constructor(canvas, tensorFlow) {
    this.canvas = canvas;
    tf = tensorFlow;

    // Min and max of circles' radii.
    this.CIRCLE_RADIUS_MIN = 5;
    this.CIRCLE_RADIUS_MAX = 20;
    // Min and max of rectangle side lengths.
    this.RECTANGLE_SIDE_MIN = 40;
    this.RECTANGLE_SIDE_MAX = 100;
    // Min and max of triangle side lengths.
    this.TRIANGLE_SIDE_MIN = 50;
    this.TRIANGLE_SIDE_MAX = 100;

    // Canvas dimensions.
    this.w = this.canvas.width;
    this.h = this.canvas.height;
  }
Constructor of ObjectDetectionImageSynthesizer.

@param {} canvas An HTML canvas object or node-canvas object.
@param {*} tensorFlow A tensorflow module passed in. This done for
  compatibility between browser and Node.js.