// create an engine

  engine = Engine.create();

  engine.world.gravity = 0.098;

​

  // 設定

  let group = Body.nextGroup(true);

  let particleOption = Common.extend({ inertia: Infinity, friction: 0.00001, collisionFilter: { group: group }});

  let constraintOption = Common.extend({stiffness: 1});

​

  // Create the net structure by using constraints to connect particles

  cloth = Composites.stack(xx, yy, numx, numy, gapx, gapy, 

      (x, y) => {

        return Bodies.circle(x, y, particleR, particleOption);  // This creates the particles (the points of the net)

    });

​

  // Convert the particles into constraints (lines)

  for (let i = 0; i < numx; i++) {

    for (let j = 0; j < numy; j++) {

      if (i < numx - 1) {

        let constraintX = Constraint.create({

          bodyA: cloth.bodies[i + j * numx],

          bodyB: cloth.bodies[i + 1 + j * numx],

          stiffness: 0.2,

        });

        Composite.add(engine.world, constraintX);

      }

      if (j < numy - 1) {

        let constraintY = Constraint.create({

          bodyA: cloth.bodies[i + j * numx],

          bodyB: cloth.bodies[i + (j + 1) * numx],

          stiffness: 0.2,

        });

        Composite.add(engine.world, constraintY);

      }

    }

  }

​

  // Add the cloth structure to the world

  Composite.add(engine.world, cloth);

  // 固定上方兩端小單元

  cloth.bodies[0].isStatic = true;

  cloth.bodies[numx - 1].isStatic = true;

​

  // ===========================

  // Create rotating beam (unchanged)

  let rbc = {x:width * 0.5, y:height * 0.5};

  rotbeam = Bodies.rectangle(rbc.x, rbc.y, width * 0.2, 5);

  let c = Constraint.create({

    pointA: rbc,

    bodyB: rotbeam,

    length: 0,

  });

  Composite.add(engine.world, [rotbeam, c]);

​

  // ===========================

  // Add mouse control (unchanged)

  var mouse = Mouse.create(canvas.elt);

  mouse.pixelRatio = pixelDensity(); 

  var mConstraint = MouseConstraint.create(engine, {

    mouse: mouse,

    constraint: { angularStiffness: 0, stiffness: 0.2 },

  });

  Composite.add(engine.world, mConstraint);

​

  // Create the engine runner (unchanged)

  runner = Runner.create();

}

​