function cover () {
    document.getElementById("bild1a").style.opacity = 1;
    document.getElementById("bild1a").style.filter = "Alpha(opacity=100)";
    document.getElementById("bild1b").style.opacity = 0;
    document.getElementById("bild1b").style.filter = "Alpha(opacity=0)";

    document.getElementById("bild2a").style.opacity = 1;
    document.getElementById("bild2a").style.filter = "Alpha(opacity=100)";
    document.getElementById("bild2b").style.opacity = 0;
    document.getElementById("bild2b").style.filter = "Alpha(opacity=0)";

    document.getElementById("bild3a").style.opacity = 1;
    document.getElementById("bild3a").style.filter = "Alpha(opacity=100)";
    document.getElementById("bild3b").style.opacity = 0;
    document.getElementById("bild3b").style.filter = "Alpha(opacity=0)";
}

function blend (nr,percent) {
    if( typeof percent == "undefined" ) {
        percent = 100;
    }
    obj=document.getElementById(nr);
    percent -= 5;
    if( percent <= 100 ) {
        obj.style.opacity = percent/100;
        obj.style.filter = "Alpha(opacity="+percent+")";
    }
    if( percent > -100 ) {
        window.setTimeout(function() { blend(nr,percent); }, 70 );
    }
}

function blendout (nr,percent) {
    if( typeof percent == "undefined" ) {
        percent = 100;
    }
    obj=document.getElementById(nr);
    percent -= 5;
    if( percent <= 100 ) {
        obj.style.opacity = percent/100;
        obj.style.filter = "Alpha(opacity="+percent+")";
    }
    if( percent > -100 ) {
        window.setTimeout(function() { blendout(nr,percent); }, 70 );
    }
}

function blendin (nr,percent) {
    if( typeof percent == "undefined" ) {
        percent = 0;
    }
    obj=document.getElementById(nr);
    percent += 5;
    if( percent <= 100 ) {
        obj.style.opacity = percent/100;
        obj.style.filter = "Alpha(opacity="+percent+")";
    }
    if( percent > -100 ) {
        window.setTimeout(function() { blendin(nr,percent); }, 70 );
    }
}