// creates a captcha string, load captcha image and changes value of a hidden formular element
// hardcoded, works only on contact form site
function captcha(){
  // possible characters
  var characters = "abdefghjmnqrtABDEFGHJLMNQRT23456789";
  // captcha set to an empty string
  var captcha="";
  // cpatcha gets created
  for (i=0;i<6;i++){
    pos = characters.length*Math.random();
    captcha=captcha+characters.substr(pos,1);
  }
  // new captcha image is set
  document.getElementById("captcha_img").src="rbx_inc/rbx_captcha.php?captcha="+captcha;
  // new captcha is set to a hidden input 
  document.getElementById("Captcha").value=captcha;
}