Thread: Need Help With Terrain Mesh Collisions

The answer: If you want realism, you have to code it.

The naive way would be to just put the viewer at the height of the nearest vertex to them. That needs no calculations but will result in jumpy step-like movement up and down as they traverse different heights.

There's an intermediate way of calculating the intersection of a line and a plane that needs quite good knowledge of matrix maths.

The proper way is to expand the problem to the collision of two objects and have a primitive collision engine resolve the difference for you.

Either way, it's a lot more than can be answered in a small post. Try the usual sources of OpenGL tutorials (e.g. NeHe Productions: Collision Detection) and look around at the maths behind what OpenGL is actually doing all the time. I can also recommend the book "Physics for Game Developers" which covers things like this as well as making basic physics engines in C++.


Personally, for a quick "cheap" answer, I'd find the nearest three/four vertex heights of the ground and average them in a quick-cheap calculation that, if I could be bothered, would average them biased towards the nearest one. Chances are it would be fine for 99% of things, but the proper way is to learn about plane intersection matrix maths.

You need to determine the height at px,py within cell C. You can do this by determining the triangle the player is in. Each quad is composed of two triangles. Once you figure out which triangle the player is in you can the perform a bilinear interpolation on the heights (usually y value) of the vertices. Place the player at y + some_height where some_height is the distance from the player's eyes to his feet and you will be able to walk over the terrain. Note that a more advanced version of this will pitch the player's view to match the slope they are on. As well the slope should be taken into account when moving the player around.

You can download source code for this from the web site for several books by Frank Luna. In the books he explains how to do exactly what you want.

http://www.d3dcoder.net/

Don't worry about the underlying API. The concept is the same in Direct3D as it is in OpenGL. The only difference is the 'handedness' of the coordinate system and the API calls you might make.