How to implement an octree in C#?

Question

First time posting here but I've been reading the site for a few years now. I'm trying to implement a simple generic type Octree in C# (using some XNA includes). I've thoroughly researched and I understand the concept, I just can't seem to make it work. Searching around yields some implementations in other languages, but they all seem custom tailored to a specific application; and I haven't really been able to make much sense out of those.

Below is my Octree class so far, the Vector3, BoundingBox, and ContainmentType are from XNA. I feed in max and min points, and a list of points that are within the boundaries. However none of the points actually get added to the tree. Any help would be much appreciated!

public class Octree<T> : ISerializable
{   
    Vector3 max;
    Vector3 min;
    OctreeNode head;

    public Octree(Vector3 min, Vector3 max, List<Vector3> values)
    {
        this.max = max;
        this.min = min;
        head = new OctreeNode( min, max,  values);           
    }

    public Octree() { }

    public Octree(SerializationInfo info, StreamingContext context)
    {
    }

    public void GetObjectData(SerializationInfo info, StreamingContext context)
    {            
    }

    internal class OctreeNode
    {            
        Vector3 max;
        Vector3 min;
        Vector3 center;
        public Vector3 position;
        public T data;

        public BoundingBox nodeBox;
        public List<OctreeNode> subNodes;
        public OctreeNode( Vector3 min, Vector3 max,List<Vector3> coords)
        {
            nodeBox = new BoundingBox(min, max);
            subNodes = new List<OctreeNode>();

            this.min = min;
            this.max = max;
            center = (min + ((max - min) / 2));

            nodeBox = new BoundingBox(min, max);
            if (coords.Count == 0)
            { return; }
            subNodes.Add(new OctreeNode(center, max));
            subNodes.Add(new OctreeNode(new Vector3(min.X, center.Y, center.Z), new Vector3(center.X, max.Y, min.Z)));
            subNodes.Add(new OctreeNode(new Vector3(min.X, center.Y, max.Z), new Vector3(center.X, max.Y, center.Z)));
            subNodes.Add(new OctreeNode(new Vector3(center.X, center.Y, max.Z), new Vector3(max.X, max.Y, center.Z)));

            subNodes.Add(new OctreeNode(new Vector3(center.X, min.Y, center.Z), new Vector3(max.X, center.Y, min.Z)));
            subNodes.Add(new OctreeNode(new Vector3(min.X, min.Y, center.Z), new Vector3(center.X, center.Y, min.Z)));
            subNodes.Add(new OctreeNode(new Vector3(min.X, min.Y, max.Z), center));
            subNodes.Add(new OctreeNode(new Vector3(center.X,min.Y,max.Z), new Vector3(max.X,center.Y,center.Z)));


            List<List<Vector3>> octants = new List<List<Vector3>>();
            for (int i = 0; i < 8; i++)
            {
                octants.Add(new List<Vector3>());
            }
            foreach (Vector3 v in coords)
            {
                int i = 0;
                foreach(OctreeNode n in subNodes)
                {
                    ContainmentType t = n.nodeBox.Contains(v);

                    if (t.Equals(ContainmentType.Contains))
                    {
                        octants[i].Add(v);
                    }
                    i++;
                }
            }

            for (int i=0;i<subNodes.Count;i++)
            {
                if (octants[i].Count > 0)
                {
                    Vector3 v = octants[i][0];
                    octants[i].Remove(v);
                    subNodes[i] = new OctreeNode(subNodes[i].min, subNodes[i].max, octants[i]);
                }
            }
        }

        public OctreeNode(Vector3 min, Vector3 max)
        {
            nodeBox = new BoundingBox(min, max);
        }            
    }
}

If you are referring to automated Unit Tests then no, I haven't. I was planning to do that once I can actually get the constructor to populate the tree in some manner. — user1927218, Dec 24 '12 at 20:44

score 7 · Answer 1 · answered Mar 15 '13 at 22:27

I pasted your code in a fresh project in Visual Studio and debugged calling the Octree constructor with a couple of point values. Here's a few easy picks that should help you get your octree working.

In OctreeNode(Vector3 min, Vector3 max, List<Vector3> coords), some of the subNodes don't have sensible min and max bounds. For example, new Vector3(min.X, min.Y, max.Z), center spans from max.Z to center.z. The upper limit is always smaller than the lower limit. Try listing the nodes systematically to reduce the possibility of such errors, like this:

subNodes.Add(new OctreeNode(new Vector3(min.X,    min.Y,    min.Z),    new Vector3(center.X, center.Y, center.Z)));
subNodes.Add(new OctreeNode(new Vector3(min.X,    min.Y,    center.Z), new Vector3(center.X, center.Y, max.Z)));
subNodes.Add(new OctreeNode(new Vector3(min.X,    center.Y, min.Z),    new Vector3(center.X, max.Y,    center.Z)));
subNodes.Add(new OctreeNode(new Vector3(min.X,    center.Y, center.Z), new Vector3(center.X, max.Y,    max.Z)));
subNodes.Add(new OctreeNode(new Vector3(center.X, min.Y,    min.Z),    new Vector3(max.X,    center.Y, center.Z)));
subNodes.Add(new OctreeNode(new Vector3(center.X, min.Y,    center.Z), new Vector3(max.X,    center.Y, max.Z)));
subNodes.Add(new OctreeNode(new Vector3(center.X, center.Y, min.Z),    new Vector3(max.X,    max.Y,    center.Z)));
subNodes.Add(new OctreeNode(new Vector3(center.X, center.Y, center.Z), new Vector3(max.X,    max.Y,    max.Z)));

In the constructor OctreeNode(Vector3 min, Vector3 max) you don't initialize the fields min, max, and center. As a result, when the final OctreeNodes have their lower and upper limits always set to zero on the line
```
   subNodes[i] = new OctreeNode(subNodes[i].min, subNodes[i].max, octants[i]);
```
On that same line, you pass in as node values all the points but the one that actually lies in the range of the node. The local variable v is the value that lies in the range. It is removed from octants after which octants is passed as node values.
The values passed in to the OctreeNode constructor are not actually ever stored anywhere but the created node is always split into smaller nodes and the values are passed on to the subnodes. Therefore fixing the above three issues will result in the code getting in an infinite recursion. To break the recursion, you'd need to implement a halting condition. Usually in octrees the condition is that if there is a sufficiently small number of values inside a node, the node is not split into subnodes but the value is stored in the node. Only if the node contains sufficiently many values, then the node is split and its values are distributed among the new subnodes.

How to implement an octree in C#?

1 Answers1