down

DOWN downsamples an image by a given factor
Usage: y=down(x,r)
 image y is obtained from image x by summing and down sampling
 image y will have the size (floor(m/r),floor(n/r)) 
 where (m,n) is the size of image x 

 Author: Ali Mohammad-Djafari
 Date: Nov. 4, 2002

 Test: 
 x=rand(9); y=down(x,2); y=down(x,3);