var available_bg = Array("foto_zespresso02.jpg","foto_zespresso01.jpg");
var current_bg_no = 0;
var last_bg_no = -1;
var bg_anim_in_progress = 0;
function preload_bg(){
    for( var a in available_bg ){
        if( a < 100 )
            document.getElementById("bg").innerHTML += "<img src='media/starter/"+available_bg[a]+"' style='width:1px; height:1px; position: absolute; left: -1000px;'>";
    }
    return;
}
function change_bg(diff){
    // zablokuj jesli animacja jest teraz uruchomiona
    if(bg_anim_in_progress!=0) return;
    else bg_anim_in_progress = 1;
    // ustal numer elementu z tlem
    current_bg_no += diff;
    if( diff > 0 ){
        current_bg_no > available_bg.length-1
            ? current_bg_no = available_bg.length-1
            : null;
    } else {
        current_bg_no < 0
            ? current_bg_no = 0
            : null;
    }
    // nie animuj dwa razy tego samego tla
    //alert(current_bg_no+","+last_bg_no);
    if( current_bg_no == last_bg_no ){
        bg_anim_in_progress = 0;
    } else {
        last_bg_no = current_bg_no;
        // uruchom efekt zanikania
        $("bg").fade({ duration: .4 });
        // uruchom efekt pojawiania sie tla
        setTimeout("document.getElementById('bg').style.backgroundImage = 'url(media/starter/'+available_bg[current_bg_no]+')';$('bg').appear({ duration: .4 });",400);
        // odblokuj funkcje po udanej animacji
        setTimeout("bg_anim_in_progress = 0;",800);
        //alert(current_bg_no+"/"+(available_bg.length-1)+": "+available_bg[current_bg_no]);
    }
    return;
}