Detecting Primary Color in Photo

For one of our clients at SmartLogic, we had to find the primary color of a photo to use as a background while the photo was loading. This was pretty easy given ImageMagick and lot of other tutorials online, but the primary color was always some extremely dark color, or gray/black.

This sidekiq worker detects the “best” primary color of the photo, skipping really dark colors and any version of gray.

This uses the convert imagemagick command to find the color histogram and sorts it by most used color. It then strips out “bad” colors to find the most common color we can use.

class ProcessPhotoWorker
  include Sidekiq::Worker

  # Regex for the output of image magic
  COLOR_REGEX =
     /\d+: \(\s{0,2}\d{1,3},\s{0,2}\d{1,3},\s{0,2}\d{1,3}\) (?<color>#.{6}).*/
  # These are all very dark colors
  COLORS_TO_IGNORE = [
    "#000000", "#000033", "#000066",
    "#003300", "#003333", "#003366",
    "#330000", "#330033", "#330066",
    "#333300", "#333333", "#333366",
    "#660000", "#660033", "#660066",
    "#663300", "#663333", "#663366",
  ]

  def perform(photo_id)
    @photo = Photo.find(photo_id)

    # Save a local version of the S3 photo
    tmpfile = Tempfile.new("photo")
    begin
      tmpfile.binmode
      tmpfile.write(@photo.image.read)
      tmpfile.rewind

      image = ::MiniMagick::Image.read(tmpfile.read)

      capture_geometry(image)
      capture_main_color(image, tmpfile)
    ensure
      tmpfile.close
      tmpfile.unlink
    end

    @photo.save
  end

  private

  def capture_geometry(image)
    @photo.image_height = image[:height]
    @photo.image_width = image[:width]
  end

  # Uses the convert tool to find the most common color
  # Output is similar to:
  #   67655: ( 50, 18, 18) #321212 srgb(50,18,18)
  #  240295: ( 15,  3,  6) #0F0306 srgb(15,3,6)
  def capture_main_color(image, tmpfile)
    output = image.run(command_to_run(tmpfile))

    sorted_colors = output_to_colors(output)
    sorted_colors = sorted_colors - COLORS_TO_IGNORE
    sorted_colors = skip_gray(sorted_colors)

    @photo.image_main_color = sorted_colors.last
  end

  def command_to_run(tmpfile)
    command = ::MiniMagick::CommandBuilder.new("convert")
    command << tmpfile.path << "-resize" << "100x100" << "+dither"
    command << "-remap" << "netscape:" << "-format" << "'%c'" << "histogram:info:"
    command
  end

  def output_to_colors(command_output)
    lines = command_output.split("\n")

    sorted_lines = lines.map do |line|
      # convert white space to single space inbetween for the regex
      # first line contains a "'"
      line.gsub(/\s+/, " ").gsub("'", "").strip
    end.sort_by(&:to_i)

    sorted_colors = sorted_lines.map do |color|
      match = COLOR_REGEX.match(color)
      match[:color] if match
    end

    sorted_colors.reject(&:blank?)
  end

  def skip_gray(sorted_colors)
    sorted_colors.reject do |color|
      color = Color::RGB.from_html(color)
      color.red == color.green &&
        color.red == color.blue &&
        color.green == color.blue
    end
  end
end

Eric Oestrich

Detecting Primary Color in Photo