var shkr_timer;
var shake_amount;
var decelerate_by;
var shift_left;
var fps;

function move_window() {
    if (shake_amount > 0) {
        if (shake_amount > 2) {
            if (shift_left) {
                window.moveBy(0-shake_amount,0-shake_amount);
                shake_amount = shake_amount * decelerate_by;
                shift_left = false;
            } else {
                window.moveBy(shake_amount,shake_amount);
                shake_amount = shake_amount * decelerate_by;
                shift_left = true;
            }
        } else {
            shake_amount = 0;
        }
        shkr_timer = setTimeout("move_window()", 1000/fps);
    }
}
function shake(amt, decel, speed) {
    // default values
    shake_amount = typeof(amt) != 'undefined' ? amt : 25;
    decelerate_by = typeof(decel) != 'undefined' ? decel : 0.70;
    fps = typeof(speed) != 'undefined' ? speed : 12;
    shift_left = false;

    shkr_timer = setTimeout("move_window()", 1000/fps);
}