MATLAB: Sliding colfilt problem with custom function - matlab

Question

I'm trying to do a custom Matlab function using colfilt where if the value of the pixel is black or white (0 or 255) the value will be the median of neighbours. Since I'm using colfilt, that means that the neighbours values are present in the same column and therefore I did this function:
function [Y] = Lab3_2(X)
n = X(5)
if(n ==255 || n ==0)
Y = median(X)
else
Y = n
end
end
And the function gets called with:
Y = uint8(colfilt(Isp,[3 3],'sliding',#Lab3_2))
Where Isp is an image with salt&pepper noise.
The problem is that I get this error:
Error using reshape To RESHAPE the number of elements must not change.
Error in colfilt (line 182)
reshape(feval(fun,x,params{:}),block(1),block(2));
I read the documentation and it says that the function must return a row vector containing a single value for each column in the temporary matrix.
I think i'm not doing this correctly (I'm refering to my output variable Y) and also I'm not sure if the line n = X(5) is correct.
Does anyone know how I can fix it?

Answer 1

If you had a loop iterating over each column of the input matrix X (or, if colfilt passed the columns to your function one by one), your approach would work. To implement the loop, you'd do something like this:
function [Y] = Lab3_2(X)
num_cols = size(X,2); % get number of columns in X
Y = zeros(1,num_cols); % preallocate row vector Y
for c = 1:size(X,2); % iterate over each column of X
Xcol = X(c);
n = Xcol(5) % check whether center pixel is 0 or 255
% assumes a 3x3 neighborhood
if(n ==255 || n ==0)
Y(c) = median(X) % yes, replace with median of column values
else
Y(c) = n % no, use original value
end
end
end
But looping over the columns is unnecessary when median and the comparison operators already work column-wise over the entire matrix. A more concise way to do the same thing would be:
function [Y] = Lab3_2(X)
Y = X(5,:) % initialize Y to current pixel
bw_indices = (Y == 255 | Y == 0); % get indices of 0,255 values
X_median = median(X); % take medians of all columns
% replace 0,255 values with their corresponding medians
Y(bw_indices) = X_median(bw_indices);
end