Mapping textures to DirectX sphere?

Question

I am writing a virtual globe using DirectX similar to Google Earth. So far, I have completed tessellation, and have tested with a wrapped texture over the entire sphere, which was successful. I have written the texture coordinates to correspond with the latitude and longitude (90lat,-180lon = 0,0 and -90lat,180lon = 1,1).

For this project, I need to layer several image tiles over the sphere. For example, 8 images spanning 90 degrees by 90 degrees. These tiles may dynamically update (i.e. tiles may be added or removed as you pan around). I have thought about using a render target view and drawing the tiles directly to that, but I'm sure there is a better way.

How would I go about doing this? Is there a way to set the texture to only span a specific texture coordinate space? I.e. from (0, 0) to (0.25, 0.5)?

Answer 1

There are three straight-forward solutions (and possibly many more sopisticated ones).

1. You can create geometry that matches the part of the sphere covered by a tile and draw those subsequently, setting the correct texture before each draw call (if the tiles are laid out in a simple way, you can also generate this geometry using instancing and a single draw call).
2. You can write a pixel shader that evaluates the texture coordinates and chooses the appropriate texture using transformed texture coordinates.
3. Render all textures to a big texture and use that to render the sphere. Whenever a tile changes, bind the big texture as a render target and draw the new tile on top of it.