return dominant color on a picture with minimagick

image_color_info.rb

class ImageColorInfo
  def initialize(image)
    @image = image
  end

  def dominant_color
    dominant_colors.first
  end

  def dominant_colors
    histogram.sort_by(&:first).reverse.map(&:last)
  end

  def histogram
    @histogram ||= histogram_result.split("\n").map do |data|
      count, color = data.split(":", 2)
      [
        count.to_i,
        color.strip.split(" ")[1][0, 7]
      ]
    end
  end

  private

  # returned format example:
  #  "    4290: (2.59281,2.51622,2.50009) #030303 srgb(1.01679%,0.986752%,0.980429%)\n    104: (3.90722,2.76358,2.89232) #040303 srgb(1.53224%,1.08376%,1.13424%)\n    215: (89.8296,20.9757,19.2815) #5A1513 srgb(35.2273%,8.22577%,7.56136%)\n    391: (173.48,38.3166,28.8013) #AD261D srgb(68.0313%,15.0261%,11.2946%)"
  def histogram_result
    temp_file = Tempfile.new
    temp_file.binmode
    temp_file.write @image.download

    begin
      convert = MiniMagick::Tool::Convert.new
      convert << temp_file.path
      convert.resize("100x100")
      convert << '-format' << '%c'
      convert << '-colors' << '6'
      convert << 'histogram:info:-'
      convert.call

    rescue MiniMagick::Error
      ""
    ensure
      temp_file.unlink
    end
  end
end

model.rb

class MyModel
  before_save :track_if_image_changed
  after_save_commit :save_background_color, prepend: true
  
  private
  
  def save_background_color
    return unless image_changed?
    return unless image.attached?
    return unless (color = ImageColorInfo.new(image).dominant_color)
    update_columns background_color: color
  end

  def track_if_image_changed
    @track_if_image_changed = attachment_changes.any?
  end

  def image_changed?
    @track_if_image_changed == true
  end
end