如何将实体或几何图形剪裁到地形的底面

我想隐藏这一点的一种方法是将“土壤”渲染为地形下的一个长方体或墙,但我需要长方体或墙的顶部来确认地形几何(红线)。 我想我可以通过使用 WallGeometry 在剔除的地形周围而不是一个长方体,并根据 sampleTerrain 打电话来。

Cesium Sandcastle link

var viewer = new Cesium.Viewer('cesiumContainer', {
        skyAtmosphere: false,
        shouldAnimate : true,
        terrainProvider: Cesium.createWorldTerrain()
    });
var globe = viewer.scene.globe;

var position = Cesium.Cartographic.toCartesian(new Cesium.Cartographic.fromDegrees(-113.2665534, 36.0939345, 100));
var distance = 3000.0;

globe.clippingPlanes = new Cesium.ClippingPlaneCollection({
    modelMatrix : Cesium.Transforms.eastNorthUpToFixedFrame(position),
    planes : [
        new Cesium.ClippingPlane(new Cesium.Cartesian3( 1.0,  0.0, 0.0), distance),
        new Cesium.ClippingPlane(new Cesium.Cartesian3(-1.0,  0.0, 0.0), distance),
        new Cesium.ClippingPlane(new Cesium.Cartesian3( 0.0,  1.0, 0.0), distance),
        new Cesium.ClippingPlane(new Cesium.Cartesian3( 0.0, -1.0, 0.0), distance)
    ],
    unionClippingRegions : true,
            edgeWidth:3,
    edgeColor: Cesium.Color.RED,
    enabled : true
});
var rockBox = viewer.entities.add({
    name : 'RockBox',
    position: Cesium.Cartesian3.fromDegrees(-113.2665534, 36.0939345, 900),
    box : {
        dimensions : new Cesium.Cartesian3(distance*2, distance*2,800.0),
        material : Cesium.Color.GRAY.withAlpha(0.4),
        outline : true,
        outlineColor : Cesium.Color.GRAY
    }
});
var waterTable = viewer.entities.add({
    name : 'WaterTable',
    position: Cesium.Cartesian3.fromDegrees(-113.2665534, 36.0939345, 440),
    box : {
        dimensions : new Cesium.Cartesian3(distance*2, distance*2, 160.0),
        material : Cesium.Color.BLUE.withAlpha(0.9),
        outline : true,
        outlineColor : Cesium.Color.BLUE
    }
});

viewer.zoomTo(viewer.entities);