convert_colors.rb

require 'rmagick'

def convert_rgb_to_xyz(r, g, b)
   var_R = ( r / 255.0 )                     # RGB from 0 to 255
   var_G = ( g / 255.0 )
   var_B = ( b / 255.0 )

   if ( var_R > 0.04045 )
      var_R = ( ( var_R + 0.055 ) / 1.055 ) ** 2.4
   else
      var_R = var_R / 12.92
   end

   if ( var_G > 0.04045 )
      var_G = ( ( var_G + 0.055 ) / 1.055 ) ** 2.4
   else
      var_G = var_G / 12.92
   end

   if ( var_B > 0.04045 )
      var_B = ( ( var_B + 0.055 ) / 1.055 ) ** 2.4
   else
      var_B = var_B / 12.92
   end

   var_R = var_R * 100
   var_G = var_G * 100
   var_B = var_B * 100

   # Observer. = 2°, Illuminant = D65
   x = var_R * 0.4124 + var_G * 0.3576 + var_B * 0.1805
   y = var_R * 0.2126 + var_G * 0.7152 + var_B * 0.0722
   z = var_R * 0.0193 + var_G * 0.1192 + var_B * 0.9505

   return { :x => x, :y => y, :z => z }
end

def convert_xyz_to_lab(x, y, z)
   # Observer= 2°, Illuminant= D65
   var_X = x / 95.047
   var_Y = y / 100.000
   var_Z = z / 108.883

   if ( var_X > 0.008856 )
      var_X = var_X ** ( 1.0/3.0 )
   else
      var_X = ( 7.787 * var_X ) + ( 16.0 / 116.0 )
   end


   if ( var_Y > 0.008856 )
      var_Y = var_Y ** ( 1.0/3.0 )
   else
      var_Y = ( 7.787 * var_Y ) + ( 16.0 / 116.0 )
   end

   if ( var_Z > 0.008856 )
      var_Z = var_Z ** ( 1.0/3.0 )
   else
      var_Z = ( 7.787 * var_Z ) + ( 16.0 / 116.0 )
   end

   l = ( 116 * var_Y ) - 16
   a = 500 * ( var_X - var_Y )
   b = 200 * ( var_Y - var_Z )

   return { :l => l, :a => a, :b => b }
end

def convert_rgb_to_lab(r, g, b)
   xyz = convert_rgb_to_xyz(r,g,b)
   lab = convert_xyz_to_lab(xyz[:x],xyz[:y],xyz[:z])
   return lab
end

def get_lab_from_pixel(pixel)
   r = pixel.red / 256
   g = pixel.green / 256
   b = pixel.blue / 256

   lab = convert_rgb_to_lab(r, g, b)
end

def get_distances(lab, colors_lab)
   distances = []

   colors_lab.each do |color|
      # Get the L*a*b values for the color to be compared
      l1 = lab[:l]
      a1 = lab[:a]
      b1 = lab[:b]

      # Get the L*a*b values for the reference color
      l2 = color[1][:l]
      a2 = color[1][:a]
      b2 = color[1][:b]

      # Calculate the Euclidean distance between the colors
      distance = ((l2 - l1) ** 2) + ((a2 - a1) ** 2) + ((b2 - b1) ** 2)

      array = [color[0], distance]
      distances << array
   end

   return distances
end

def compare_pixel_to_colors(pixel)
   lab_to_compare = get_lab_from_pixel(pixel)

   colors = ["#660000", "#990000", "#cc0000", "#cc3333", "#ea4c88",
      "#993399", "#663399", "#333399", "#0066cc", "#0099cc", "#66cccc",
      "#77cc33", "#669900", "#336600", "#666600", "#999900", "#cccc33",
      "#ffff00", "#ffcc33", "#ff9900", "#ff6600", "#cc6633", "#996633",
      "#663300", "#000000", "#999999", "#cccccc", "#ffffff"];

   colors_lab = []

   colors.each do |color|
      pixel = Magick::Pixel.from_color(color)
      lab = get_lab_from_pixel(pixel)
      array = [color, lab]
      colors_lab << array
   end

   distances = get_distances(lab_to_compare, colors_lab)

   distances.sort_by! { |k| k[1] }

   return distances.first
end

image = Magick::Image.read("https://s3.amazonaws.com/artburst-dev/artworks/medium/1/bubblegun.jpg").first

hist = image.quantize(8).color_histogram

pixels_array = []

pixels = hist.keys.sort_by {|pixel| hist[pixel]}

pixels.each do |pixel|
   match = compare_pixel_to_colors(pixel)
   pixels_array << [ pixel.to_color(Magick::AllCompliance,false,8), hist[pixel], match[0], match[1] ]
end
pixels_array.reverse!
p pixels_array # [["#34689A", 42766, "#0099cc", 490.8731192616437], ["#242526", 4910, "#000000", 203.08699403851878], ["#FCFCFC", 2256, "#ffffff", 1.0733110124134708], ["#364350", 1201, "#000000", 865.6590484167668], ["#375C9C", 493, "#0066cc", 502.90135927320506], ["#A8270D", 308, "#990000", 53.58203429701537], ["#75A91B", 243, "#669900", 35.025636152203894], ["#A2A4AE", 183, "#999999", 49.0313510152735]]