arrow.rb

require "rubygems"
require "RMagick"

#
# draw an arrow at the point
#
def draw_arrow( image, point, direction="south" )
  arrow = Magick::Draw.new

  # make the value of "point" the center (origin)
  arrow.translate( point.first, point.last )
  
  # rotate based on the direction
  case direction
    when "north" then arrow.rotate(  0)
    when "east"  then arrow.rotate( 90)
    when "south" then arrow.rotate(180)
    when "west"  then arrow.rotate(270)
    else raise "Not a valid direction: #{direction}"
  end

  # calculate the arrow dimensions
  tail_height = 0.100 * image.rows
  arm_height  = 0.050 * image.rows
  arm_width   = 0.025 * image.columns

  # draw the arrow
  # NOTE
  # We are always drawing a south-facing arrow, the "direction"
  # takes care of rotating it to point in the right ... direction
  arrow.stroke( "black" )
  arrow.stroke_width(2.5)
  arrow.line(          0, tail_height, 0, 0 ) # line going down
  arrow.line(  arm_width,  arm_height, 0, 0 ) # line from the right
  arrow.line( -arm_width,  arm_height, 0, 0 ) # line from the left
  arrow.draw( image )

  return image
end

[ "north", "east", "south", "west" ].each do |direction|
  draw_arrow( Magick::Image.new(200,100), [100,50], direction ).write("./arrow/#{direction}.png")
end

This is drawing a rather sensible looking arrow. The length of the arrows is fixed at the moment but at least the fundamental reason for writing this method (pointing the arrow in any -- one of four -- direction) is being accomplished.