Matrix dimensions must agree error

Hello!

I am getting this error for this code to filter images. I am getting a matrix dimensions must agree error and I'm unsure where/what the exact issue is.

Here is my code:

function img_out = edge_detect(img_in)
    x = [-1 0 1];
    y = [-1 0 1];
    
    Gxmat = [-1 0 1;-2 0 2; -1 0 1];
    Gymat = [1 2 1; 0 0 0; -1 -2 -1];
    
    Gx = zeros(size(img_in));
    Gy = zeros(size(img_in));
    
    fix = img_filter(img_in);
        
    for k = 1:3
        for n = 1:3
            Gx = Gx + Gxmat.*img_shift(fix,x(n),y(k));
            Gy = Gy + Gymat.*img_shift(fix,x(n),y(k));
        end
    end
    
    img_out = rms(Gx,Gy);
    
end

function img_out = img_shift(img_in, horiz, vert)
   x = horiz;
   y = vert;
   
   img_out = zeros(size(img_in));
   
   if x == 0 && y == 0
       img_out = img_in;
   elseif x == 1 && y == 0
       img_out(:,2:end) = img_in(:,1:end-1);
   elseif x == 1 && y == 1
       img_out(1:end-1,2:end) = img_in(2:end,1:end-1);
   elseif x == 1 && y == -1
       img_out(2:end,2:end) = img_in(1:end-1,1:end-1);
       
   elseif x == -1 && y == 0
       img_out(:,1:end-1) = img_in(:,2:end);
   elseif x == -1 && y == 1
       img_out(1:end-1,1:end-1) = img_in(2:end,2:end);
   elseif x == -1 && y == -1
       img_out(2:end,1:end-1) = img_in(1:end-1,2:end);
       
   elseif x == 0 && y == 1
       img_out(1:end-1,:) = img_in(2:end,:);
   elseif x == 0 && y == -1
       img_out(2:end,:) = img_in(1:end-1,:);
   end
    img_in = img_out;
    img_out = img_in;
end

function img_out = img_filter(img_in)
    x = [-1 0 1];
    
    y = [-1 0 1];
    
    img_out = zeros(size(img_in));
    
    for k = 1:3
        for n = 1:3
            img_out = img_out + img_shift(img_in,x(k),y(n)); %indexing so you essentailly are taking the image out
            %and running it with the shift but accounting with x and y. 
        end
    end
    
    img_out = img_out./9;
end

And here is the code to call to put into the command window:

dmat = zeros(10);
dmat(5:6,5:6) = 1;
dmat(:,1)=1;
dmat(:,end)=1;
dmat(1,:)=1;
dmat(end,:)=1;

img_out = edge_detect(dmat);
imagesc(img_out);

Thank you very much for your help!

[Star Strider]

‘I am getting a matrix dimensions must agree error...’

Where?

[Arshia Nazerani Houshmand]

It's indicating that the error is on this line:

            Gx = Gx + Gxmat.*img_shift(fix,x(n),y(k));
            Gy = Gy + Gymat.*img_shift(fix,x(n),y(k));

[Star Strider]

I cannot run your code, and because there are no comments or any other description, I have no idea what it does.

Unless the output of ‘img_shift’ is a (3x3) matrix, you will continue to get that error.

Think carefully about what you want to be doing, and then write your code to do exactly that.

[Arshia Nazerani Houshmand]

My bad, essentially it's a code for edge filteration of a image in order to reduce noise. The first function is the edge filteration, the second function is to shift the image for filtering and the final function is to filter the main filteration.

I'm unsure if img_shift is a (3x3) matrix so I'll confirm that. Thanks for the help!

[Walter Roberson]

Your code assumes that img_in is a 2d array. Not, for example, an rgb image. In the code before this have you tested to be sure you will be passing 2d?

[Arshia Nazerani Houshmand]

Here is the question prompt:

Write a function which will take, as input, a 2D image in double format, apply the averaging filter, and generate an edge-detected image using the Sobel method. There are two kernels used in the edge detection filter process, Gx and Gy as shown in the Edge Detection section. function img_out = edge_detect(img_in) Your function should use the img_shift() and img_filter() functions you wrote in the previous problems.

The code before this which is the 3rd function: img_out = img_filter(img_in) , runs perfectly and provides the correct answer in MatLab grader. This whole problem is a precurser to being able to take an BW image and running a filter on it. Unfortunetly, I cannot run a BW image filter unless I get this current 2D array code to work.

Answer 1

[Raunak Gupta]

Hi,

By running the example, I see that in edge_detect function, img_shift returns same size image that is inputted in it. So the size of img_shift(fix,x(n),y(k)) will be 10x10 as per example but the size of Gxmat and Gymat is 3x3. That is why you are not able to do elementwise multiplication. From what you are trying to implement I can see that the elementwise multiplication should be a 2-D convolution which can be done using conv2. So, I think replacing the Gx and Gy lines to below ones can clear the matrix dimension error.

Gx = Gx + conv2(img_shift(fix,x(n),y(k)),Gxmat,'same'); 
Gy = Gy + conv2(img_shift(fix,x(n),y(k)),Gymat,'same'); 

For using rms, I think only one input matrix is required the dimension input. I am not sure what is the logic behind using rms with two matrices of same size.