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• Revision 423e8a9727 : Fix allocation of context buffers on frame resize The patch : https://gerrit.chr

  24 juillet 2014, par Adrian Grange

  Changed Paths :
   Modify /vp9/decoder/vp9_decodeframe.c

  
  
  Fix allocation of context buffers on frame resize

  The patch :
  https://gerrit.chromium.org/gerrit/#/c/70814/
  changed the test that determined whether the context
  frame buffers needed to be reallocated or not.

  The code checked for a change in total frame area
  to signal the need to reallocate context buffers.
  However, the above_context buffer needs to be
  resized i:xf only the width of the frame has increased.

  Change-Id : Ib89d75651af252908144cf662578d84f16cf30e6

• FFMPEG : Videos converted from FLV to MP4 does not play in iPod but works in iPhone

  3 juin 2012, par Shakti Singh

  I used below command to convert videos from FLV,M4V to MP4.

  ffmpeg -y -i video_1336406262.flv -vcodec libx264 -vpre slow -vpre
  ipod640 -b 250k -bt 50k -acodec libfaac -ac 2 -ar 48000 -ab 64k -s
  480x320 video_1336406262.mp4

  The videos converted from M4V to MP4 are playing very well in both iPhone and iPod but the videos converted from FLV to MP4 does not work in iPod but does in iPhone.

  In the video area of HTML5 page iPod even does not show the play symbol.

  Could someone help here ?

  I am using the same command to convert from both FLV and M4V to MP4.

  Thanks

• Why is ffmpeg's conversion to YUV420 so poor ?

  8 novembre 2020, par Hugues

  I have been using ffmpeg and other compression tools to compare rate-distortion curves for YUV420-resampled video.
In these comparisons, results from ffmpeg are consistently worse, with PSNR values that are 0.5-1.0 dB lower.

  


  I tracked the problem to ffmpeg's conversion between RGB and YUV420.
To simplify, let us assume "lossless compression" and therefore consider only RGB -> YUV420 -> RGB.
Also, we operate on a single PNG image frame.

  


  # Use some default options.
ffmpeg="ffmpeg -nostdin -hide_banner -v error"

# Obtain a source image.
wget -nv -O original.png https://i.stack.imgur.com/8J1qY.png
size="256x256"

# Compare it with itself to verify that we get an infinite average PSNR.
$ffmpeg -v info -i original.png -i original.png -lavfi psnr -f null - |& grep PSNR
# average:inf

# Convert the image to YUV420, and convert back to RGB.
$ffmpeg -i original.png -pix_fmt yuv420p -f rawvideo -y temp1.yuv420
$ffmpeg -f rawvideo -s $size -pix_fmt yuv420p -i temp1.yuv420 -y result1.png

# Compare it with the original image to measure the PSNR (in dB).
$ffmpeg -v info -i result1.png -i original.png -lavfi psnr -f null - |& grep PSNR
# average:36.894551


  


  Now, as an alternative, we perform the RGB <-> YUV420 chroma resampling manually :

  &#xA;

  yuv444_to_yuv420="extractplanes=y+u+v[y][u][v];\
  [u]scale=w=iw/2:h=ih/2:flags=area[u];\
  [v]scale=w=iw/2:h=ih/2:flags=area[v];\
  [y][u][v]mergeplanes=0x001020:yuv420p"
yuv420_to_rgb="extractplanes=y+u+v[y][u][v];\
  [u]scale=w=iw*2:h=ih*2:flags=neighbor[u];\
  [v]scale=w=iw*2:h=ih*2:flags=neighbor[v];\
  [y][u][v]mergeplanes=0x001020:yuv444p,format=rgb24"

$ffmpeg -i original.png -pix_fmt yuv444p -f rawvideo - | \
  $ffmpeg -f rawvideo -pix_fmt yuv444p -s $size -i - \
    -lavfi "$yuv444_to_yuv420" -f rawvideo -y temp2.yuv420
$ffmpeg -f rawvideo -pix_fmt yuv420p -s $size -i temp2.yuv420 \
  -lavfi "$yuv420_to_rgb" -y result2.png

# Measure PSNR by comparing with the original image.
$ffmpeg -v info -i result2.png -i original.png -lavfi psnr -f null - |& grep PSNR
# average:37.536444
# This is an improvement of 0.64 dB!


  &#xA;

  This brings up two questions :

  &#xA;

  &#xA;
  1. Why doesn't ffmpeg implement a better conversion to/from yuv420p by default ?

  &#xA;

  2. Is there any simpler way to obtain or express this improved conversion ?

  &#xA;

  &#xA;