OpenGL find distance to a point

I have a virtual landscape with the ability to walk around in first-person. I want to be able to walk 开发者_如何学Goup any slope if it is 45 degrees or less. As far as I know, this involves translating your current position out x units then finding the distance between the translated point and the ground. If that distance is x units or more, the user can walk there. If not, the user cannot. I have no idea how to find the distance between one point and the nearest point in the negative y direction. I have programmed this in Java3D, but I do not know how to program this in OpenGL.

Barking this problem at OpenGL is barking up the wrong tree: OpenGL's sole purpose is drawing nice pictures to the screen. It's not a math library!

Depending you your demands there are several solutions. This is how I'd tackle this problem: The normals you calculate for proper shading give you the slope of each point. Say your heightmap (=terrain) is in the XY plane and your gravity vector g = -Z, then the normal force is terrain_normal(x,y) · g. The normal force is, what "pushes" your feet against the ground. Without sufficient normal force, there's not enough friction to convey your muscles force into a movement perpendicular to the ground. If you look at the normal force formula you can see that the more the angle between g and terrain_normal(x,y) deviates, the smaller the normal force.

So in your program you could simply test if the normal force exceeds some threshold; correctly you'd project the excerted friction force onto the terrain, and use that as acceleration vector.

If you just have a regular triangular hightmap you can use barycentric coordinates to interpolate Z values from a given (X,Y) position.