comparing 3x3 block with center pixel

i have a matrix as shown below

i wanted to take 3x3 pixel and take 3 pixels at a time along with the center pixel,

and compare the selected 3 pixel value with the center pixel,

and if 2 or morepixel has value greater than the center pixel and i wanted to assign 1 to it else 0

So in this case i will get 0-1-1-0 and then convert the binary 0110 to its corresponding decimal value = 6

Then the next 3x3 pixel

[Matt J]

Do you take the 3x3 blocks in non-overlapping tiles or is it a sliding window?

[Elysi Cochin]

sir i'm doing the loop as

for i=2:Row-1
    for j=2:Col-1
        %"computation"
    end 
end

[KALYAN ACHARJYA]

You did not answered the @Matt J question.

Also is the start pixel at 70 or 200?

What about boundary pixels?

[Elysi Cochin]

same as in LBP code

I need overlapping 3x3 blocks

So the above for loop goes overlapping right?

Boundary pixels also needed, i wrote so, thinking to take 3x3 pixel, i can do i-1, j-1, i, j, i+1, j+1

Am i wrong? Or should i rewrite the for loop as

for i=1:Row
    for j=1:Col
        %"computation"
    end 
end

Answer 1

[Matt J]

Here, X is the input matrix.

result=0;
result=result+1*rot90(fkernel(rot90(X,+1)),-1);
result=result+2*rot90(fkernel(rot90(X,+2)),-2);
result=result+4*fkernel(X);
result=result+8*rot90(fkernel(rot90(X,-1)),+1);

function Y=fkernel(X)

  [m,n]=size(X);
  
  Y=nan(m-2,n-2);
  
  for i=1:n-2

      B=X(:,i:i+2);
      
      Y(:,i)=sum(B(1:m-2,:)>=B(2:m-1,2),2)>=2;
      
  end

end

Answer 2

[KALYAN ACHARJYA]

Self Declaration: Odd Way but simpler

for i=2:Row-1
    for j=2:Col-1
        if (mat(i,j)<mat(i-1,j-1)+mat(i,j)<mat(i,j-1)+mat(i,j)<mat(i+1,j-1))=>2
            data(1)=1;
        else 
            data(1)=0;
        end 
        if (mat(i,j)<mat(i-1,j-1)+mat(i,j)<mat(i-1,j)+mat(i,j)<mat(i-1,j+1))=>2
            data(2)=1;
        else 
            data(2)=0;
        end 
        if (mat(i,j)<mat(i+1,j-1)+mat(i,j)<mat(i_1,j)+mat(i,j)<mat(i+1,j+1))=>2
            data(3)=1;
        else 
            data(3)=0;
        end 
        if (mat(i,j)<mat(i-1,j+1)+mat(i,j)<mat(i,j+1)+mat(i,j)<mat(i+1,j+1))=>2
            data(4)=1;
        else 
            data(4)=0;
        end 
        mat(i,j)=bin2dec(logical(data));
    end 
end

[Elysi Cochin]

when i did as below, i'm getting the follwoing error

"Error using bin2dec 
Input must be a character vector."

How to rectify it?

[KALYAN ACHARJYA]

mat = [70 20 100 24 89; 230 200 220 78 55; 210 80 10 57 34; ...
    110 94 67 88 28; 32 36 89 78 58; 59 73 54 68 48];
[Row, Col] = size(mat);
for i=2:Row-1
    for j=2:Col-1
        if (mat(i,j)<mat(i-1,j-1)+mat(i,j)<mat(i,j-1)+mat(i,j)<mat(i+1,j-1))>=2
            data(1)=1;
        else 
            data(1)=0;
        end 
        if (mat(i,j)<mat(i-1,j-1)+mat(i,j)<mat(i-1,j)+mat(i,j)<mat(i-1,j+1))>=2
            data(2)=1;
        else 
            data(2)=0;
        end 
        if (mat(i,j)<mat(i+1,j-1)+mat(i,j)<mat(i-1,j)+mat(i,j)<mat(i+1,j+1))>=2
            data(3)=1;
        else 
            data(3)=0;
        end 
        if (mat(i,j)<mat(i-1,j+1)+mat(i,j)<mat(i,j+1)+mat(i,j)<mat(i+1,j+1))>=2
            data(4)=1;
        else 
            data(4)=0;
        end 
        mat(i,j)=bi2de(logical(data))
    end 
end

[KALYAN ACHARJYA]

use Logical, no need to combine separately

[Jan]

Easier:

for i=2:Row-1
    data = zeros(1, 4);
    for j=2:Col-1
        if (mat(i,j)<mat(i-1,j-1)+mat(i,j)<mat(i,j-1)+mat(i,j)<mat(i+1,j-1))>=2
            data(1) = 8;
        end 
        if (mat(i,j)<mat(i-1,j-1)+mat(i,j)<mat(i-1,j)+mat(i,j)<mat(i-1,j+1))>=2
            data(2) = 4;
        end 
        if (mat(i,j)<mat(i+1,j-1)+mat(i,j)<mat(i-1,j)+mat(i,j)<mat(i+1,j+1))>=2
            data(3) = 2;
        end 
        if (mat(i,j)<mat(i-1,j+1)+mat(i,j)<mat(i,j+1)+mat(i,j)<mat(i+1,j+1))>=2
            data(4) = 1;
        end 
        mat(i,j) = sum(data);
    end 
end

[Matt J]

I think Jan's modification would really need to be as follows,

    out=zeros(size(mat));
    data = zeros(1, 4);
    for i=2:Row-1   
        for j=2:Col-1
     
            if (mat(i,j)<mat(i-1,j-1)+mat(i,j)<mat(i,j-1)+mat(i,j)<mat(i+1,j-1))>=2
                data(1) = 8;
            end 
            if (mat(i,j)<mat(i-1,j-1)+mat(i,j)<mat(i-1,j)+mat(i,j)<mat(i-1,j+1))>=2
                data(2) = 4;
            end 
            if (mat(i,j)<mat(i+1,j-1)+mat(i,j)<mat(i-1,j)+mat(i,j)<mat(i+1,j+1))>=2
                data(3) = 2;
            end 
            if (mat(i,j)<mat(i-1,j+1)+mat(i,j)<mat(i,j+1)+mat(i,j)<mat(i+1,j+1))>=2
                data(4) = 1;
            end 
            out(i,j) = sum(data); data(:)=0;
        end 
    end

[Jan]

@Matt J: You are right, modifying mat inside the loop, which uses mat as input also is not wanted. If you declare data before the loops and reset the values to 0 by data(:)=0 or if it is recreated by zeros(1,4) does not really matter. For large arrays one of the methods might be faster than the other.

[Matt J]

@Jan: It does matter however, that you were recreating data outside the j-loop. It needs to be reset inside both loops after each (i,j) is processed.