gistfile1.rb

require 'tk'

class Pendulum 
  def initialize
    setup
    show
    @after_id = @root.after(500) {animate}
 
    @canvas.bind('<Destroy>') do
      @root.after_cancel(@after_id)
    end
 
    Tk.mainloop
  end


  def setup
    @root = TkRoot.new("title" => "Pendulum Animation")
    @canvas = TkCanvas.new($root) do
      width 320
      height 200
      create TkcLine,   0,  25, 320,  25, 'tags' => 'plate', 'width'   => 2,       'fill' => 'grey50'
      create TkcOval, 155,  20, 165,  30, 'tags' => 'pivot', 'outline' => "",      'fill' => 'grey50'
      create TkcLine,   1,   1,   1,   1, 'tags' => 'rod',   'width'   => 3,       'fill' => 'black'
      create TkcOval,   1,   1,   2,   2, 'tags' => 'bob',   'outline' => 'black', 'fill' => 'yellow'
    end

    @canvas.raise('pivot')
    @canvas.pack('fill' => 'both', 'expand' => true)

    @Theta = 45.0
    @dTheta = 0.0
    @length = 150
    @home = {
             :x => 160,
             :y => 25
            }
  end
 
  def show
    angle = @Theta * Math::PI / 180
    x = @home[:x] + @length * Math.sin(angle)
    y = @home[:y] + @length * Math.cos(angle)
    @canvas.coords('rod', @home[:x], @home[:y], x, y)
    @canvas.coords('bob', x-15, y-15, x+15, y+15)
  end
 
  def recompute_angle
    scaling = 3000.0 / (@length ** 2)
    # first estimate
    firstDDTheta = -Math.sin(@Theta * Math::PI / 180) * scaling
    midDTheta = @dTheta + firstDDTheta
    midTheta = @Theta + (@dTheta + midDTheta)/2
    # second estimate
    midDDTheta = -Math.sin(midTheta * Math::PI / 180) * scaling
    midDTheta = @dTheta + (firstDDTheta + midDDTheta)/2
    midTheta = @Theta + (@dTheta + midDTheta)/2
    # again, first
    midDDTheta = -Math.sin(midTheta * Math::PI / 180) * scaling
    lastDTheta = midDTheta + midDDTheta
    lastTheta = midTheta + (midDTheta + lastDTheta)/2
    # again, second
    lastDDTheta = -Math.sin(lastTheta * Math::PI/180) * scaling
    lastDTheta = midDTheta + (midDDTheta + lastDDTheta)/2
    lastTheta = midTheta + (midDTheta + lastDTheta)/2
    # Now put the values back in our globals
    @dTheta  = lastDTheta
    @Theta = lastTheta
  end
 
  def animate
    recompute_angle
    show
    @after_id = @root.after(15) {animate}
  end
end

Pendulum.new