filterDog2d

 Difference of Gaussian (Dog) Filter.

 Adapted from code by Serge Belongie.  Takes a "Difference of Gaussian" -
 all centered on the same point but with different values for sigma. Also
 serves as an approximation to an Laplacian of Gaussian (LoG) filter (if
 order==1).

 USAGE
  G = filterDog2d( r, var, order, [show] )

 INPUTS
  r       - Final filter will be 2*r+1 on each side
  var     - variance of central Gaussian
  order   - should be either 1-LoG or 2-difference of 3 Gaussians
  show    - [0] figure to use for optional display

 OUTPUTS
  G       - filter

 EXAMPLE
  G = filterDog2d( 40, 40, 1, 1 ); %order=1 (LoG)
  G = filterDog2d( 40, 40, 2, 3 ); %order=2

 See also FILTERDOOG, FILTERGAUSS

 Piotr's Computer Vision Matlab Toolbox      Version 2.0
 Copyright 2014 Piotr Dollar.  [pdollar-at-gmail.com]
 Licensed under the Simplified BSD License [see external/bsd.txt]