MATLAB: What’s the problem in the loop

for loopimage processing

A=imread('cameraman.tif');
B=zeros(255,255);
for j=0:255
    for k=0:255
        B(j,k)=A(j,k+1)-A(j,k-1);
    end
end
figure,imshow(B);

error: Attempted to access A(0,1); index must be a positive integer or logical.

Error in tryfebin (line 6)
        B(j,k)=A(j,k+1)-A(j,k-1);

Best Answer

Your code should go like this:

A=imread('cameraman.tif');
B=zeros( size(A) );
for j=1:size(A,1)
    for k=2:(size(A)-1)  %edge pixels of B matrix has to be treated differently
        B(j,k)=A(j,k+1)-A(j,k-1);  %you can not apply this algorithm to edge pixels
    end
end
figure,imshow(B);

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MATLAB: “Index exceeds matrix dimensions.” But it doesn’t

You set m and n to size() of the image. You have for j=1:n . Your loop tries to index the image a(i,j+1) . But when j reaches n in the loop, reaches the number of columns in the image, then a(i,j+1) tries to go one past the number of columns.

MATLAB: Problm in imwriting a arrayimage

rohan - when you step through the attached code, does the following line get called 18 times (I'm guessing this is how many images you are reading in based on the first for loop):

str = ['C:\Users\Public\Videos\Sample Videos\pro\Ne\im' num2str(m) '.jpg'] ;

If this line is invoked for every iteration of k, is it distinct on each iteration? I'm guessing it isn't, and here is why. Your first for loop iterates as:

for k = 1:18

However, there are several instances in the code where you re-use k for other purposes, and set it to 1 and then iterate again:

    k=1;
    for i=1:1:n
        for j=1:2:n
            if (j<=n-1)
                s(i,k)=(b(i,j)+b(i,j+1))/2;
                d(i,k)=b(i,j)-b(i,j+1);
            end
            k=k+1;
        end
        k=1;
    end

This happens a handful of times including near the end:

    for j=1:1:n/8
        for i=1:2:n/4
            if i<=n/4-1
                s(k,j)=(b(i,j)+b(i+1,j)/2);
                d(k,j)=b(i,j)-b(i+1,j);
            end
            k=k+1;
        end
        k=1;
    end
    % some stuff and then:
    b=imcrop(b,[0 0 64 64]);
    m=k;

Note how k is reset to one after every completion of the inner for loop (on i). And then m is initialized to k…which means that m is always one and so probably im1.jpg is the only image ever being saved. (Is that correct?)

To correct this, k should never be modified by any inner loop within the code, just choose some other name for this alternate looping/indexing variable.

Best Answer

Related Solutions

MATLAB: “Index exceeds matrix dimensions.” But it doesn’t

MATLAB: Problm in imwriting a arrayimage

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