if all cubical grids have been related, the procedure ends. [0042] FIG. 5 is a detailed flowchart illustrating the step S13 in FIG. 3, namely how to search near points of the selected point. [0043] In step S130, the near points searching module 23 searches the serial number "xxx" of the cubical grid which confines the selected point, and searches the twenty-six cubical grids which are adjacent to the cubical grid "xxx" from the cubical figure according to the serial number "xxx". [0044] In step S131, the judging module 26 judges whether the quantity of the points confined in the twenty-seven cubical grids (including the cubical grid "xxx" and the adjacent twenty-six cubical grids) is more than a predetermined value, more than 9 points, for example. [0045] If the quantity is less than the predetermined value, in step S132, the near points searching module 23 searches other cubical grids which are adjacent to one of the twenty-six cubical grids that have been searched. [0046] Until the quantity of the points confined in the cubical grid "xxx" and the searched cubical grids is more than the predetermined value, in step S133, the near points searching module 23 computes a distance between the selected point and each of the points which confined in the cubical grid "xxx" and the searched cubical grids to obtain a plurality of near points, 9~15 points is better, according to the distances. [0047] FIG. 6 is a detailed flowchart illustrating step S14 in FIG. 3, namely how