(转)画图绘制虚线

 //function kindly provided by phrogz at:  
//http://stackoverflow.com/questions/4576724/dotted-stroke-in-canvas 
       var CP = window.CanvasRenderingContext2D && CanvasRenderingContext2D.prototype;
        if (CP && CP.lineTo) {
            CP.dashedLine = function (x, y, x2, y2, dashArray) {
                if (!dashArray) dashArray = [10, 5];//虚线长度，虚线间隔
                var dashCount = dashArray.length;
                this.moveTo(x, y);
                var dx = (x2 - x), dy = (y2 - y);
                var slope = dy / dx;
                var distRemaining = Math.sqrt(dx * dx + dy * dy);
                var dashIndex = 0, draw = true;
                while (distRemaining >= 0.1) {
                    var dashLength = dashArray[dashIndex++ % dashCount];
                    if (dashLength > distRemaining) dashLength = distRemaining;
                    var xStep = Math.sqrt(dashLength * dashLength / (1 + slope * slope));

                    var signal = (x2 > x ? 1 : -1);

                    x += xStep * signal;
                    y += slope * xStep * signal;
                    this[draw ? 'lineTo' : 'moveTo'](x, y);
                    distRemaining -= dashLength;
                    draw = !draw;
                }
            }
        }