Convert one super-sized (ideally iPad landscape @2x) image into the slew of images required to implement a non-repeating nav bar background at all sizes and resolutions for iOS.

navbar.rb

#!/usr/bin/env ruby

# Takes one Retina iPad landscape nav bar image, and creates all of
# the requisite sizes for use on each device and orientation. Crops
# part of the image for iPhone portrait size.
#
# This is ONLY necessary for non-repeating or non-resizable (i.e. with
# cap insets) nav bar images. This program is an illustration of why
# you should discourage your designers from doing this.
#
# Requires Ruby (obviously) and ImageMagick (the "convert" tool)
#
# Usage: navbar.rb source_image


infile = ARGV[0]

raise "Source image ${infile} does not exist" unless File.exist?(infile)

dims = {
  "portrait~ipad" => [768, 44],
  "landscape~ipad" => [1024, 44],
  "portrait~iphone" => [320, 44],
  "landscape~iphone" => [480, 32],
  "landscape-568h" => [568, 32]
}

dims.each do |name, dim|
  scales = [2]
  scales << 1 if !(name =~ /568h/)
  
  scales.each do |s|
    px_dim = dim.map { |x| x * s }
    outname = if s == 2
                name.sub(/(~|$)/, "@2x\\1")
              else
                name
              end
    outfile = "navbar-#{outname}.png"
    dim_str = px_dim.join("x")

    opts = []
    if dim == [320, 44]
      opts << "-shave '30%x0'"
    end
    opts << "-resize '#{dim_str}!'"
    puts `convert "#{infile}" #{opts.join(' ')} "#{outfile}"`
  end
end