如何使用 Cesium 中的可用性间隔定义多边形实体？

Question

I am defining a realy huge polygon that changes its position and shape using inertial reference frame.

First, I tryed to define a set of CZML polygons, each one with an availability and its coordinates like the "california" object in this sandcastle example , but this exceeds the maximum memory limit and blocks the browser and eventually the computer. Then, I had it working using only one polygon in CZML providing a list with an interval and the polygon shape coordinates, like the "dynamicPolygon" in the same sandcastle example .

Now I am trying to change the CZML implementation to use entities but the entity documentation allows to use a PolygonGraphics that allows to use a Property or a PolygonHierarchy to configure the shape, and I can not figure out how to do the same "dynamicPolygon" using entities.

Answer 1

You can use a CallbackProperty (that is a type of Property ) in the hierarchy constructor option. There should not be issues with memory due to there is only one object and the positions are being calculated dynamically. You also can use a collection like TimeIntervalCollectionProperty and then ask for the value for an specific time.

Try to add the next code fragment in the Hello World Cesium sandcastle :

viewer.entities.add({
    id: "dynamicPolygon",
    name: "dynamicPolygon",
    polygon: new Cesium.PolygonGraphics({
      hierarchy: new Cesium.CallbackProperty(function (time, result) {
        var n = time.secondsOfDay%10;
        result = new Cesium.PolygonHierarchy([
            Cesium.Cartesian3.fromDegrees(2*n, n),
            Cesium.Cartesian3.fromDegrees(-2*n, n),
            Cesium.Cartesian3.fromDegrees(-2*n, -n),
            Cesium.Cartesian3.fromDegrees(2*n, -n)
        ]);
        return result;
      }, false),
      material: Cesium.Color.WHITE
  })
});