"No Colors Anymore I Want Them To Turn Black"...and White

I’m currently implementing a Digital Asset Management (DAM) system for Universal Uclick, the company I work for. One of the requirements is to keep both color and black and white versions of our creators’s works. This hasn’t been a problem with our previous system because we’ve used naming conventions to distinguish between the two versions. In this newer version, I’ve decided to use the same name, regardless of color or grayscale, and just store said parameter in the database. The problem I ran into, however, is determining whether an image is color or grayscale. 

I discovered that the answer is to create a black and white version (in memory) of every image and compare the two versions using ImageMagick’s “difference” method. But before I get into to that, we first have to deal with one little problem…

I’m using Ruby’s implementation of ImageMagick (RMagick). According to the docs, I’m supposed to use the “quantize” method for converting color to black and white. The only problem is, it doesn’t work with images like this Non Sequitur strip:

The solution, I found, is to use the “modulate” method, setting brightness to 1.0, saturation to 0.0, and hue to 0.5. Just for giggles I benchmarked the two methods and found modulate to be significantly faster. Here are the results from that:

           user     system      total        real
quantize  4.620000   6.820000  11.440000 ( 11.831151)
modulate  1.270000   0.060000   1.330000 (  1.125789)

Which one would you use? Yeah, I thought so. (Note: I did not run the benchmark against the above image, but rather, on a Garfield strip).

So now the image is being changed to black and white correctly. The next step is to compare the original to the new B&W version. To do that, use ImageMagick’s “difference” method and look at the resulting “normalized_mean_error”. If it’s 0.0, the image is already black and white, if not, it’s color. Here’s a snippet of code to do this:

require 'rubygems'
require 'RMagick'

class Chromatic
  attr_reader :image
  
  def initialize(img)
    @image = img
  end
  
  def chromatic?
    grayscale_img = self.image.modulate(1,0,0.5)
    self.image.difference(grayscale_img)
    
    self.image.normalized_mean_error > 0
  end
end

img = Magick::Image::read(ARGV.shift).first
img2 = Chromatic.new(img)
puts img2.chromatic?

Two birds; one rolling stone. You can now make B&W images faster and determine if an image is color or black and white. Outstanding.