cubicBezier.js

class CubicBezier {
  constructor() {
    const precision = 100;
    this.controlPoints = [0, 0, 0, 0];
    this.coords = this.getCoordsArray(precision);
  }
  setControlPoints(x1, y1, x2, y2) {
    this.controlPoints = [x1, y1, x2, y2];
  }
  getCoord(t) {
    // 如果t取值不在0到1之间，则终止操作
    if (t > 1 || t < 0) return;
    const _t = 1 - t;
    const [x1, y1, x2, y2] = this.controlPoints;
    const coefficient1 = 3 * t * Math.pow(_t, 2);
    const coefficient2 = 3 * _t * Math.pow(t, 2);
    const coefficient3 = Math.pow(t, 3);
    const px = coefficient1 * x1 + coefficient2 * x2 + coefficient3;
    const py = coefficient1 * y1 + coefficient2 * y2 + coefficient3;
    // 结果只保留三位有效数字
    return [parseFloat(px.toFixed(3)), parseFloat(py.toFixed(3))];
  }

  getCoordsArray(precision) {
    const step = 1 / (precision + 1);
    const result = [];
    for (let t = 0; t <= precision + 1; t++) {
      result.push(this.getCoord(t * step));
    }
    this.coords = result;
    return result;
  }

  getY(x) {
    if (x >= 1) return 1;
    if (x <= 0) return 0;
    let startX = 0;
    for (let i = 0; i < this.coords.length; i++) {
      if (this.coords[i][0] >= x) {
        startX = i;
        break;
      }
    }
    const axis1 = this.coords[startX];
    const axis2 = this.coords[startX - 1];
    const k = (axis2[1] - axis1[1]) / (axis2[0] - axis1[0]);
    const b = axis1[1] - k * axis1[0];
    // 结果也只保留三位有效数字
    return parseFloat((k * x + b).toFixed(3));
  }
}