SONFControl

  Reads, sets or clears specified bits in a filter mask structure

  Implemented through SONFControl.dll

  [VAL FILTERMASK]=SONFCONTROL(FILTERMASK, LAYER, ITEM, ACTION)

  INPUTS: FILTERMASK, filter mask structure
          LAYER, the layer to change (0 to 3) or 'ALL' for  all layers
          ITEM, the item to read or change (0 to 255) or 'ALL' for all items
          ACTION the action to take 'READ', 'SET', 'CLEAR' or 'INVERT'.

  OUTPUTS: VAL
              For a read: set to the state of the item or, if LAYER or ITEMS
              are 'ALL' set to 1 if all set and 0 if all are cleared
              For a write: returns 0 or a negative error
              FILTERMASK is a copy of the input filter with the required bits
              set, cleared or inverted.

  N.B. any non-numeric input is assumed to represent 'ALL' above. For
  string inputs only the first character is used.


  Author:Malcolm Lidierth
  Matlab SON library:
  Copyright © The Author & King's College London 2005-2006