indeed, the octree cell "code" is a streaming value that represents the position of the cell relatively to its parent for successive level of subdivisions (3 bits for each level). You have a quick description of this in this article: http://www.danielgm.net/phd/isprs_laser ... 05_dgm.pdfBy the way, this is why in the standard version is limited to 10 levels of subdivision (10*3 = 30 bits < 32 bits) as codes are stored as 32 bits integers.
The cell index is simply the index of the cell code inside the sorted
cell code list (= the true octree structure). Once we have computed all cell codes (on for each 3D point), we sort them so that all the point lying in the same cell are next to each other, and their neighbors are not very far (depending on the subdivision scheme and subdivision level). So, in order to be able to know which cell code corresponds to which point, we associate a code with an index.