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Exclude central pixel from sliding mean calculation of surrounding pixels in a matrix

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I have got a matrix (M) where I need to calculate the average of the surrounding (3x3) pixels for each pixel, excluding the central one.
I have tried colfilt, but this calculates the mean including the central pixel:
avg = (colfilt(M,[3 3],'sliding',@nanmean));
What I need to do is to exclude the central pixel of each 3x3 matrix from the mean calculation, and assign to that central pixel the value of the average of the surrounding 8 pixels. Schematically:
Any help would be massively appreciated!

Accepted Answer

Image Analyst
Image Analyst on 18 Jan 2023
Try setting the central pixel of the kernel to 0.
kernel = ones(3, 3) / 8;
kernel(2, 2) = 0;
avg = conv2(M, kernel, 'same');

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