How do I project lines dynamically on to 3D terrain?

Question

I'm working on a game in XNA for Xbox 360. The game has 3D terrain with a collection of static objects that are connected by a graph of links. I want to draw the links connecting the objects as lines projected on to the terrain. I also want to be able to change the colors etc. of links as players move their selection around, though I don't need the links to move. However, I'm running into issues making this work correctly and efficiently.

Some ideas I've had are:

1) Render quads to a separate render target, and use the texture as an overlay on top of the terrain. I currently have this working, generating the texture only for the area currently visible to the camera to minimize aliasing. However, I'm still getting aliasing issues -- the lines look jaggy, and the game chugs frequently when moving the camera EDIT: it chugs all the time, I just don't have a frame rate counter on Xbox so I only notice it when things move.

2) Bake the lines into a texture ahead of time. This could increase performance, but makes the aliasing issue worse. Also, it doesn't let me dynamically change the properties of the lines without much munging.

3) Make geometry that matches the shape of the terrain by tessellating the line-quads over the terrain. This option seems like it could help, but I'm unsure if I should spend time trying it out if there's an easier way.

Is there some magical way to do this that I haven't thought of? Is one of these paths the best when done correctly?

Answer 1

Your 1) is a fairly good solution. You can reduce the jagginess by filtering -- first, make sure to use bilinear sampling when using the overlay. Then, try blurring the overlay after drawing it but before using it; if you choose a proper filter, it will remove the aliasing.

If it's taking too much time to render the overlay, try reducing its resolution. Without the antialiasing filter, that would just make it jaggier, but with a good filter, it might even look better.

I don't know why the game would chug only when moving the camera. Remember, you should have a separate camera for the overlay -- orthogonal, and pointing down onto the terrain.

Answer 2

Does XNA have a shadowing library? If so, yo could just pretend the lines are shadows.