accessing surrounding elements in an array

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!

Answer 1

[Sean de Wolski]

%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;

[Gordon]

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]

Yes, you need to convert A to double first. See the first line of my answer.

[Sean de Wolski]

Also, note: this answer is replacing border elements. It would be straightforward to change it.

Answer 2

[Andrei Bobrov]

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;

[Gordon]

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]

typo in the line "out = A;" must be "Aout = A;"

[Sean de Wolski]

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]

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.