[FFmpeg-user] Thank you, Paul - Decimation with ppsrc=1 malfunctions

[Mark Filipak (ffmpeg)]

On 12/28/2020 03:01 PM, Mark Filipak (ffmpeg) wrote:
-snip-> Thank you, Paul. I still see one small problem (which may be in the source) which I'd like to
> pursue, but the 2 major faults are now gone. Congratulations!

ffmpeg -report -i preprocessed.mkv -i source.mkv -filter_complex "decimate='ppsrc=1':dupthresh=1.0" 
"target dupthresh=1.0.mkv"
ffmpeg -report -i preprocessed.mkv -i source.mkv -filter_complex "decimate='ppsrc=1':dupthresh=1.5" 
"target dupthresh=1.5.mkv"

I performed a careful review, below, using the latest build. Note that decimate=ppsrc=1 removes the 
2nd of the repeat-pair except for the first instance (frames 938 & 939). In the first instance, the 
the 1st of the repeat-pair is removed. That bothers me a bit. I anticipated that the first frame of 
'target dupthresh=1.0.mkv' & 'target dupthresh=1.5.mkv' would be frame 938. That it's 939 makes me 
wonder about ppsrc's initialization.

  'source.mkv' frame numbers
   |          'target dupthresh=1.0.mkv' frame numbers
   |           |    'target dupthresh=1.5.mkv' frame numbers
   |           |     |
  938
  939 repeat  939   939
  940         940   940
  941 combed  941   941
  942 combed  942   942
  943         943   943
  944 repeat
  945         945   945
  946 combed  946   946
  947 combed  947   947
  948         948   948
  949 repeat
  950         950   950
  951 combed  951   951
  952 combed  952   952
...etc.

How do I know it was frame 938 that was removed? Because the frame numbers are burned into 'source.mkv'.