1. A method for generating or decoding a bitstream representing a 3D model, comprising: determining a surface smoothness measure for each one of a plurality of sub-cells of a cell in an octree, the octree being representative of the 3D model, wherein the determining the surface smoothness measure comprises, for a particular sub-cell of the cell: forming a plurality of triangles in response to the particular sub-cell and neighboring cells of the cell, wherein each one of the triangles is defined by a point representative of the particular sub-cell and two points representative of two cells of the neighboring cells, determining an area for the each one of the triangles, and determining the surface smoothness measure for the particular sub-cell in response to the areas of the plurality of triangles; and determining a traversal order of the sub-cells of the cell in response to the surface smoothness measures of the sub-cells.