JS ellipse 转 PathData

绘制Path

function ellipse2path(cx, cy, rx, ry, degree) {
    //cx cy:圆心
    //rx ry:x y 轴长
    //degree:度数，顺时针方向为正
    //思路：求出直线与椭圆的交点，然后从一端绘制到另一端
    //非数值单位计算，如当宽度像100%则移除 
    if (isNaN(cx - cy + rx - ry))
        return;
    //逆时针绘制
    var radian = (Math.PI * 2 / 360) * degree
    var x1 = cx + rx * Math.cos(radian)
    var y1 = cx + rx * Math.sin(radian) //计算机Y轴正向向下
    var x2_relative = 2 * rx * Math.cos(radian)
    var y2_relative = -2 * rx * Math.sin(radian)

    var path = 'M' + x1 + ' ' + y1 +
        'a' + rx + ' ' + ry + ' ' + degree + ' 1 1 ' + x2_relative + ' ' + y2_relative + ' ' +
        'a' + rx + ' ' + ry + ' ' + degree + ' 1 1 ' + -1 * x2_relative + ' ' + -1 * y2_relative + ' ' + 'z';
    return path;
}