 do_guides){
    spline = document.getElementById("spline");
    var spline_context = spline.getContext("2d");
    spline_context.font = "16px sans-serif";
    // background
    spline_context.fillStyle="#eeeeee";
    spline_context.fillRect(0, 0, w, h);

    // grid
    draw_grid_on(spline_context);

    // spline1
    var A1 = [];
    var B1 = [];
    for(var i = 0; i < 4; i += 2) {
        A1[i] = [];
        for(var j = 0; j < 4; ++j){
            A1[i][j] = Math.pow(spline_xs[i], j);
        }
        