Vector3

• cross() overwrites the vector with the cross product, while dot() returns a number.
• distanceToSquared() avoids a square root when only comparing distances.
• project() and unproject() convert between world and normalized device coordinates.

const a = new THREE.Vector3(1, 2, 3);
const b = new THREE.Vector3(4, 0, -2);
const mid = new THREE.Vector3().addVectors(a, b).multiplyScalar(0.5);
const offset = new THREE.Vector3().subVectors(b, a);
const dist = offset.length();
const dir = offset.normalize();   // 注意：normalize 会改写 offset

mesh.position.copy(mid);
mesh.lookAt(b);

const R = 6371000;   // 地球平均半径，单位米

function lonLatToVector3(lon, lat, radius) {
  const phi = THREE.MathUtils.degToRad(90 - lat);
  const theta = THREE.MathUtils.degToRad(lon);

  return new THREE.Vector3(
    radius * Math.sin(phi) * Math.cos(theta),
    radius * Math.cos(phi),
    radius * Math.sin(phi) * Math.sin(theta)
  );
}

const beijing = lonLatToVector3(116.39, 39.9, 1);

• Assuming Vector3 picks a unit: choose meters or degrees and never mix scales in one scene.
• equals() compares exact floats, so near-identical results may report false.