var speed = 50;         // speed
var max_opacity = 100;  // max
var min_opacity = 10;   // min
var do_step  =  5;      // step
var op_img_idx = 0;

function change_opacity(obj,do_option)
{
    if(obj.doing)
        clearInterval(obj.doing);
    if(!obj.id) obj.id = 'op_img_id_'+ (op_img_idx++);
    obj['cur_op'] = ( do_option == 'down' ? max_opacity : min_opacity ) ;
    obj.doing = setInterval(do_option+"_alpha('"+obj.id+"')",speed);
}

function do_opacity(a,v)
{
  if(!a.style) a.setAttribute('style','');
  if("opacity" in a.style){
            a.style.opacity=v/100;
        }else if("MozOpacity" in a.style){
            a.style.MozOpacity=v/100;
        }else if("filter" in a.style){
            a.style.filter="alpha(opacity="+v+")";
        }

}
function down_alpha(obj_index)
{
    var obj = document.getElementById(obj_index);
    obj['cur_op'] -= do_step;
    if(obj['cur_op'] < min_opacity ){
        clearInterval(obj.doing);
        obj.doing = false;
    }
     do_opacity(obj,Math.max(obj['cur_op'],min_opacity));
}

function up_alpha(obj_index)
{
    var obj = document.getElementById(obj_index);
    obj['cur_op'] += do_step;
    if(obj['cur_op'] > max_opacity ){
        clearInterval(obj.doing);
        obj.doing = false;
    }
     do_opacity(obj,Math.min(obj['cur_op'],max_opacity));
}