Purpose

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  This procedure copies a rectangular subregion of a rectangular
  array to another one, handling all the crappy edge cases.

Inputs

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  src: The source array (to be copied from)
  x1: The lo x coordinate of the subregion to copy
  y1: The lo y coordinate of the subregion to copy
  dest: The destination array (to be copied to)
  x2: The x coordinate in dest where x1 in src should be copied
  y2: The y coordinate in dest where y1 in src should be copied
  dx: The x size of the subregion to copy
  dy: The y size of the subregion to copy

Keyword Parameters

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  add: Set to add the src region to dest
  sub: Set to subtract the src region from dest
  mult: Set to multiply the src region to dest
  div: Set to divide the src region from dest
  BEHAVIOR:
  Assuming both images are big enough, the program extracts a
  dx-by-dy postage stamp from src at position (x1,y1), and inserts it
  into position (x2,y2) of dest. If any part of the postae stamp
  falls outside of either image, those pixels are quietly ignored.

Modification History

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  December 2010: Written by Chris Beaumont
  December 13 2010: Changed implementation for speed. The lower and
  upper bounds for each array are computed directly, instead of using
  index arrays and WHERE. cnb.
  Aug 24 2011: Fixed a bug in handling y trimming. cnb