How to drape Slope Ramp on User added Terrain Provider?

1. A concise explanation of the problem you’re experiencing.

When I add my own DTM, the slope covers the defined DTM and also the part of ellipsoid that does not have DTM data. How to restrict the slope material to user added DTM rectangle only?

2. A minimal code example. If you’ve found a bug, this helps us reproduce and repair it.

3. Context. Why do you need to do this? We might know a better way to accomplish your goal.

4. The Cesium version you’re using, your operating system and browser.

The slope and aspect should work only on terrain, not on ellipsoid. These components have meaning over the elevated points, whereas ellipsoid will be flat and lack elevation heights, all around the ellipsoid surface.