cesium中有好多种物体拾取，在这里记录一下，方便查阅。

1、viewer.scene.pick(windowPosition);

　　官网介绍：

Returns an object with a `primitive` property that contains the first (top) primitive in the scene at a particular window coordinate or undefined if nothing is at the location. 
Other properties may potentially be set depending on the type of primitive and may be used to further identify the picked object.
When a feature of a 3D Tiles tileset is picked, pick returns a Cesium3DTileFeature object.

返回scene中指定位置的顶端的primitive属性的的一个对象。
适用于选取3D tiles，改变3Dtiles的属性，比如颜色等。