MATLAB: Apply a Local Averaging Mask

image processingImage Processing Toolbox

How can you apply a 3×3 local averaging mask to enhance the sharpness in an image. How can you set the mask? I'd like to get a better understanding in how this method of filtering an image works and how to compute the algorithm.

Best Answer

Local averaging:

conv(double(YourImage), ones(3,3)/9, 'same')

ones(3,3)/9 is not usually the mask used for sharpening. You tend to sharpen horizontally and vertically separately, using a mask such as [0, 1, 0; -1 0 -1; 0 1 0]

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Use this kernel for smoothing the image

A = [1,1,1;1,1,1;1,1,1]/9;

Also, it is better to use the image as the first input to conv2 and the kernel as second

B = conv2(J,A)

in the default setting, there is no difference. However, if you specify a different 'shape' option, then your approach can cause problems.

If you have, image processing toolbox, you can also use imfilter(): https://www.mathworks.com/help/releases/R2020a/images/ref/imfilter.html

You can also create the kernel using following command

A = fspecial('average',3) % requivalent: A = [1,1,1;1,1,1;1,1,1]/9;

MATLAB: Sharpening image using first order derivative

You can sharpen the image by adding the Laplacian to the original image. This can all be done in one convolution:

windowWidth = 3;
kernel = -1 * ones(windowWidth);
middleRow = ceil(windowWidth / 2);
kernel(middleRow, middleRow) = 2 * windowWidth ^ 2 - 1;
sharpenedImage = conv2(double(grayImage), kernel, 'same');
imshow(sharpenedImage, []);

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