Question about drawing an ellipse and about cartesian3 angles

CesiumJS
1

Question 1:

In Cesium.Shapes there is a static function named "computeEllipseBoundary". One of the params it takes is "rotation". I understand what this value is, but I'm not sure how to get it without hard-coding.

I want the rotation value to change depending on where my mouse is at a given time. For example, if my mouse is at a 45 degree angle on the 3D globe, I want to know that. I'm trying to constantly re-draw an ellipse until I release the left click on the mouse, so I need my params to constantly update until I'm finished drawing the ellipse.

Question 2:

In Cesium.Cartesian3 there is a static function named "angleBetween". The documentation says it returns "the angle between the Cartesians". What does that mean? What angle? I was hoping it would return a value I could use as the "rotation" in my first question, but it doesn't look like that is what it's trying to do.

Any help on either question is much appreciated!

2

In Cesium an ellipse is drawn along a line of longitude that passes through the center point.
Noting that positive x points East and positive y points North you can do something like this which is a modification of the Ellipse example in SandCastle.

// Green ellipse with height
// Default orientation of ellipse is along north/south
// Remember that positive x is East and positive y is North
var north = Cesium.Cartesian3.fromArray([0.0, 1.0, 0.0]);
var mouse = Cesium.Cartesian3.fromArray([1.0, 1.0, 0.0]);
var rotationAngle = Cesium.Cartesian3.angleBetween(north, mouse);

ellipseGeometry = new Cesium.EllipseGeometry({
    center : ellipsoid.cartographicToCartesian(Cesium.Cartographic.fromDegrees(-95.0, 35.0)),
    semiMinorAxis : 200000.0,
    semiMajorAxis : 400000.0,
    rotation : rotationAngle,
    height: 200000.0,
    vertexFormat : Cesium.PerInstanceColorAppearance.VERTEX_FORMAT
});