在Autodesk Forge Viewer中重新计算边界框可以使用以下代码示例：

function computeBoundingBox(viewer, model) {
  const modelInstanceTree = model.getData().instanceTree;
  const rootId = modelInstanceTree.getRootId();

  let boundingBox = new THREE.Box3();
  const fragIds = [];

  modelInstanceTree.enumNodeFragments(rootId, (fragId) => {
    fragIds.push(fragId);
  });

  fragIds.forEach((fragId) => {
    const fragBoundingBox = new THREE.Box3();
    modelInstanceTree.getWorldBounds(fragId, fragBoundingBox);
    boundingBox.union(fragBoundingBox);
  });

  const min = boundingBox.min;
  const max = boundingBox.max;

  const worldMin = new THREE.Vector3(min.x, min.y, min.z);
  const worldMax = new THREE.Vector3(max.x, max.y, max.z);

  const worldSize = worldMax.clone().sub(worldMin);

  const center = worldMin.clone().add(worldSize.clone().multiplyScalar(0.5));

  const size = Math.max(worldSize.x, worldSize.y, worldSize.z);

  const newBoundingBox = new THREE.Box3(
    new THREE.Vector3(center.x - size / 2, center.y - size / 2, center.z - size / 2),
    new THREE.Vector3(center.x + size / 2, center.y + size / 2, center.z + size / 2)
  );

  viewer.impl.invalidate(true, true, true);

  return newBoundingBox;
}

const viewer = new Autodesk.Viewing.Private.GuiViewer3D(document.getElementById('viewer'));
const options = {
  env: 'AutodeskProduction',
  getAccessToken: getForgeToken,
};
Autodesk.Viewing.Initializer(options, () => {
  viewer.start();
  viewer.loadModel('your-model-url', (model) => {
    const boundingBox = computeBoundingBox(viewer, model);
    console.log('New Bounding Box:', boundingBox);
  });
});

请确保在上述代码中将'your-model-url'替换为您要加载的模型的URL。

这段代码通过遍历模型的所有碎片（fragments）并获取其世界边界框（world bounds），然后合并所有碎片的边界框以计算出新的边界框。最后，它使用新的边界框更新了Viewer的显示，并返回计算出的新边界框对象。

请注意，此代码示例假设您已经通过合适的方式（例如使用Forge OAuth认证）获取了访问令牌（access token）。如果您还没有设置获取访问令牌的函数getForgeToken，则需要相应地实现它。