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• Saving frames in different formats yields different MSE

  8 septembre 2022, par Mattia

  I am trying to find the frame in a video that is the most different from its previous and next one. Selecting this frame would give me the best chance of synchronizing two videos coming from different sources.
I am using the psnr filter between the target video and its delayed version, i.e., with ss=1/fps.

  


  Everything seems to work, but something strange happens when I am trying to see which frame (and surrounding frames) was chosen.

  


  I extract the frames as follows :

  


  ffmpeg -ss  -i  -frames 1 tmp/.png -y


  


  Doing this for the 2 frames before and the 2 frames after the selected frame, I get mse_avg and psnr_avg from the psnr filter as follows :

  


  mse_avg  psnr_avg          comment
2099.68     14.91     prev2 - prev
3365.92     12.86  prev - keyframe
2716.40     13.79  keyframe - next
2581.58     14.01     next - next2


  


  Instead, when dealing with the video, as well as when extracting single frames in .mp4 format I get :

  


  mse_avg  psnr_avg          comment
 942.57     18.39     prev2 - prev
1611.20     16.06  prev - keyframe
1348.51     16.83  keyframe - next
1208.34     17.31     next - next2


  


  I thought that saving frames in lossless image format would preserve their visual characteristics, thus providing the same performance.
Why does changing the frame output to PNG format yield different MSE ?

  


• avformat/utils : return pending IO error on EOF in av_read_frame()

  24 août 2019, par Marton Balint

  avformat/utils : return pending IO error on EOF in av_read_frame()
  avio_feof() returns true both in case of actual EOF and in case of IO errors.
  
  Some demuxers (matroska) have special handling to be able to return the proper
  
  error for this exact reason, e.g. :
  if (avio_feof(pb)) 
  
       if (pb->error) 
  
           return pb->error ;
  
        else 
  
           return AVERROR_EOF ;
  
       
  
  
  However, most of the demuxers do not, and they simply return AVERROR_EOF if
  
  avio_feof() is true, so there is a real chance that IO errors are mistaken for
  
  EOF.
  We might just say that the API user should always check the IO context error
  
  attribute on EOF to make sure no IO errors happened, but not even ffmpeg.c does
  
  this. It should be more intuitive to the API user if we simply return the IO
  
  error as the return value of av_read_frame() instead of AVERROR_EOF.
  Signed-off-by : Marton Balint <cus@passwd.hu>
  

  • [DH] libavformat/utils.c

• avcodec/h264_mp4toannexb_bsf : fix missing PS before IDR frames

  19 mai 2023, par Zhao Zhili

  avcodec/h264_mp4toannexb_bsf : fix missing PS before IDR frames
  If there is a single group of SPS/PPS before an IDR frame, but no
  
  SPS/PPS after that, we will miss the chance to reset
  
  idr_sps_seen/idr_pps_seen. No SPS/PPS are inserted afterwards.
  This patch saves in-band SPS/PPS and insert them before IDR frames
  
  when necessary.
  Signed-off-by : Zhao Zhili <zhilizhao@tencent.com>
  

  • [DH] libavcodec/h264_mp4toannexb_bsf.c
  • [DH] tests/fate/h264.mak