[gameprogrammer] Re: selection / picking on terrain geometry

Hi Shaun,
That sounds like a quick and effective way of doing it. My terrain is heightmap generated and stored in a grid also (for now anyway). I have figured out how to create the ray from the camera, but was still looking for an optimal solution for searching all of the polys in the terrain for an intersection. I was thinking I might need an octtree, but thats something else I havent done yet :)
If you have some sample code that would be great.

Much appreciated.
Josh

Shaun Simpson wrote: 

Hey Josh ..

for a university project a friend and i divised a test for working out terrain intersection for a 3d rts we were creating, our terrain was in a grid format (stored in a 2d array) with each point containing a height value, we worked out the intersection by translating the camera's coordinates onto the terrain and then firing a ray from the camera, by stepping forward along the ray at grid sized intervals we would check to see if it was lower than the height of the terrain if so that was our intersection tile., it is fairly accurate as long as you have small size tiles.. i still have have the code and can search out and forward the code if you wish, im not sure how applicable it is but for ourselves it was the simpliest solution and very cost effective.

Shaun
shaun_simpson_@xxxxxxxxxxx


From: Josh Stewart <aek@xxxxxxxxx>
Reply-To: gameprogrammer@xxxxxxxxxxxxx
To: gameprogrammer@xxxxxxxxxxxxx
Subject: [gameprogrammer] Re: selection / picking on terrain geometry
Date: Fri, 31 Mar 2006 00:18:39 +0800

I think that I will do this method with the ray/poly intersection test.
I will have to think of a way to limit the testing to only terrain polygons that are actually visible in the window (the terrain isn't split up so non visible terrain isnt being culled atm)

If I choose the method where I read the depth buffer and transform that back to world coordinates, I think that I may have problems clicking on the terrain if there is ever anything drawn over the top of it (trees, particles, units), as the depth buffer only contains the last Z value of the item closest to the camera.

Although maybe I can use some depth buffer hackery with glDepthFunc(GL_GREATER), assuming the terrain is always the furthest thing away, but that will surely mess with my rendering.

Just have to find a good ray / poly intersection test that returns the point of intersection. My maths book has a sample from Graphics Gems I ... I do wonder if there are newer methods though.

Js

Simon Mihevc wrote:
You could do it by searching for an intersection betwen your terrain and a ray. Get the ray by projecting the mouse coordinates onto 2 different z planes, which gives the starting point and the direction vector for the ray. Then find the point on the terrain, that is closest to the ray for various ray lengths.
Finding the point that is nearest to the ray is the problem here since there are a lot of points and there's no time to check them all. A bit of knowledge of linear algebra and algorithms helps here.

I must be working on a project because, thats right folks, I have another question :)
:)

Assuming standard 3D RTS with 3D terrain and a 3rd person 45 degree camera (fixed or not, I havent decided yet)

Using OpenGL selection / picking it seems that selection of units in an RTS game, using a mouse, is relatively easy.
However I am having trouble figuring out the best way to click on a point on the screen where there are no units, and have the game know the point on the terrain, where the user clicked.
The terrain is 1 large piece of geometry that is generated from heightmap and optimised. I cannot just give it a name as clicking anywhere will just return the selection stack with the name of the terrain.
How can I find the point on the terrain where I was directing my click.

An example might be selecting some units and then clicking on an empty space to "move here"

Any help or suggestions, including further reading links to books or sites that tackle the subject, is appreciated.
Thankyou

Josh Stewart



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