public static int PackVoxelEdgeId(VolumeHeader hd, Int3 a3, Int3 b3)
    {
        int a = hd.CubicToLinear(a3);
        int b = hd.CubicToLinear(b3);

        return(a + (hd.TotalCount * b));
    }
    public static int PackVoxelEdgeIdSorted(VolumeHeader hd, Int3 a3, Int3 b3)
    {
        int a = hd.CubicToLinear(a3);
        int b = hd.CubicToLinear(b3);

        if (a < b)
        {
            return(PackVoxelEdgeId(hd, a3, b3));
        }
        else
        {
            return(PackVoxelEdgeId(hd, b3, a3));
        }
    }