var height = 600;
var width = 700;

function open_cart(form)
{
  xpos = (screen.width - width) / 2;
  ypos = (screen.height - height) / 2;

  var f = form;
  var action = f.action.value;
  f.action.value = 'cart';
  f.target = 'cart';

  window.open('', 'cart', 'width=' + width + ',height=' + height + ',left=' + xpos + ',top=' + ypos + ',resizable=yes,status=yes,toolbar=no,location=no,menubar=no,scrollbars=yes');

  f.submit();
  f.action.value = action;
  f.target = '_self';
  return false;
}

function window_open(url)
{
  xpos = (screen.width - width) / 2;
  ypos = (screen.height - height) / 2;

  options = "width=" + width + ",height=" + height + ",left=" + xpos + ",top=" + ypos + ",statusbar=no,menubar=no,toolbar=no,scrollbars=yes,resizable=yes";
  window.open(url, '', options);
}