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So here's what I'm trying to do. I have a terrain, various heights yada yada. When I click and drag the mouse, I am trying to place what kind of looks like a zone from simcity 3, 4 and so forth. My question is, how do I map/position the plane/zone object so that it conforms to the surface of the terrain? I'm just looking for hypotheticals and methods here, I want to write the code myself.

There are a number of ways to achieve it.

1) Subdivided plain which wraps over the terrain as you drag
2) Edit diffuse values of the terrain vertices (works well if your terrain has a consistently high poly count)
3) Use a decal shader to draw another texture the area you're dragging
4) Just use a simple plain but disable z-depth (flat terrains would work the best with this method)

You probably already know to use pick object to get the mouse coordinates on the terrain.

TheComet

Here's a more in-depth follow-up.

(Oh and yes, I use the pick commands with Sparky's for better terraforming of the terrain)

What I was thinking about doing is use objects to simulate the vertices of the zone plane. I would draw the poly's to these locations. So say I have a huge zone I'm drawing over a mountain top. If there are numerous locations that are the same height, then the poly over that area would match the size. Think of it like a line of objects at varying heights to the north, south, east and west edges of the zone plane. Then within this "box" of vertices, when a height is less than or greater than the draw area, another vertex object is placed. The result would be a plane object that is just slightly higher than the terrain (a hard-forced z bias would make this look nice) This plane would only be viewable while it's drawn and then would be hidden. It would be re-shown if the user wishes to just see the zones.

EDIT - the trouble with this method is, if the terrain is terraformed, the old zone would need to be updated. Terraformed areas in the zone would delete the vertex object locations and keep the remaining ones, deleting the old polys. The result would be a zone with a hole in it - which works.

I've looked into decals, but haven't found one that I adequetley understand. I haven't even figured out how to make different textures on the terrain appear without killing the frame rate. Shaders nearly kill it, and trying to memblock draw on a texture is nice, but I can't scale the textures prior to painting or post, using that method. So for now, I am working on what I can surely do.

What I need to know how to do though, is ideally draw the thing, store the locations of the vertex objects per zone and have it all in a function that allows me to color the zone plane and specify what kind of objects are spawned within it.

@TheComet: Is there an inbuilt decal function available in Dark Basic Pro ? How do do they draw the decals in FPSC ?