Generate 1x1 PNG files with a particular background colour and opacity

tsp.rb

#!/usr/bin/env ruby
require 'oyster'

spec = Oyster.spec do
  name "tsp -- Generate 1x1 PNG files with a particular background colour and opacity"
  
  description <<-EOS
    tsp is a command-line utility for generating 1x1 pixel PNG files with a given
    background colour and opacity. Due to the relative lack of support for RGBA
    amongst web browsers, it's useful to be able to generate background images
    with the requisite properties.
    
    This program requires that the ImageMagick command-line tool `convert` be
    installed.
  EOS
  
  string :colour, :default => "ffffff",
         :desc => "The colour of the image"
  
  float  :opacity, :default => 1.0,
         :desc => "The opacity of the image"
  
  author "Benedict Eastaugh <benedict@eastaugh.net>"
  
  copyright <<-EOS
    Copyright 2010 Benedict Eastaugh. This program is free software, distributed
    under the 3-clause BSD license.
  EOS
end

begin; opts = spec.parse
rescue Oyster::HelpRendered; exit
end

colour  = opts[:colour].sub(/^#/, "").slice(0,6)

if colour.length == 3
  colour = colour * 2
elsif colour.length != 6
  puts "Colour must be a 3- or 6-character hexadecimal string. You tried: " + colour
  exit
end

red     = colour[0,2].hex
green   = colour[2,2].hex
blue    = colour[4,2].hex

opacity = opts[:opacity]

if opacity > 1.0
  puts "Opacity must be a floating-point number less than 1.0. You tried: " + opacity.to_s
end

op_pc   = (opacity * 100).to_i
op_pc   = op_pc < 100 ? "0" + op_pc.to_s : "100"

if red && blue && green
  `convert -size 1x1  xc:'rgba(#{red},#{green},#{blue},#{opacity})' #{colour}-#{op_pc}.png`
end