ImageMagick and RMagick for Testing Colors

ImageMagick and RMagick for Testing Colors


2 min read

A couple weeks ago I started working on a pure-Ruby GPIO library for the Raspberry Pi to see whether I could get rid of a persistent glitch I was experiencing while driving the ShiftBrite LED modules. I still haven't been able to pinpoint the issue but while I was at it I put together some color conversion classes. Since I couldn't test the libraries on the LED modules directly, I needed a simple way to check that the conversion worked beyond the numbers looking OK and matching up with a few test cases from Photoshop.

I have never used the Ruby RMagick library before so killing two birds with one stone seemed a nice way to learn something new and test whether my converters were working properly.

Unfortunately, I had some troubles getting RMagick to compile properly on Mountain Lion, despite a bunch of links on doing just what I was looking to do. Luckily, I finally found this post in Japanese. Luckily, while I don't speak Japanese, code can be a universal language and the author's trick worked for me. Here it is again in the hopes that it will pop up a bit higher on the search indices:

Mac OS X, Mountain Lion, ImageMagick and RMagick installation using Homebrew and Rubygems:

brew update
brew install imagemagick
ln -s /usr/local/lib/libMagickCore-Q16.dylib /usr/local/lib/libMagickCore.dylib
gem install rmagick
rm /usr/local/lib/libMagickCore.dylib

Excellent, we're in business! Pulled up irb and tested a few commands, got my head around the Magick::Draw class, and wrote up a test script. The simplest test case I could think of was to render up a full HSV spectrum with the hue angle on the X-axis and the saturation and value levels along the Y-axis. Here is the complete source for the test.

#!/usr/bin/env ruby

$:.unshift '.'
require 'rmagick'
require 'color'

canvas =, 100)
gc =
gc.fill_opacity 1

(0..359).each do |hue|
  (0...50).each do |saturation|
    hsv = hue, saturation * 2, 100
    rgb = hsv.to_rgb
    gc.fill "rgb(#{rgb.r * 100}%, #{rgb.g * 100}%, #{rgb.b * 100}%)"
    gc.rectangle hue, saturation, hue + 1, saturation + 1

  (0...50).each do |value|
    hsv = hue, 100, 100 - value * 2
    rgb = hsv.to_rgb
    gc.fill "rgb(#{rgb.r * 100}%, #{rgb.g * 100}%, #{rgb.b * 100}%)"
    gc.rectangle hue, value + 50, hue + 1, value + 51

gc.draw canvas
canvas.write 'output.png'

The results of the run showed what I wanted to see:

HSV Test Conversion Image


I've also been putting up this and my other Raspberry Pi sources up on GitHub. Give them a look, fork them, send me pull requests, let me know if it helps you.