Codebrawl selective color

README.md

Codebrawl Selective Color entry

All the code so far is in select.rb. I set up a few helper functions at the top of the file to get color components in floating point for the hue calculations. The hue calculation itself is ripped straight from Wikipedia HSL and HSV, implemented in pure Ruby. I also took advantage of ChunkyPNG's already-implemented grayscale conversion (after rolling my own first, then realizing my mistake).

The commands used to generate all the versions are:

./select.rb 20 65 input.png yellow.png
./select.rb 0 18 input.png red.png
./select.rb 65 90 input.png green.png
./select.rb 185 215 input.png blue.png
./select.rb 235 245 input.png violet.png

where the command line syntax is:

select.rb <lower hue limit, 0-360.0> <upper hue limit, 0-360.0> <input file> <output file>

Since I'm not a GUI expert, I haven't wrapped this up with its own color picker - I used the ColorSync Utility built-in to my MacBook to estimate Hue value ranges for the different crayons.

blue.png

Gemfile

# A sample Gemfile
source "http://rubygems.org"

gem "chunky_png"

# gem "rails"

Gemfile.lock

GEM
  remote: http://rubygems.org/
  specs:
    chunky_png (1.2.4)

PLATFORMS
  ruby

DEPENDENCIES
  chunky_png

green.png

red.png

select.rb

#!/usr/bin/env ruby

require 'bundler'
require 'chunky_png'

def r_f(color)
  ChunkyPNG::Color.r(color) / 255.0
end
def g_f(color)
  ChunkyPNG::Color.g(color) / 255.0
end
def b_f(color)
  ChunkyPNG::Color.b(color) / 255.0
end

def hue(color)
  return 0.0 if ChunkyPNG::Color.grayscale? color

  alpha = 0.5 * (2*r_f(color) - g_f(color) - b_f(color))
  beta = Math.sqrt(3)/2.0 * (g_f(color) - b_f(color))
  rad = Math.atan2(beta, alpha)
  rad = 2*Math::PI + rad if rad < 0
  rad / (2*Math::PI) * 360.0
end

unless ARGV.length == 4
  puts <<HELP
Usage:
  select.rb <lower hue limit, 0-360.0> <upper hue limit, 0-360.0> <input file> <output file>
HELP
  exit(0)
end

lower_limit = Float(ARGV[0])
upper_limit = Float(ARGV[1])
input_file = ARGV[2]
output_file = ARGV[3]

puts "reading from #{input_file}"
puts "writing to #{output_file}"

input = ChunkyPNG::Image.from_file(input_file)

puts "keeping pixels with #{lower_limit} <= hue <= #{upper_limit}"

(0...input.height).each do |y|
  puts "row #{y}"
  (0...input.width).each do |x|
    h = hue input[x,y] 
    unless lower_limit <= h && h <= upper_limit
      input[x,y] = ChunkyPNG::Color.grayscale ChunkyPNG::Color.grayscale_teint input[x,y]
    end
  end
end

input.save(output_file)

violet.png

yellow.png