GIS 算法，计算箭头

根据两个点，计算箭头。

                            var x2 = points[length - 1].x;
                            var y2 = points[length - 1].y;
                            var x1 = points[length - 2].x;
                            var y1 = points[length - 2].y;

                            var distance = Math.sqrt(Math.pow((x2 - x1),2) + Math.pow((y2 - y1),2));
                            var yb = y2 - s * (x2 - x1) / (Math.SQRT2 * distance) + s * (y1 - y2) / (Math.SQRT2 * distance);
                            var xb = x2 - s * (x2 - x1) / (Math.SQRT2 * distance) - s * (y1 - y2) / (Math.SQRT2 * distance);
                            var ya = y2 + s * (x2 - x1) / (Math.SQRT2 * distance) + s * (y1 - y2) / (Math.SQRT2 * distance);
                            var xa = x2 - s * (x2 - x1) / (Math.SQRT2 * distance) + s * (y1 - y2) / (Math.SQRT2 * distance);


                            feature.geometry.components.push(new OpenLayers.Geometry.Point(xa, ya));
                            feature.geometry.components.push(new OpenLayers.Geometry.Point(x2, y2));
                            feature.geometry.components.push(new OpenLayers.Geometry.Point(xb, yb));
                            /*
                            var x1 = points[length - 1].x;
                            var y1 = points[length - 1].y;
                            var x2 = points[length - 2].x;
                            var y2 = points[length - 2].y;
                            if (x2 != x1) {
                            var k = (y2 - y1) / (x2 - x1);
                            if (k > 0) {
                            if (x2 > x1) {//1象限
                            var angle = Math.PI / 2 - Math.atan(k);
                            var angleL = angle - Math.PI / 4;
                            if (angleL < 0) {
                            angleL += 2 * Math.PI;
                            }
                            var angleR = angle + Math.PI / 4;
                            }
                            else {//3
                            var angle = 1.5 * Math.PI - Math.atan(k);
                            var angleL = angle - Math.PI / 4;
                            var angleR = angle + Math.PI / 4;
                            }
                            } else {
                            if (y2 > y1) {//4
                            var angle = 1.5 * Math.PI + Math.atan(-k);
                            var angleL = angle - Math.PI / 4;
                            var angleR = angle + Math.PI / 4;
                            if (angleR > 2 * Math.PI) {
                            angleR -= 2 * Math.PI;
                            }
                            }
                            else {//2
                            var angle = 0.5 * Math.PI + Math.atan(-k);
                            var angleL = angle - Math.PI / 4;
                            var angleR = angle + Math.PI / 4;
                            }
                            }
                            } else {
                            var angle = 0;
                            var angleL = angle - Math.PI / 4;
                            var angleR = angle + Math.PI / 4;
                            }
                            var xn1 = x1 + s * Math.sin(angleL);
                            var yn1 = y1 + s * Math.cos(angleL);
                            var xn2 = x1 + s * Math.sin(angleR);
                            var yn2 = y1 + s * Math.cos(angleR);
                            feature.geometry.components.push(new OpenLayers.Geometry.Point(xn1, yn1));
                            feature.geometry.components.push(new OpenLayers.Geometry.Point(x1, y1));
                            feature.geometry.components.push(new OpenLayers.Geometry.Point(xn2, yn2));*/