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• Using Mencoder to Convert Sequence of Images Sorted Numerically to Video

  7 décembre 2017, par txizzle

  I want to use mencoder to convert a sequence of PNG files to video. The PNG files are named like file1.png, file2.png, ... file1000.png. However, if I run this :

  mencoder mf://*.png -mf fps=25 -ovc copy -oac copy output.avi

  The resulting video will not stitch together the frames in the correct order (correct : 1,2,3,.... incorrect : 1, 10, 100, ...). Answers to a similar question suggest that I can manually input the range of frames to have mencoder use the correct numerical order :

  mencoder mf://file{1..1000}.png -mf fps=25 -ovc copy -oac copy output.avi

  or to manually sort the files with something like ls -v and then save the results to a list (say, list.txt), and then pass that into mencoder :

  mencoder mf://@list.txt -mf fps=25 -ovc copy -oac copy output.avi

  However, my question is : Is there a way to do this in a single command without specifying the last frame ?

  Ideally, I would want an equivalent to mf://file*.png that uses a numerical sorting as opposed to the default sorting. For example, ffmpeg has the option to specify input files like file%d.png, which would order the files numerically.

  Thanks !

• How to install ffmpeg for a django app on heroku ?

  10 janvier 2018, par yndolok

  I’d like to use ffmpeg to extract a frame from a video to use it as a poster. This is my first time deploying an app, let alone on heroku, so I’m not sure how to install ffmpeg on the server.

  I’ve found this build of ffmpeg with instructions to ’vendor’ it into my app, and then adjust my app’s configuration settings / path. What does it mean to vendor something ? I have a feeling it’s a rails thing, because I can’t seem to find an explanation by googling.

  Also, what would be the django equivalent of the instructions to run
  heroku config:set PATH=bin:vendor/ffmpeg/bin:<...> -a yourapp and heroku config:set LD_LIBRARY_PATH=vendor/ffmpeg/lib:/usr/local/lib -a yourapp ?

• How to compute the number of extra samples added by LAME or FFMPEG

  24 janvier 2018, par actuallyaswin

  I am attempting to build a MP3 decoder / parser in Python which supports files encoded by LAME or FFMPEG.

  My encoding shell script is shown here :

  #!/bin/bash
  
  for i in wav/*.wav; do
  
      i=${i##*/};
  
      lame --nores --strictly-enforce-ISO -t --cbr -b 64 -h "wav/${i}" "mpeg/lame/${i%.wav}.mp3";
  
      ffmpeg -i "wav/${i}" -codec:a libmp3lame -qscale:a 2 "mpeg/ffmpeg/${i%.wav}.mp3";
  
  done

  This scripts reads WAVE files located in ./wav/ and produces a controlled-bitrate MP3 of 64kbps in my ./mp3/lame/ directory, and a variable-bitrate MP3 of quality 2 in my ./mp3/ffmpeg/.

  I have written a Python script that iterates through both resultant MP3s, counting the number of frames and samples. Both the LAME and FFMPEG results are equivalent (in terms of frames and samples), but their binary files are different.

  The LAME/FFMPEG sample count was done by iterating through the binary MP3 files, locating and parsing the frame header, then using the MP3 spec to determine the number of samples per frame.

  • Number of MP3 data-frames : 112 (ignoring the Xing/Info first frame)
  • Number of output frames : 112*576 = 64512

  Here is a comparison of the sample count for a single 4-second input file :

  • Input WAV # of samples = 62996
  • Output LAME/FFMPEG # of samples = 64512
  • Difference = 1516

  I understand that according to the LAME FAQ file, resultant MP3 files are zero padded in the front and back to make sure the inverse MDCT is performed properly, but also because the windows overlap.

  What I can’t ascertain from the above FAQ, or from any previous StackOverflow post, is how to compute the number of artificially added samples. If I can be sure that all 1516 of these samples are zeros, and I can be sure of their position in the bytestream, I’d like to be able to confidently toss them out. Since there are 1516 "extra" samples and there are 576 samples per frame for a V2LIII encoding, then there must be more than two (but less than three) erroneous MPEG frames here.

  Is anyone here savvy enough with MPEG encoding/decoding to know how many samples are added, and in which frames those samples will be in ? In other words, will the first frame and last frame always contain blank data, or are there more frames ?