rmagick annotate on arc

annotate_on_arc.rb

require 'rmagick'
iclude Math

def annotate_on_arc(img, str, wc, r, x0, y0, theta0, font, fill, stroke, size)
# img 		rmagick draw Macick::Image object
# str		annotate string
# wc		width of one charator[px]
# r, x0 ,y0	radius and center point[px] of circle(arc)
# theta0	angle at center of string[rad]
# font, size	font familiy and font size[pt?]
# fill, stroke	color of font fill and stroke
  draw = Magick::Draw.new
  l = wc * str.length # l is length of string[px]
  angle_string = l / r  # [rad]
  angle_charactor = wc / r #[rad]
  charr = str.split("")
  ii = 0
  for ch in charr do
    theta_c0 = theta0 + angle_string * 0.5 - ii * angle_charactor
    #theta_c0 is angle of charactor annotate angle that used in calc x, y
    theta_cc = theta_c0 - 0.5  * angle_charactor
    #theta_cc is angle of charactor center angle that used in rotation
    x = x0 + r * cos(theta_c0)
    y = y0 - r * sin(theta_c0)
    ch = ch.gsub(/^@/, '\@')
    draw.annotate(img, wc, wc, x, y, ch) do
      self.font = font
      self.fill = fill
      self.stroke = stroke
      self.pointsize = size
      self.gravity = Magick::NorthWestGravity
      self.rotation = -1.0 * (theta_cc * 180 / PI - 90)
    end
    ii += 1;
  end
end