extract_colors_from_image.rb

require 'rubygems'
require 'rmagick'
require 'color_namer'


img = Magick::ImageList.new('photo.jpg')

img.resize_to_fit!(500) # resize the image to have faster quantization
quantized = img.quantize(16) # quantize the photo to reduce number of colors
img.destroy! # prevent memory leaks

colors_hash = {}
quantized.color_histogram.each_pair do |pixel, frequency| # grab list of colors and frequency
  
  shade = ColorNamer.name_from_html_hash(
            pixel.to_color(Magick::AllCompliance, false, 8, true)
          ).last # get shade of the color
  
  # group the colors by shade
  
  if colors_hash[shade].nil?
    colors_hash[shade] = frequency.to_i
  else
    colors_hash[shade] += frequency.to_i
  end
  
end

quantized.destroy! # prevent memory leaks

# normalize color frequency to percentage

sum = colors_hash.inject(0){ |s,c| s + c.last.to_i }.to_f
colors_hash.map{ |c| c[1] = (c.last.to_f / sum * 100).round; c }.inject({}){ |h,c| h[c[0]] = c[1]; h } 
# returns {"Blue"=>19, "Green"=>39, "Brown"=>22, "Yellow"=>12, "Grey"=>8}