在Cesium.js中围绕顶点旋转锥体

参考资料得知可以设置偏移量, 有没有办法计算任何滚动值的正确偏移量？
我还想在旋转锥体时延长锥体的长度，这样当它旋转30度时，锥体的底部仍然会在那个方向上到达地面，而顶点仍然保持在原来的位置，我不知道这有多可行。

var viewer = new Cesium.Viewer('cesiumContainer');

var position = Cesium.Cartesian3.fromDegrees(-75, 40, 90); 

//Original, non-rotated cone for comparison.
viewer.entities.add(new Cesium.Entity({
    position: position,
    point: {
        color: Cesium.Color.YELLOW,
        show: true,
        pixelSize: 20
    },
    cylinder: {
        topRadius: 0,
        bottomRadius: 45,
        length: 180,
        material: Cesium.Color.YELLOW.withAlpha(0.5)
    }
}));

var heading = Cesium.Math.toRadians(0.0);
var pitch = Cesium.Math.toRadians(0.0);
var roll = Cesium.Math.toRadians(90.0);
var hpr = new Cesium.HeadingPitchRoll(heading, pitch, roll);

//Create a rotation
var orientation = Cesium.Transforms.headingPitchRollQuaternion(position, hpr);

// offset the rotation so it's rotating from the apex of the cone, instead of the centre
var offset = new Cesium.Cartesian3(0, 90, 90);

//Create a transform for the offset.
var enuTransform = Cesium.Transforms.eastNorthUpToFixedFrame(position);

//Transform the offset
Cesium.Matrix4.multiplyByPointAsVector(enuTransform, offset, offset);

//Add the offset to the original position to get the final value.
Cesium.Cartesian3.add(position,  offset,  position);

viewer.entities.add(new Cesium.Entity({
    position: position,
    orientation: orientation,
    point: {
        color: Cesium.Color.YELLOW,
        show: true,
        pixelSize: 20
    },
    cylinder: {
        topRadius: 0,
        bottomRadius: 45,
        length: 180,
        material: Cesium.Color.YELLOW.withAlpha(0.5)
    }
}));

viewer.zoomTo(viewer.entities);