zh/builder/src/zrender/graphic/shape/Ellipse.js

/**
 * 椭圆形状
 * @module zrender/graphic/shape/Ellipse
 */
import Path from '../Path';
export default Path.extend({
  type: 'ellipse',
  shape: {
    cx: 0,
    cy: 0,
    rx: 0,
    ry: 0
  },
  buildPath: function (ctx, shape) {
    var k = 0.5522848;
    var x = shape.cx;
    var y = shape.cy;
    var a = shape.rx;
    var b = shape.ry;
    var ox = a * k; // 水平控制点偏移量

    var oy = b * k; // 垂直控制点偏移量
    // 从椭圆的左端点开始顺时针绘制四条三次贝塞尔曲线

    ctx.moveTo(x - a, y);
    ctx.bezierCurveTo(x - a, y - oy, x - ox, y - b, x, y - b);
    ctx.bezierCurveTo(x + ox, y - b, x + a, y - oy, x + a, y);
    ctx.bezierCurveTo(x + a, y + oy, x + ox, y + b, x, y + b);
    ctx.bezierCurveTo(x - ox, y + b, x - a, y + oy, x - a, y);
    ctx.closePath();
  }
});