Ciao Francesco,
ho scritto un semplice esempio che dissolve un elemento con opacity=1 e fa comparire un elemento con opacity=0.
- Dissolve.Add( id_name, step ); permette di "dissolvere" o "far comparire" l'elemento con id = idname.

Spero che l'esempio sia chiaro e di esser stato di aiuto.
Tony


codice:
<!DOCTYPE>
<html>
  <head>
<style>

body { background-color:#FFD;}

div {
  position:absolute;left:50%;top:50%;
  text-align:center;
  font:16px Arial;
}
 
  
</style>
<script>


function f_act_dissolve(){
                                                  
  Dissolve.Add("id_btn");
  Dissolve.Add('id_home',0.05);  
  setTimeout("Dissolve.Add('id_tonenge');",1000);    // dopo 1 secondo
}


//------------------------------------------------------------------------------
Dissolve={
  act:0,
  aidn:[],
  tm:50,
  ostep:0.03,

Add:function(idn,stp){
with(Dissolve){
    if(!stp) stp=ostep;
    var d,o,j=document.getElementById(idn);
    o=parseInt(j.style.opacity);
    if(!o) o=0;
    d=(o>0.5)?-1:1;   
    aidn.push([j,o,d,stp]); 
    if(!act) { act=1; Cycle(); } 
}},

Cycle:function(){
with(Dissolve){
  var r,n=aidn.length;
  if(!n) { act=0; return; }
  for(r=0;r<n;r++) Dis(r);
  for(r=n-1;r>=0;r--) if(!aidn[r].length) aidn.splice(r,1);    
setTimeout(function(){ Cycle(); },tm);  
}},

Dis:function(r){
with(Dissolve){
   var D=aidn[r];
   D[1] += (D[3]*D[2]);
   if(D[1]<0) D[1]=0;
   if(D[1]>1) D[1]=1;
   D[0].style.opacity=D[1];
   if(!D[1] || D[1]==1) aidn[r]=[];
}}

}
//------------------------------------------------------------------------------


</script>  
  </head>
  <body>

<!-- div contenitore della Home page -->
<div id='id_home' style='margin:-300px;width:600px;height:600px;background-color:#48E;opacity:0.0;'>
  <br><span onclick='f_act_dissolve();' style='cursor:pointer;text-decoration:underline;'>Torna indietro</span>
</div>

<!-- div contenitore pulsante -->
<div id='id_btn' style='margin:-60px;width:120px;height:120px;background-color:#FF0;opacity:1.0;'>
  <br>Test<br>
  <br>
  <button onclick='f_act_dissolve();' style='cursor:pointer;'>Entra nel sito:</button>
</div>


<div id='id_tonenge' style='top:10px;font:bold 24px Arial;width:300px;margin-left:-150px;color:#B00;opacity:0.0;'>Tonenge</div>

  </body>
</html>