2]displace' OUTPUT.mp4

  I used the example provided in the documentation of ffmpeg, since the expression used in geq is irrelevant for the purposes of the problem.

  At the and of the computation, I get the pixels of the input video not properly displaced, meaning that I can clearly see a sort of ghost carrying-color-information video under a displaced but b/w one.
  After some tests, I noticed that the displacemnt map created had only the luma plane displaced correctly while the chrominance planes were displaced, but differently from luma, which is the origin of the planes disalignment in the intput video as you can see in the following extract frames :

  Planes separated

  I also noticed that the video describing the Cb and Cr planes of the displacement maps have half resolution of the luma plane.

  My question is : how can i setup correctly the Cr and Cb planes in the geq definition so that they are exactly identical to the luma plane ?

  It would be also great if someone could explain me why ffmpeg gives me an output so much different for luma and Cb, Cr planes even if the function provided is the same.

  If, it can help, i’m using ffmpeg 3.3-static build.

  Thanks for your time.

• Merge commit '4de220d2e3751c459f8739a08ac6ca52e63eba30'

  28 septembre 2017, par James Almer

  Merge commit '4de220d2e3751c459f8739a08ac6ca52e63eba30'
  * commit '4de220d2e3751c459f8739a08ac6ca52e63eba30' :
  
    frame : allow align=0 (meaning automatic) for av_frame_get_buffer()
  See https://ffmpeg.org/pipermail/ffmpeg-devel/2017-September/215834.html
  Merged-by : James Almer <jamrial@gmail.com>
  

  • [DH] doc/APIchanges
  • [DH] libavutil/frame.c
  • [DH] libavutil/frame.h
  • [DH] libavutil/version.h

• checkasm : Remove unnecessary const on scalar parameters

  15 décembre 2023, par Martin Storsjö

  checkasm : Remove unnecessary const on scalar parameters
  The ffmpeg coding style doesn't usually use const on scalar
  
  parameters (or on the pointer values - as opposed to the type
  
  that is pointed to, where it has a semantic meaning), contrary
  
  to the dav1d coding style (where this was imported from).
  This avoids warnings about differences in the type signatures
  
  between declaration and definition of this function, with older
  
  versions of MSVC.
  The issue was observed with one version of MSVC 2017,
  
  19.16.27024.1, with warnings like these :
      src/tests/checkasm/checkasm.c(969) : warning C4028 : formal parameter 3 different from declaration
  The warning itself is bogus as the const here is harmless, and
  
  newer versions of MSVC no longer warn about this.
  Signed-off-by : Martin Storsjö <martin@martin.st>
  

  • [DH] tests/checkasm/checkasm.c
  • [DH] tests/checkasm/checkasm.h