在html画布上绘制贝塞尔曲线

我需要绘制一条由N个控制点定义的贝塞尔曲线，这些控制点存储在一个数组中。我有一个500px x 500px的画布。这里是JSFiddle的链接:jsfiddle.net/HswXy整个JS代码:

<script>
    window.onload=function(){
        var old_n=0,n=0;
        var nrSelect = document.getElementById("mySelect");
        var submit = document.getElementById("submit");
        nrSelect.addEventListener("change",function(){
            old_n=n;
            n = nrSelect.selectedIndex;
            var inputx,inputy,br;
            if(document.getElementById("pointsdiv"))
            {
                for(i=1;i<=n;i++){
                    inputx = document.createElement('input');
                    inputy = document.createElement('input');
                    br = document.createElement('br');
                    inputx.type = "text";
                    inputy.type = "text";
                    inputx.size = 3;
                    inputy.size = 3;
                    inputx.id = "x_" + i;
                    inputy.id = "y_" + i;
                    inputx.value = "x_" + i;
                    inputy.value = "y_" + i;
                    inputx.addEventListener("focus",function(){if(this.value==this.id) this.value="";});
                    inputy.addEventListener("focus",function(){if(this.value==this.id) this.value="";});
                    document.getElementById("pointsdiv").appendChild(inputx);
                    document.getElementById("pointsdiv").appendChild(inputy);
                    document.getElementById("pointsdiv").appendChild(br);
                }
                document.getElementById("pointsdiv").id="pointsdiv_after";
            }
            else
            {
                if( old_n < n )
                {
                    for(i=old_n+1;i<=n;i++){
                        inputx = document.createElement('input');
                        inputy = document.createElement('input');
                        br = document.createElement('br');
                        inputx.type = "text";
                        inputy.type = "text";
                        inputx.size = 3;
                        inputy.size = 3;
                        inputx.id = "x_" + i;
                        inputy.id = "y_" + i;
                        inputx.value = "x_" + i;
                        inputy.value = "y_" + i;
                        inputx.addEventListener("focus",function(){if(this.value==this.id) this.value="";});
                        inputy.addEventListener("focus",function(){if(this.value==this.id) this.value="";});
                        document.getElementById("pointsdiv_after").appendChild(inputx);
                        document.getElementById("pointsdiv_after").appendChild(inputy);
                        document.getElementById("pointsdiv_after").appendChild(br);
                    }
                }
                else
                {
                    var parent;
                    for(i=n+1;i<=old_n;i++){
                        parent = document.getElementById("pointsdiv_after");
                        parent.removeChild(parent.lastChild);
                        parent.removeChild(parent.lastChild);
                        parent.removeChild(parent.lastChild);
                    }
                }
            }
        });

        //BEZIER CURVE
        function factorial(n){
            var result=1;
            for(i=2;i<=n;i++){
                result = result*i;
            }
            return result;
        }
        function Point(x,y){
            this.x=x;
            this.y=y;
        }
        var points = new Array();
        function getPoint(t){
            var i;
            var x=points[0].x;
            var y=points[0].y;
            var factn = factorial(n);
            for(i=0;i<n;i++){
                var b = factn / (factorial(i)*factorial(n-i));
                var k = Math.pow(1-t,n-i)*Math.pow(t,i);
                // console.debug( i+": ",points[i] );
                x += b*k*points[i].x;
                y += b*k*points[i].y;
            }
            return new Point(x, y);
        }
        //--BEZIER CURVE
        submit.addEventListener("click",function(){
            if(n){
                for(i=1;i<=n;i++){
                    var px = document.getElementById("x_"+i);
                    var py = document.getElementById("y_"+i);
                    points.push(new Point(parseInt(px.value,10),parseInt(py.value,10)));
                    // console.debug( points[i-1] );
                }
                var canvas = document.getElementById('myCanvas');
                var context = canvas.getContext('2d');
                context.beginPath();
                console.debug( points[0].x, points[0].y );
                context.moveTo(points[0].x, points[0].y);
                var t=0.01;
                while (t<=1)
                {
                    //get coordinates at position
                    var p=getPoint(t);
                    // console.debug( p.x, p.y );
                    //draw line to coordinates
                    context.lineTo(p.x, p.y);
                    //increment position
                    t += 0.01;
                }
                context.stroke();
            }
        });
    }
</script>

问题是它似乎不能正常工作。我现在测试了2个点，它总是从画布的左上角开始(即使我的第一个点是(250,250)，并且线的长度不是应该的像素数)。这是更长的时间。