function switch_filter() {
    if(document.getElementById("filter").style.display == "block") {
        document.getElementById("filter").style.display    = "none";
        document.getElementById("filter_switch").src = "/img/filter_use.gif";
    } else {
        document.getElementById("filter").style.display    = "block";
        document.getElementById("filter_switch").src = "/img/filter_hide.gif";
    }
}