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• ffmpeg output seeking not accurate even after -i [closed]

  3 septembre 2023, par hl037_

  In the documentation of ffmpeg, it is stated that if -ss is passed after -i, then it is frame accurate (Other questions are for -ss before -i).

  


  Here is my command :

  


  ffmpeg -i vid.mkv -ss 53 -codec copy test.mkv -y 


  


  However, it is still not accurate, and seems to jump to the nearest keyframe. Am i missing something ?

  


  Note : the problem only occurs when the video steam is copied. If reencoded or discarded, it works as expected

  


  Note2 (As I am doing other tries) : There are also strange things with audio :

  


  ffmpeg -i vid.mkv -ss 53 -codec copy test1.mkv -y 
ffmpeg -i vid.mkv -ss 54 -codec copy test2.mkv -y 
ffmpeg -i test1.mkv -vn -c:a copy test1a.mkv -y
ffmpeg -i test2.mkv -vn -c:a copy test2a.mkv -y


  


  

  • test1.mkv is reported to be the right lenght, but the first frame correspond to the frame at 0:50 in original (end is good)

  


  • test2.mkv is reported to be the right lenght, but the first frame correspond to the frame at 0:55 in original (end is good)

  


  • test1a.mkv is the expected audio (starts at 0:53 of the original)

  


  • test2a.mkv is the expected audio (starts at 0:54 of the original)

  


  


• Tour of Part of the VP8 Process

  18 novembre 2010, par Multimedia Mike — VP8

  My toy VP8 encoder outputs a lot of textual data to illustrate exactly what it’s doing. For those who may not be exactly clear on how this or related algorithms operate, this may prove illuminating.

  Let’s look at subblock 0 of macroblock 0 of a luma plane :

   subblock 0 (original)
    92  91  89  86
    91  90  88  86
    89  89  89  88
    89  87  88  93
  

  Since it’s in the top-left corner of the image to be encoded, the phantom samples above and to the left are implicitly 128 for the purpose of intra prediction (in the VP8 algorithm).

   subblock 0 (original)
       128 128 128 128
   128  92  91  89  86
   128  91  90  88  86
   128  89  89  89  88
   128  89  87  88  93
  

  
  Using the 4×4 DC prediction mode means averaging the 4 top predictors and 4 left predictors. So, the predictor is 128. Subtract this from each element of the subblock :

   subblock 0, predictor removed
   -36 -37 -39 -42
   -37 -38 -40 -42
   -39 -39 -39 -40
   -39 -41 -40 -35
  

  Next, run the subblock through the forward transform :

   subblock 0, transformed
   -312   7   1   0
      1  12  -5   2
      2  -3   3  -1
      1   0  -2   1
  

  Quantize (integer divide) each element ; the DC (first element) and AC (rest of the elements) quantizers are both 4 :

   subblock 0, quantized
   -78   1   0   0
     0   3  -1   0
     0   0   0   0
     0   0   0   0
  

  The above block contains the coefficients that are actually transmitted (zigzagged and entropy-encoded) through the bitstream and decoded on the other end.

  The decoding process looks something like this– after the same coefficients are decoded and rearranged, they are dequantized (multiplied) by the original quantizers :

   subblock 0, dequantized
   -312   4   0   0
      0  12  -4   0
      0   0   0   0
      0   0   0   0
  

  Note that these coefficients are not exactly the same as the original, pre-quantized coefficients. This is a large part of where the “lossy” in “lossy video compression” comes from.

  Next, the decoder generates a base predictor subblock. In this case, it’s all 128 (DC prediction for top-left subblock) :

   subblock 0, predictor
    128 128 128 128
    128 128 128 128
    128 128 128 128
    128 128 128 128
  

  Finally, the dequantized coefficients are shoved through the inverse transform and added to the base predictor block :

   subblock 0, reconstructed
    91  91  89  85
    90  90  89  87
    89  88  89  90
    88  88  89  92
  

  Again, not exactly the same as the original block, but an incredible facsimile thereof.

  Note that this decoding-after-encoding demonstration is not merely pedagogical– the encoder has to decode the subblock because the encoding of successive subblocks may depend on this subblock. The encoder can’t rely on the original representation of the subblock because the decoder won’t have that– it will have the reconstructed block.

  For example, here’s the next subblock :

   subblock 1 (original)
    84  84  87  90
    85  85  86  93
    86  83  83  89
    91  85  84  87
  

  Let’s assume DC prediction once more. The 4 top predictors are still all 128 since this subblock lies along the top row. However, the 4 left predictors are the right edge of the subblock reconstructed in the previous example :

   subblock 1 (original)
      128 128 128 128
   85  84  84  87  90
   87  85  85  86  93
   90  86  83  83  89
   92  91  85  84  87
  

  The DC predictor is computed as (128 + 128 + 128 + 128 + 85 + 87 + 90 + 92 + 4) / 8 = 108 (the extra +4 is for rounding considerations). (Note that in this case, using the original subblock’s right edge would also have resulted in 108, but that’s beside the point.)

  Continuing through the same process as in subblock 0 :

   subblock 1, predictor removed
   -24 -24 -21 -18
   -23 -23 -22 -15
   -22 -25 -25 -19
   -17 -23 -24 -21
   subblock 1, transformed
  
   -173  -9  14  -1
  
      2 -11  -4   0
  
      1   6  -2   3
  
     -5   1   0   1
   subblock 1, quantized
  
   -43  -2   3   0
  
     0  -2  -1   0
  
     0   1   0   0
  
    -1   0   0   0
   subblock 1, dequantized
  
   -172  -8  12   0
  
      0  -8  -4   0
  
      0   4   0   0
  
     -4   0   0   0
   subblock 1, predictor
  
    108 108 108 108
  
    108 108 108 108
  
    108 108 108 108
  
    108 108 108 108
   subblock 1, reconstructed
  
    84  84  87  89
  
    86  85  87  91
  
    86  83  84  89
  
    90  85  84  88
  

  I hope this concrete example (straight from a working codec) clarifies this part of the VP8 process.

• Replace part of a video (without replacing the audio) in ffmpeg [closed]

  13 février 2021, par Real Noob

  I have a video file that is divided into three different sections. I want to replace the middle part of this video with another clip. However, I still want to keep the audio of the original video. Can I do that in ffmpeg ?

  


  Here are my requirements :

  


  

  1. Original Video is 60 seconds long.

  


  2. The part I want to replace is from 25 to 45 seconds.

  


  3. I want to keep the original audio from 25 to 45 seconds and just replace the visual part with some other clip.

  


  4. The generated video will also be 60 seconds long. However, it will have the new video from 25 to 45 seconds.

  


  


  Thanks.