What you are asking is to use the DeepZoom control as a “dynamic texture” to feed the Viewport3D (representing round ground). This is an interesting thought, but I don’t think it will work.
Viewport3D will display the deepzoom control to display with uniform resolution throughout, and then use the resulting texture. If you zoom in to Europe, for example, depth control will also be offered to draw Australia, with the same resolution, even if they are on the other side of the Earth. Theoretically, viewport3D could calculate the enclosing box of all visible graphic elements, but in typical scenarios for Viewport3D this will not improve performance in a noticeable way, so I doubt that they will. This is not entirely trivial.
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