Question

I’m making a Minecraft clone as my very first OpenGL project and am stuck at the box selection part. What would be the best method to make a reliable box selection?

I’ve been going through some AABB algorithms, but none of them explain well enough what they exactly do (especially the super tweaked ones) and I don’t want to use stuff I don’t understand.

Since the world is composed of cubes I used octrees to remove some strain on ray cast calculations, basically the only thing I need is this function:

float cube_intersect(Vector ray, Vector origin, Vector min, Vector max)
{
    //???
}

The ray and origin are easily obtained with

Vector ray, origin, point_far;
double mx, my, mz;

gluUnProject(viewport[2]/2, viewport[3]/2, 1.0, (double*)modelview, (double*)projection, viewport, &mx, &my, &mz);
point_far = Vector(mx, my, mz);
gluUnProject(viewport[2]/2, viewport[3]/2, 0.0, (double*)modelview, (double*)projection, viewport, &mx, &my, &mz);
origin = Vector(mx, my, mz);
ray = point_far-origin;

min and max are the opposite corners of a cube.

I’m not even sure this is the right way to do this, considering the number of cubes I’d have to check, even with octrees.

I’ve also tried gluProject, it works, but is very unreliable and doesn’t give me the selected face of the cube.

---

EDIT

So this is what I’ve done: calculate a position in space with the ray:

float t = 0;
for(int i=0; i<10; i++)
{
    Vector p = ray*t+origin;
    while(visible octree)
    {
        if(p inside octree)
        {
            // then call recursive function until a cube is found
            break;
        }
        octree = octree->next;
    }
    if(found a cube)
    {
        break;
    }
    t += .5;
}

It’s actually surprisingly fast and stops after the first found cube.

As you can see the ray has to go trough multiple octrees before it finds a cube (actually a position in space) – there is a crosshair in the middle of the screen. The lower the increment step the more precise the selection, but also slower.

Answer 1

Working with boxes as primitives is overkill in memory requirements and processing power.
Cubes are fine for rendering and even there you can find more advanced algorithms that give you a better final image (Marching cubes). Minecraft’s graphics are very primitive in this sense as voxel rendering has been around for a long time and significant progress has been made.

Basically you should exploit the fact that all your boxes are equally spaced and of the same size. These are called voxels.
Raycasting in a grid is trivial in comparison to what you have – a broad-phase oct-tree and a narrow phase AABB test. I suggest you research a bit on voxels and voxel set collision detection/raycasting as you will find both algorithms that are easier to implement and that would run faster.