NAME:   mesh_to_ucd - 2D field mesh to quad UCD structure
  
  Type:   mapper
  
  Author: Ian Curington, Advanced Visual Systems, Inc.
  
  Source: mesh_to_ucd.c
  
  Description:
  ------------
    A mesh_to_ucd makes a 2D UCD surface showing surface height variation
    from one field.
    Each cell is drawn as per field to mesh, (linear interpolation),
    or in block mode with descrete non-interpolated colors.
    It will optionally leave holes in the UCD output
    where the data goes out of range for the float dials, allowing
    mask values to control cell visibility, or region boundaries.
    It is basically a variation on field_to_mesh, allowing more
    advanced UCD type visualizations and data processing.
  
  Inputs:
    Field:
      "field 2D uniform/irregular scalar float"
      This controls the height map of the surface.
      It also controls the scalar values, on the UCD nodes
      Can be 2-space or 3-space.
 
  Output:
   UCD, with coords of mesh, and scalar value as node data.
  
  Parameters:
  -----------
  
  Z-Scale
    This controls the relative offset from the base plane of each
    cell, creating the wavy mesh_to_ucd effect.
  
  Leave Holes
    This will allow drop outs in the field where the field
    values go out of range for the Lower-Upper dials.
    This means UCD cells will be missing from the structure
    if any corner touches an invalid node.
 
  Lower
    A float value to determin the range of valid data at nodes
    if "leave holes" mode is on.
  
  Upper 
    A float value to determin the range of valid data at nodes
    if "leave holes" mode is on.
  
  Example Network:
  ----------------
  
                    Read Image (=avs.x)
                       |
                    Downsize    = 12
                       |
                    field to float
                       |
                    extract scalar
                       |
                    mesh_to_ucd
                       |
  gen colormap         | 
        |              |
        |    |-------- |
     ucd_contour       |
         |----------|  |
                 ucd to geom
                    |
                 geometry viewer