maskSphere

 Creates an 'image' of a n-dimensional hypersphere.

 Useful for testing visualization procedures.

 Create a d-dimensional matrix mask of dimensions [s x s x s x ... x s]
 where s=2r+1.  Each element mask(x1,x2...,xd) is 1 if (x1,...xd) falls
 inside the centered hypersphere of rad r.

 In 1d this corresponds to a vector of the form [.. 0 1 1 1 1 1 0 ..]
 In 2d this corresponds to an image of a white circle.
 In 3d and 4d, well try it and 'see' what it looks like.
 
 USAGE
  mask = maskSphere( d, r, [show] )

 INPUTS
  d         - dimension (any positive integer)
  r         - sphere integer radius 
  show      - [] figure in which to display results

 OUTPUTS
  mask      - [s x s x s x ... x s] hypersphere image

 EXAMPLE
  mcircle = maskSphere( 2, 20, 1 );
  msphere = maskSphere( 3, 10, 2 );
  msphere = maskSphere( 4, 10, 3 );

 See also MASKCIRCLE, MONTAGE2

 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]