MATLAB: Low-pass filter

Question

how to add a Low-pass filter the watermarked picture, i.e., replace each pixel by the average of its

NxN pixel neighborhood (for border pixels, you could e.g. extend the picture to the outside

by gray pixels with pixel value 128).

I use my watermarked picture as

wmarked = imread('markedimage.png');

Answer 1

hsize=??          % define
h=fspecial('average',hsize);
filter_wmarked=imfilter(wmarked,h);