accessing surrounding elements in an array

2011 5 17
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Hi
I'm currently a complete newbie to MATLAB and I'm having trouble with a few things. I have several png files and I've counted the number of pixels that are above a certain input. I want to smooth the image/data by accessing each in the array and averaging it with its surrounding elements, excluding the boundaries of the image.
I know how to access each individual element in an array using its coordinates, and average them out using M = mean(A) - the problem I have is with accessing the neighbouring elements!
Any help would be great!

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Sean de Wolski
Sean de Wolski 2011-5-17

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%Boundaries included
A2 = conv2(double(A),ones(3),'same')/9;
idx = A > 200; %simulating you're threshold
A(idx) = A2(idx);
This will yield a matrix with a windowing averaged matrix (A2) and select points in A that don't meet a criteria and replace them.
%Boundaries kept
A2 = conv2(double(A),ones(3),'valid')/9;
Apart = A(2:end-1,2:end-1);
idx = Atemp > 200; %simulating you're threshold
Atemp(idx) = A2(idx);
A(2:end-1,2:end-1) = Atemp;

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Gordon
Gordon 2011-5-17
I tried this using a given threshold - and it issued a similar warning to the one above. I'm not really sure what I'm supposed to do with the double and single function...
Sean de Wolski
Sean de Wolski 2011-5-17
Yes, you need to convert A to double first. See the first line of my answer.
Sean de Wolski
Sean de Wolski 2011-5-17
Also, note: this answer is replacing border elements. It would be straightforward to change it.

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Andrei Bobrov
Andrei Bobrov 2011-5-17

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variant
Aout = A;
Aout(2:end-1,2:end-1) = conv2(A,ones(3),'valid')/9;
% or "8" and [1 1 1;1 0 1;1 1 1]?
MORE EDIT
2 variants
1.
Aout(2:end-1,2:end-1) = conv2(A,ones(3),'valid')/9;
2.
Aout(2:end-1,2:end-1) = conv2(A,[1 1 1;1 0 1;1 1 1],'valid')/8;

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Gordon
Gordon 2011-5-17
im still a little at a loss to what I'm doing, but so far I've tried using your little bit of code there in an if statement (so I can process several images simultaneously) and it returned an error for my Aout variable as 'undefined.' :
for i = 1:10;
if i < 10
A = imread(sprintf(['image00' num2str(i) '.png']));
out = A;
Aout(2:end-1,2:end-1) = conv2(A,ones(3),'valid')/8;
end
end
Andrei Bobrov
Andrei Bobrov 2011-5-17
typo in the line "out = A;" must be "Aout = A;"
Sean de Wolski
Sean de Wolski 2011-5-17
Why don't you just run your for loop from 1:9?
for ii = 1:9
A = imread(...)
A2 = conv2(A,ones(3),'same')/9;
idx = A > 200; %simulating you're threshold
A(idx) = A2(idx);
%Do something with A now that it's smoothed
end
Gordon
Gordon 2011-5-17
I've been issued the following warning:
Warning: CONV2 on values of class UINT8 is obsolete.
Use CONV2(DOUBLE(A),DOUBLE(B)) or CONV2(SINGLE(A),SINGLE(B))
instead.

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