Bresenham's Line Algorithm in DragonRuby

main.rb

# brief demonstration of Bresenham's line algorithm in DragonRuby

BRUSH_SIZE = 16

def tick(args)
  args.state.last_mouse_x ||= args.inputs.mouse.x
  args.state.last_mouse_y ||= args.inputs.mouse.y

  # set the start of the line
  x0 = args.state.last_mouse_x
  y0 = args.state.last_mouse_y

  # set the end of the line
  x1 = args.inputs.mouse.x
  y1 = args.inputs.mouse.y

  # draw the line
  draw_line(args, x0, y0, x1, y1)

  # remember the last position of the mouse
  args.state.last_mouse_x = args.inputs.mouse.x
  args.state.last_mouse_y = args.inputs.mouse.y
end

# draw a line using Bresenham's line algorithm
def draw_line(args, x0, y0, x1, y1)
  dx = (x1 - x0).abs
  sx = x0 < x1 ? 1 : -1
  dy = -(y1 - y0).abs
  sy = y0 < y1 ? 1 : -1
  err = dx + dy

  while true
    args.outputs.sprites << [x0, y0, BRUSH_SIZE, BRUSH_SIZE, 'sprites/circle-black.png']
    break if x0 == x1 && y0 == y1

    e2 = 2 * err
    if e2 >= dy
      err += dy
      x0 += sx
    end
    if e2 <= dx
      err += dx
      y0 += sy
    end
  end
end