How do I apply a filter in MATLAB?

According to the documentation, you can use imfilter

to apply the filter:

I = imread('cameraman.tif');
kLap = fspecial('laplacian');
filtered = imfilter(I,kLap,'replicate');
imshow(filtered); title('Filtered Image');

      

        
        
        
      

    

EDIT: I just figured out what you were asking for :

I = imread('cameraman.tif');

% simple high pass filter
kLap = [-1, -1, -1;
        -1,  8, -1;
        -1, -1, -1];

filtered_3x3 = imfilter(I,kLap,'replicate');
imshow(filtered_3x3); title('Filtered Image (3x3)');
pause();

% another simple high pass filter
kLap = [-1 -3 -4 -3 -1;
        -3  0  6  0 -3;
        -4  6 20  6 -4;
        -3  0  6  0 -3;
        -1 -3 -4 -3 -1];

filtered_5x5 = imfilter(I,kLap,'replicate');
imshow( filtered_5x5 ); title('Filtered Image (5x5)');