I just don't know what to do. Please help me!

Dito.
Could you be more concrete about what you're talking ? Maybe some screenshots ?

Here are some keywords you should goolge for:
- hi-res modelling
- texture baking (hi to low poly)
- displacement mapping
- sculpting
- zbrush
- normal mapping

I suspect that to get exactly what you want, you are going to have to do some scripting. I use Blender, and I know that in Blender you can use the compositing Node system to output the depthmap of a scene pretty easily, but that's maybe not exactly what you want. In Blender you can also bake a displacement map from one object to another, but again that doesn't seem exactly like what you want.

One problem I can see with your approach (or maybe I just mis-understand what you mean) is the assumption that the UV-map "islands" (clusters of faces in the UV unwrapping) each correspond to a view of the object from a particular angle. It is possible, of course, since you can orient the object to a view and project faces when UV-unwrapping, but this isn't really the best way to UV-unwrap an object, since faces that are oblique to the view when projected get very little texture space in the final map regardless of their size, in the worse case even turning "degenerate" (ie, projecting to a line in the UV map, rather than a triangle/quad), which results in severe stretching or distortion when rendered. The best ways to UV-unwrap an object are to unwrap it so that faces get space in the texture map proportionate to their actual size; in this case, then, the projected islands will not correspond to any particular view orientation.

One way of doing what you want might be to write a script that projects a series of rays. The rays are projected as eye or camera rays, using the desired projection/perspective settings. When an intersection is calculated, the UV coordinates of a face are interpolated to find the texture coordinates, then the calculated depth value of the ray intersection is written into the texture map. While this wouldn't really be too difficult to implement as a script, I do not know of any "standard" means by which it can be done without scripting.

But if you are using mapped boxes for LOD primitives, do you really need the baked heightmap to correspond to the UV map?

Maybe I just don't understand exactly how your LOD scheme really works. Maybe a more detailed explanation? I know from your previous posts that it involves a heightmap and a normal map and a 3D box. Are the maps actually applied to the box, or are you guys instead using a sort of beefed up billboard system, where the maps are used to add the illusion of depth to the billboard plane? If this is the case, I wouldn't think your height/normal maps for the billboard would need to correlate to the UV mapping of the object, but again I can't really picture what you are doing.

You wouldn't use a raycast to build the UV map, the best solution for that is just grunt work with a 3D editor. The raycast would just populate a texture image with data. The UV mapping would need to be done already. I just sort of tossed that idea out there, but I have no idea if its even relevant.

For space games often "depth sprites" are used. In fact these are simple billboards + a depth map + normal map . Using a deferred render you could use this depth sprite for much better light interaction.

In this case you can simply render the object to the camera and use 3 render passes: standard texture only pass, a depth map pass(maybe you need to use nodes as JTippetts mentioned it) and a normal map pass (object space?).

To get the billboard texture right use a RGBA target format and turn off the sky to get a proper alpha value for your rendered models.

[attachment=2416:depthnormal.png]


Is this roughly like what you need? On the left is a normal map baked to a plane, in the center is the asteroid, on the right is a depth map baked to a plane. Of course, the depth map would need some post-processing to invert the depth and move the outside regions to black, but that is easy to do in the Gimp or Photoshop.

To make this image, I sculpted a quick asteroid then used Blender's Bake menu to bake an object-space normal map to one plane, and a displacement map to another. You can also bake texture (color) information to a third image, ambient occlusion map to a fourth, and so on.