How do you actually MAKE an octree(for voxels)?

Question

I have seen code that creates an octree, adds and remove data from them, but how does one actually build the octree? Is there 3d voxel software that will save to an array of some sort that can then be converted to an octree? or can you save directly to an octree?

Answer 1

Each node in an octree has one point. These nodes are broken up into (you guessed it) eight children nodes. These nodes in turn contain each, one single point.

In general, you don't add all of the your vertices to an octree unless you are doing some ungodly collision detection where every single vertex counts... Not that you can't make it fast then, but it's still slower than the approximation given by a smaller number of nodes. (This is true for nearly everything, approximation is faster).

Again, if you are doing rendering at a high quality octrees should probably have as many nodes as you have points.

Now on to the answer:

1. Create a root node with the bounding box that will enclose it from the center.

2. Insert each point. This should subdivide the octree in the relevant directions.

   1. As you insert these points, the data will move further down into leaf nodes that more closely encapsulate your model.
   2. Also, as you subdivide the bounding box is halved for each subsequent node down.
3. If you really want to save it, you can save the vertices (numbered), and then have your program write down the various connections between nodes, and vertices onto a disk, where they will be able to be loaded in roughly the same time it takes to build the octree from scratch in the first place.

Anyway, I hope I answered your question.

Answer 2

Depends on your implementation -

If you are using oct-trees to subdivide space then typically you'll throw a bunch of V3s at it, and once a node has more than a certain amount of points in it, you subdivide, and redistribute them.

If you are looking for a way to store minecraft style voxels, then you'll subdivide until you reach 1:1 with your voxel size, and store your data in the leaf nodes.

Where the data comes from is up to you - Octrees are a way of storing, manipulating and searching for data, not a file-format as such.