Cesium 使用primitive绘制几何图形，相机拉近（视野放大），几何图形破损显示不完整

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分析：此原因是因为地形导致，可以尝试将primitive设置高程（高度）如5000m，此时图形位置由于高于山峰等地形，图形可以显示完整。

但是如果几何图形必须要贴地（高度为0m）呢？

解决方法：

1.设置scene.globe.depthTestAgainstTerrain = false;即不进行地形深度测试，也就忽略了地形。这时可以正常显示几何图形。

2.使用GroundPrimitive。顾名思义，这是帮助你将几何图形贴地的类。

scene.primitives.add(new Cesium.GroundPrimitive({

		geometryInstances : instances,
		appearance : new Cesium.PerInstanceColorAppearance()
	}));

这里给出官方的解释：

A ground primitive represents geometry draped over the terrain in the Scene. The geometry must be from a single GeometryInstance. Batching multiple geometries is not yet supported.

A primitive combines the geometry instance with an Appearance that describes the full shading, including Material and RenderState. Roughly, the geometry instance defines the structure and placement, and the appearance defines the visual characteristics. Decoupling geometry and appearance allows us to mix and match most of them and add a new geometry or appearance independently of each other. Only the PerInstanceColorAppearance is supported at this time.

For correct rendering, this feature requires the EXT_frag_depth WebGL extension. For hardware that do not support this extension, there will be rendering artifacts for some viewing angles.

Valid geometries are CircleGeometry, CorridorGeometry, EllipseGeometry, PolygonGeometry, and RectangleGeometry.

这里给出官方demo:

var rectangleInstance = new Cesium.GeometryInstance({
  geometry : new Cesium.RectangleGeometry({
    rectangle : Cesium.Rectangle.fromDegrees(-140.0, 30.0, -100.0, 40.0)
  }),
  id : 'rectangle',
  attributes : {
    color : new Cesium.ColorGeometryInstanceAttribute(0.0, 1.0, 1.0, 0.5)
  }
});
scene.primitives.add(new Cesium.GroundPrimitive({
  geometryInstances : rectangleInstance
}));