function get_top_pos(element,pos){
var monElement = document.getElementById(element);
var tEcran = parseInt(screen.height); // taille de l'écran
var tpage = 770; // taille de la page
var tpos = 0 ;
if (tEcran > tpage )
	tpos = ((tEcran - tpage) / 2 );
 tpos = tpos + pos;
 alert(pos);
 alert(tEcran);
  alert(tpage);
 alert(tpos);
monElement.style.position = "absolute";
monElement.style.margin = "auto";

monElement.style.top = tpos + "px";
}
function get_left_pos(element,pos){
var monElement = document.getElementById(element);
var tEcran = parseInt(screen.width); // taille de l'écran
var tpage = 1000; // taille de la page
if (tEcran > tpage ) var lpos = ((tEcran - tpage) / 2  );
 lpos += parseInt(pos);
monElement.style.position = "absolute";
monElement.style.left = lpos + "px";

}

function centrer(element,x,y){
 if ( typeof this.cpt == 'undefined' ) this.cpt = 0;
 alert(this.cpt);
if (cpt == 0){
get_top_pos(element,y); 
get_left_pos(element,x);

}
this.cpt++;
}