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• Matlab VideoReader reading frames as green images or just not at all

  2 mai 2019, par Jordan

  I’m trying to read a video file and store the frames as a series of images. I’m using VideoReader but for some reason I’m having trouble. I want to store the frames of two videos encoded differently and measure the structural similarity and PSNR between the two on a frame-by-frame basis.

  Essentially I’ve three video files, an original (which reads fine), one compressed with VP9 using ffmpeg, and one compressed with H.624 using ffmpeg. The original video was originally just a set of frames merged into a .avi video using VirtualDub. The compressed videos are also .avi container.

  The VP9 video appeared to work fine but when I open the images using imshow() they appear to be just a solid green color. The video opens fine on VLC so I’m not sure what the problem is.

  The H.264 video doesn’t read at all. When it attempts to enter the "while hasFrame()" loop, it skips over it which leads to believe Matlab thinks the video frames aren’t there ? Again, this video opens fine in VLC and the three videos look almost identical.

  Anyone got any ideas why this is happening ? Is it to do with the way Matlab decodes the video or some parameters set by ffmpeg ?

  Original vs VP9 in Matlab

  Console output for ffmpeg - VP9

  Console output for ffmpeg - H264

  Main file :

  test_vid = 'vp9.avi';
  
  images = readVideos(test_vid);
  
  
  
  for i=1:length(images)
  
  
  
    % Convert from cells to matrices
  
    image1 = cell2mat(images(1,i));
  
    image2 = cell2mat(images(2,i));
  
  
  
    % Do stuff with the images here
  
  
  
    subplot(1,2,1);
  
    imshow(image1);
  
    subplot(1,2,2);
  
    imshow(image2);
  
  end

  ReadVideos() :

  function images = readVideos(test_video)
  
  
  
      % Video directories
  
      test_video_dir = strcat('src/', test_video);
  
      v_original = VideoReader('src/input.avi');
  
      v_test = VideoReader(test_video_dir);
  
  
  
      % Read original video
  
      i = 1;
  
      v_original.CurrentTime = 5;
  
      while hasFrame(v_original)
  
          frame = readFrame(v_original);
  
          originalImages{i} = frame;
  
          i = i + 1;
  
      end
  
  
  
      % Read test video
  
      i = 1;
  
      v_test.CurrentTime = 5;
  
      while hasFrame(v_test)
  
           frame = readFrame(v_test);
  
           testImages{i} = frame;
  
           i = i + 1;
  
      end
  
  
  
      images = cat(1, originalImages, testImages);
  
  end

  On a side note, is Matlab the best choice for the task or is there specialised software out there for doing this ?

• Download a youtube video without storing it on the server

  6 mai 2019, par Baraque Obahamas

  I have a personal php script that allows me to download the videos of my choice. Currently, this runs youtube-dl and if necessary convert the video, ffmpeg then.

  I’m looking for a way to allow my script to download/convert the video without even storing it on the server. Make sure that youtube-dl/ffmpeg acts as a gateway but without downloading the file.

  Do you have any idea how to do this ? The idea would be to run ffmpeg on the youtube link and not on the video after it has been downloaded, and allow the user to download it at the same time as the conversion is in progress.

  We can see that this site for example www.onlinevideoconverter.com uses this method, because if we ask for a video of 2 hours or more, it allows us to download it immediately.

• How to add a caption with video using ffmpeg ?

  15 mai 2019, par Sandson Costa

  I have a video in.mkv which is dual audio and has built-in subtitles. I want to convert it to output.mp4 and I used the following command :

  ffmpeg -rtbufsize 150M -i entrada.mkv' -acodec aac -map 0:0 -map 0:2 -vcodec h264 -c:s mov_text -map 0:3 "saída.mp4"

  With this command I was able to add a caption, but it was not recorded in any video, so I had a good choice. Video is not a video being part of the video itself.

  How I do ?
  Image

  I tried the command

  ffmpeg.exe -rtbufsize 150M -i 'entrada.mkv' -acodec aac -map 0:0 -map 0:2 -vcodec h264 -vf:0:3 "saída.mp4"

  But there is the mistake

  At least one output file must be specified