def get_touch_density(cur,key="pixels_norm"):
    """
    Returns list of 'touch_density' measures;
    [cell1,cell2,touch_density]

    Parameters:
    -----------
    cur : MySQLdb cursor
    key : str
     touch density measure [pixels,pixels_norm,segments,segments_norm]
    
    """
    sql = ("select pre,post,%s "
           "from touch_density" %key)
    cur.execute(sql)
    return [(a[0],a[1],float(a[2])) for a in cur.fetchall()]