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• blank transparent pngs behave like black canvas

  10 novembre 2015, par Anay Bose

  I am trying to create a scrolling image with imagemagick’s roll and crop functions. Its a frame-by—frame animation. The source image is a transparent png with some text written on it. The following code works and it creates the required frames, including some blank png images at first for a nice, smooth effect. These blank pngs are what creating problems when I am trying to convert my image frames into video with ffmpeg. FFMPEG is seemingly considering these blank pngs as black/dark images, so the resulting video contains a blackout for a few seconds in the beginning—which I do not want.

  I am using png codec with bgra pixel format. My ffmpeg command (shown below) creates smooth, clear animation with images that have some text on it.

  "ffmpeg -i trans/trans-%d.png -vcodec png -pix_fmt bgra overlay-0.mov";

  The blank pngs behave like black canvas, but as I try to write some text on them or surround them with a border (i.e. colorize in some way) the problem disappears—which seems very strange. I have tested all my images ; they are all transparent. For the past few hours, I have been searching for a solution, no luck so far. Please note those blank pngs are required in the beginning for a smooth effect, and I cannot omit them. I have uploaded a sample video in Youtube. Please note the black fade out.

  https://www.youtube.com/watch?v=Te3LuItxcDk&feature=youtu.be

  https://youtu.be/Te3LuItxcDk

  PHP code :

     $increment = 40;
  
     $count = 0; 
  
  
  
     for ($x=40; $x <= 640 ; $x+=$increment) 
  
     {
  
       $roll = new Imagick(DOCROOT . '/composite-0.png');
  
       $roll->rollImage($x, 0);
  
       $roll->writeImage(DOCROOT . '/roll/roll-' . $x . '.png');
  
  
  
      $crop = new Imagick(DOCROOT . '/roll/roll-' . $x . '.png');
  
      $crop->cropImage($x, 720, 0, 0);
  
      $crop->writeImage(DOCROOT . '/roll/crop-' . $x . '.png');
  
  
  
      $extent = new Imagick(DOCROOT . '/roll/crop-' . $x . '.png'); 
  
      # $extent->setImageBackgroundColor(new ImagickPixel('none'));
  
      $extent->setImageBackgroundColor(new ImagickPixel('transparent'));
  
      $extent->extentImage(640, $extent->getImageHeight(), 0, 0);
  
      $extent->setImageFormat('png'); 
  
      $extent->writeImage(DOCROOT . '/trans/trans-' . $count . '.png');
  
  
  
      $count++; 
  
     }

• avfilter/avf_showcwt : do not return initial black frames with negative timestamps

  11 novembre 2023, par Paul B Mahol

  avfilter/avf_showcwt : do not return initial black frames with negative timestamps
  

  • [DH] libavfilter/avf_showcwt.c

• ffmpeg puts black frames in the first few seconds of the video

  30 juin 2018, par Joe T. Boka

  I am adding an audio file to a video file, using ffmpy, which is a python wrapper for ffmpeg.

  The input audio file voice.wave and the input video file video.avi are the same length.

  When I use the below code, the output video output.ts contains both audio and video, as expected. The problem : the audio is running fine, but the first few seconds of the output video output.ts is black.

  from ffmpy import FFmpeg
  
  ff = FFmpeg(inputs={'video.avi': None, 'voice.wav': None}, outputs={'output.ts': '-c:v h264 -c:a ac3'})
  
  ff.cmd
  
  'ffmpeg  -i voice.wave -i video.avi -c:v h264 -c:a ac3 output.ts'
  
  ff.run()