    #! /usr/local/bin/ruby -w

    # Demonstrate the map, append, and composite methods by
    # mapping the colors in three separate images into the
    # 216 "Web-safe" colors.

    require 'RMagick'

    group = Magick::ImageList.new

    # Read three images.
    snapshots = Magick::ImageList.new("images/Hot_Air_Balloons.jpg","images/Violin.jpg","images/Polynesia.jpg")

    # Scale each image to 250 pixels high & proportionately wide
    snapshots.each { |shot|
        group << shot.scale(250.0/shot.rows)
        }

    # "Read" the Netscape 216-color cube
    map = Magick::ImageList.new "netscape:"

    # Map the group of snapshots into the Netscape colors
    puts "Mapping... This may take a few seconds..."
    mapped = group.map map, false

    # Use the append method to arrange the unmapped images
    # side-by-side into a single image. Repeat for the mapped images.
    old = group.append false
    new = mapped.append false

    # Show before & after on the same image.
    # Crop the top half of the "after mapping" images away.
    half_height = old.rows / 2
    new = new.crop 0, half_height, new.columns, half_height

    # Composite the "after" images over the "before" images.
    demo = old.composite new, 0, half_height, Magick::OverCompositeOp

    # Draw a black line across the middle to help
    # distinquish "before" (top) and "after" (bottom)
    line = Magick::Draw.new
    line.line 0, demo.rows/2, demo.columns, demo.rows/2
    line.stroke "black"
    line.draw demo

    #demo.display
    puts "Writing map.jpg..."
    demo.write "map.jpg"
    exit