MATLAB: MATLAB conv2 giving wrong answer as compare to manual with Valid

image processingMATLAB

When I use conv2 the 'valid' its gives negative answers.. like

A= 3 0 1 2 7 4

1 5 8 9 3 1

2 7 2 5 1 3

0 1 3 1 7 8

4 2 1 6 2 8

2 4 5 2 3 9

b= 1 0 -1

1 0 -1

manually conv of A with b =

-5 -4 0 8

-10 -2 2 3

0 -2 -4 -7

-3 -2 – 3 -16

but MATLAB i get

5 4 0 -8

10 2 -2 -3

0 2 4 7

3 2 3 16

i cant understand the reason

Best Answer

You did not fliplr() the convolution kernel when you calculated by hand. MATLAB uses the convention that the convolution kernel is mirrored; this convention fits with the definition of integral convolution involving integral of f(tau)*g(t-tau) -- notice the implicit time reversal of g

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MATLAB: How to modify a matrix to change the entries >=0 and multiply it by 5 AND to change the entries <0 and add 2 to those entries

Start with "If an entry in B is greater than or equal to zero": You can either run a loop over all elements of B or use "logical indexing":

% Linear indexing:
for k = 1:numel(B)
   if B(k) >= 0
     ...
   end
end

Or two indices for rows and columns:

for r = 1:size(B, 2)
  for c = 1:size(B, 1)
    if B(r, c) >= 0
       ...
    end
  end
end

Or "logical indexing":

index = (B >= 0);
disp(B(index))

Now try to use this to modify B. Use the mentioned terms to search in the documentation and in the internet.

MATLAB: Please read the instructions and give the correct expression

Hello Sir David Hill

Can you check my answers? I attached pdf of my answers to matlab problems. I am hoping that you will give a comment.

Best Answer

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MATLAB: How to modify a matrix to change the entries >=0 and multiply it by 5 AND to change the entries <0 and add 2 to those entries

MATLAB: Please read the instructions and give the correct expression

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