XFDTD TUTORIAL - Extraction of Pieces from Large Grids

When working with extremely large grids, it may be desirable to extract
a smaller piece of the grid.  This can save hard disk space and greatly
decrease simulation times.

To extract part of a grid, you must do the following steps

1.  Determine the size of the smaller grid.  It must be large
enough to include the desired extraction.  In addition, xfdtd 
requires at least 6 grid points of free space around the outside 
of the extraction to prevent instabilities.

2.  Determine the starting (X,Y,Z) coordinate point of the section
to be extracted.  This will be the offset you will need to select 
when merging.

3.  Create a new grid space as determined in section 1

4.  Select File-Open Geometry File and Merge with Present Geometry

5.  When it asks for an offset, negate the (X,Y,Z) coordinate point
found in section 2.  This is because you are actually merging the 
large geometry into the new geometry.

6.  If you did everything correct you should now have the smaller 
geometry.  Using the editing tools, clear 6 cells around the outside
of the new geometry.

Example

Suppose I want to extract the nose from the human head model. 

1.  I determine where the nose is located
          Start     Stop
      x    40        65    
      y    95       117
      z    25        43

2.  Increase the size by 6 on all sizes
          Start    Stop
      x    34       71    
      y    89       123
      z    19       49
      
3.  Create a new grid with the number of cells 
    in section two using the formula
    size = Stop - Start + 1
    
    New size 
    38x35x31
    
4.  Merge using -34, -89, -19 as the offset.

6.  Clear the outside cells and save with new file name.