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• Does FFmpeg video filter(eq filter) color correction work same as function sws_setColorspaceDetails?

  25 October 2019, by Jahwan Oh

  I’m need to use color filter with two different implementation.
  first one is using the function below. with conversion

  double m_brightness = -0.18;
  
  double m_contrast = 1.45;
  
  double m_saturation = 1.2;
  
  SwsContext *swsctx = sws_getCachedContext(NULL, vStreams[1]->codec->width, vStreams[1]->codec->height, vStreams[1]->codec->pix_fmt, dst_width, dst_height, dst_pix_fmt, SWS_BICUBIC, NULL, NULL, NULL);
  
      int *inv_table, srcrange, *table, dstrange, b, c, s;
  
      sws_getColorspaceDetails(swsctx, &inv_table, &srcrange, &table, &dstrange, &b, &c, &s);
  
      b = int((1 << 16) * m_brightness);
  
      c = int(c * m_contrast);
  
      s = int(s * m_saturation);
  
      ret = sws_setColorspaceDetails(swsctx, table, srcrange, inv_table, dstrange, b, c, s);

  and the other one is using FFmpeg -vf eq color correction on command line.

  ffmpeg -i original.jpg -vf "eq=brightness=-0.18:contrast=1.45:saturation=1.2" -c:a copy original_filter.jpg

  the problem is that I need to have a same result(filtered) with this two tools. with just same brightness/contrast/saturation parameters, it showed different result, so I tried to convert these parameter between these two tools.

  I thought these two color correction work same reading the link below. I’ve tried to convert the swscale filter parameter to ffmpeg -vf eq filter parameter based on the link, but it seems it is not a conversion between them.
  but, it is more like a conversion of m_brightness <—> b.(in the code I’ve attached)
  https://github.com/google/sagetv/blob/71d122229105e164cd410fafed2a5e7483869913/third_party/mplayer/libmpcodecs/vf_scale.c#L393

  you can see the each results with different parameter.

  https://drive.google.com/drive/folders/181_LhkvpB4XbEf05_ecuLoy5nSSEcRiX?usp=sharing

  How can I convert the parameters so that it produce same filtered result? or is it totally different algorithm?

• FFMPEG Replacing black color fragments with alpha channel transparency

  31 December 2017, by kostya572

  I have file.mov video. It has a lot of blank black color fragments that have different durations.

  I need to replace black color fragments that show up for more than 5 seconds with transparency. Is that possible to add some alpha channel for that purpose?

  Comment to Mulvya’s answer:

  Amazing solution. With #2 section everything ok.

  With #1 section I created this code:

  output=$(ffprobe -f lavfi -i "movie=file.mov,blackdetect=d=3.5" -show_entries tags=lavfi.black_start,lavfi.black_end -of compact=p=0 -v 0|awk '!/^$/')
  
  echo $output

  using awk '!/^$/' to remove empty lines.

  Here is the output I get:

  

  tag:lavfi.black_start repeats with same value several times, not having right structure with closing tag:lavfi.black_end

  If I change blackdetect=d=3.5 to other value, for exaple d=10, it outputs the same result as d=3.5. How could I solve this issue having right tag:lavfi.black_start,tag:lavfi.black_end synthax with correct grepping d= value?

• FFMPEG Determine average color of an area of a video

  12 November 2019, by Naved Khan

  I have a use case where I’d want to insert one of two watermarks - one designed for a dark-ish background, the other for a light background into a video. Let’s say that I’d want to do this on the top right corner of the video.

  How do I determine the average color of the top right section of the video? Post this, how do I determine which watermark to use by looking at the average color?

  I have a solution right now where I am taking equally spaced screenshots and then measuring the average color, but it’s excruciatingly slow, especially for longer videos.

  # Calculate average color
  
      black_distances = []
  
      white_distances = []
  
  
  
      movie = FFMPEG::Movie.new(video_file)
  
      (0..movie.duration / 10).each do |second|
  
  
  
        # extract a frame
  
        filename = "tmp/watermark/#{SecureRandom.uuid}.jpg"
  
        movie.screenshot filename.to_s, seek_time: second
  
  
  
        # analyse frame for color distance
  
        frame = MiniMagick::Image.open(filename)
  
        frame.crop('20%x20%+80%+0')
  
        frame.resize('1x1')
  
        pixel = frame.get_pixels.flatten
  
  
  
        distance_from_black = Math.sqrt(((black[0] - pixel[0])**2 + (black[1] - pixel[1])**2 + (black[2] - pixel[2])**2))
  
        distance_from_white = Math.sqrt(((white[0] - pixel[0])**2 + (white[1] - pixel[1])**2 + (white[2] - pixel[2])**2))
  
  
  
        black_distances.push distance_from_black
  
        white_distances.push distance_from_white
  
  
  
        File.delete(filename) if File.exist?(filename)
  
      end
  
  
  
      average_black_distance = black_distances.reduce(:+).to_f / black_distances.size
  
      average_white_distance = white_distances.reduce(:+).to_f / white_distances.size

  I am also confused about how to use the resulting average_black_distance and average_white_distance to determine which watermark to use.