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• Using FFmpeg to detect alpha channel in PR4444 file [closed]

  8 décembre 2023, par Justin Myers

  I wanted to comment on a thread that already exists on this topic in general, but I just signed up to SO and have not earned my contribution credits yet ! If anyone has advice on how to better handle that in the future, I am all ears !!

  


  None-the-less, here is that thread :

  


  A good way to detect alpha channel in a video using ffmpeg/ffprobe

  


  I tried both the answers listed in that post to detect the presence of an alpha channel in a PR4444 video file.

  


  For Gyan's answer, I get the following pixel format from any PR4444 file (regardless if encoded with alpha or no alpha), when running part 1 :

  


  yuva444p12le

  


  As you will see in Gyan's notes, even if a FFprobe returns an "a" in the return string (for the call for pixel format), that does not equate to the presence of an alpha channel. Hence their note for the subsequent (part 2) call...

  


  I then plugged that into the part 2 of Gyan's answer, but that produced an empty from FFmpeg, and a message providing valid arguments to pair with grep. It seems as though -oP is not a valid arg for grep ? Because it wasn't listed in the returned list of options. This is the template I used (pulled verbatim from Gyan's posted solution) :

  


  ffprobe -v 0 -show_entries pixel_format=name:flags=alpha -of compact=p=0 | grep "$PIX_FMT|" | grep -oP "(?<=alpha=)\d"

  &#xA;

  Where $PIX_FMT is to be replaced with yuva444p121e (per Gyan's instructions).

  &#xA;

  I then tried Benji's solution for part 2, but that just returns the following :

  &#xA;

  pix_fmt=yuva444p12le

  &#xA;

  Of course, this isn't any more useful than part 1...

  &#xA;

  I suspect Gyan is on the right track as he mentions part 2 should produce a boolean result for the presence of an alpha channel. However, either I am misunderstanding the syntax of his template, or something has changed in FFprobe since. There is so limited information out there for this specific task. Hoping someone with more experience and knowledge can help shed some light for me ??

  &#xA;

• Frames per second Conceptual Details

  25 février 2014, par Whoami

  i am a newbie and trying to understand the concepts behind ffmpeg/video.

  FPS means frame per second , for example

  25fps -> 25 frames captured in a second

  From the display perpective

  25 frames need to be displayed in a second.

  correct me if i am wrong ?

  Now i have written a simple video player in ffmpeg to display. i read
  frames by av_read_frame(), if fps is 25, then does

   av_read_frame()

  returns 25 frames per second ? how can i relate it ?

• exec function not working as expected

  6 mars 2016, par Albert

  I am running a shell function from php to convert a video file to flash once it is uploaded. The exec function is like this :

  exec("ffmpeg -i " . WWW_ROOT . "files" . DS . "videos" . DS . $id . $ext . " -ar 22050 -r 32 -f flv -s 640x480 " . WWW_ROOT . "files" . DS . "videos" . DS . $id . ".flv");
  
  
  
  // if I echo the string it returns:
  
  // ffmpeg -i /home/vceroot/public_html/example.com/test/intraweb/app/webroot/files/videos/516c1203-0484-417a-b047-5488c40293e9.mpg -ar 22050 -r 32 -f flv -s 640x480 /home/vceroot/public_html/example.com/test/intraweb/app/webroot/files/videos/516c1203-0484-417a-b047-5488c40293e9.flv
  
  // the locations are correct

  However, it does not convert the video.

  After it uploaded the file, it just goes back to the index action. There is no converted file although the uploaded file is there.

  My permissions on the folder is 755. And the folder owner is apache. I don’t know what else to do. Can anyone assist me ?

  UPDATE

  So I’ve changed my exec query a bit and echoed the output :

  exec("ffmpeg -i " . WWW_ROOT . "files" . DS . "videos" . DS . $id . $ext . " -ar 22050 -r 32 -f flv -s 640x480 " . WWW_ROOT . "files" . DS . "videos" . DS . $id . ".flv 2>&1", $output, $return);
  
  die(print_r($output));

  This returned the following error :

  Array ( [0] => sh: ffmpeg: command not found ) 1

  So I changed it a bit to this :

  exec("/usr/local/bin/ffmpeg -i " . WWW_ROOT . "files" . DS . "videos" . DS . $id . $ext . " -ar 22050 -r 32 -f flv -s 640x480 " . WWW_ROOT . "files" . DS . "videos" . DS . $id . ".flv 2>&1", $output, $return);
  
  die(print_r($output));

  And now it returns :

  Array ( [0] => /usr/local/bin/ffmpeg: error while loading shared libraries: libavdevice.so.55: cannot open shared object file: No such file or directory ) 1

  Where do I go from here ?