Understanding Entity's `orientation` property

CesiumJS
1

I've been playing with Entities and orientation lately, and I have what I believe is a simple question.

I've modified the Interpolation example to draw the "basis" of the Entity as "long" light red, blue and green lines from the entity's position. As well, I modified it to draw "short" lines along the x, y and z axis. Finally, I've added a button to toggle the orientation property of the entity on and off (the calculated value and undefined).

I've attached an capture of the modified example. [1] First, I toggle (on, off, on) drawing the reference frame for the Entity's primitive. These match the basis (the "long" lines, that they align with in the demo), but when I toggle the orientation property off (you see the airplane change orientation), they don't match the drawn X, Y and Z vectors (you can see the reference frame no longer aligns).

Is the default orientation for an Entity (Primitive) not the identity rotation? I must be missing something as this is not what I was expecting. Is the orientation not referenced against the ECF frame? Could someone explain to me how this is working?

Thanks!

cap.gif

2

The default orientation on an entity when undefined is East-North-Up.

https://github.com/AnalyticalGraphicsInc/cesium/blob/master/Source/DataSources/Entity.js#L566

https://github.com/AnalyticalGraphicsInc/cesium/blob/master/Source/Core/Transforms.js#L74

3

Thanks for the links, Scott. I added a GridImageryProvider to the globe so I could get a better sense for the center of the Earth, and it made everything very clear. This is with the "identity rotation" as the orientation of the Entity (that is, no rotation), and it's plainly visible that orientation is referenced against the earth-centered/earth-fixed frame; the X and Y axes (green and red) enters and exit at the same latitude.