Terrain Cursor

Has anybody used FreeWorld3D (FreeWorld3D.org)?

It is a program with potential. Anyway the author uses a “cursor” that rolls over the terrain as you move it. The cursor is comprised of two circles, an inner-outer pair. The two circles follow the contour of the terrain pretty good.

I was wondering if anybody could tell me how the author accomplished this?

I wanted to include an image here, but I don’t know how (sorry).

Thanks in advance for your help,


There are many ways to do it … of the top of my head one would be to use projective texture mapping and another to have a hovering subdivided quad, manually updated on each frame to follow the ground shape.


I thought about the texture approach too, but the “cursor” does not look like one.

And the “hovering quad” is basically the strategy I am employing now. However, sometimes the terrain is above the item I am rendering.

Basically I am looking for a surefire way of rendering vector data over topology. From what I observed with the “cursor” it seems to match exactly.


If you are talking about this circle then it is clearly s subdivided set of lines which is pretty much the same approach as with the hovering quad.

That’s it.

And it does look like subdivided lines. Place the cursor farther back in that image where the slope is really varied, and it hugs the terrain pretty good.

When I was looking at it, the divisions are not necessarily are 1-to-1 match. Yet the cursor is never hidden by the terrain.

That’s because they probably calculate exact slope ( and height) of the terrain at each end of line segment and adjust accordingly.

Given enough segments and a reasonable offset from the ground , you should never see clipping.