矢印を書いてみる

ruby/GD 
 #!/usr/bin/ruby -Ke
 require "GD"
 require "iconv"
 class Arrow
   def initialize(len=100)
     a=GD::Polygon.new
     a.addPt(100,50)
     a.addPt(50,100)
     a.addPt(75,100)
     a.addPt(75,100+len)
     a.addPt(125,100+len)
     a.addPt(125,100)
     a.addPt(150,100)
     @arrow=a
   end
   def move(x=100,y=100)
     @arrow.offset(x,y)
     self
   end
   def scale(x=0.5,y=0.5)
     @arrow.scale(x,y)
     self
   end
   def rotate(x=10,y=10)
 #    @arrow.transform(2,x,2,y,30,50)
     @arrow.transform(1,0.3,0.3,0.5,30,50)
     self
   end
   def to_p
     @arrow
   end
 end
 im = GD::Image.new(800,400)
 white = im.colorAllocate(255,255,255)
 black = im.colorAllocate(0,0,0)
 red = im.colorAllocate(255,0,0)
 blue = im.colorAllocate(0,0,255)
 im.polygon(Arrow.new(100).to_p,blue)
 im.polygon(Arrow.new(100).rotate.to_p,red)
 print "content-type: image/png\n\n"
 im.png STDOUT

回転の公式
座標、(x,y) にある点を、座標 (a,b) を中心に角度 r 回転すると、
x'= (x-a)cos(r)-(y-b)sin(r)+a
y'=(x-a)sin(r)+(y-b)cos(r)+b