Find the center of specific picture area

Hi, I'm quite new to Matlab.

I want to identify the center of a specific picture area.

In my case, I have 2 pictures which I determined the difference between each other (Subtraction). Resulting of a white area, marking the difference. Here's my problem, I cannot find roughly the center of my white area. Note that I have no error in the execution, just a problem with this calculus. Can someone help me with this please. Here's a sample of my code:

clear all; close all;

%Access to the pictures
[filename, pathname] = uigetfile({'*.jpg;*.tif;*.PNG;*.gif;*.bmp','All Image Files';...
          '*.*','All Files' },'img1',...
          'C:\Users\CAP\Desktop\SCHOOL\MATLAB\cible.jpg') 

img1 = imread(filename); 
if (length(size(img1))>2)
    img1=img1(:,:,1);
end
[filename, pathname] = uigetfile({'*.jpg;*.tif;*.PNG;*.gif;*.bmp','All Image Files';...
          '*.*','All Files' },'img2',...
          'C:\Users\CAP\Desktop\SCHOOL\MATLAB\cibleref.jpg')  image

img2 = imread(filename); 
if (length(size(img2))>2)
    img2=img2(:,:,1);
end

%Determination of the difference between the 2 pictures by Subtraction
[ligne colonne]=size(img1);

image_result=uint8(zeros(ligne,colonne));
for i = 1: ligne
    for j = 1 : colonne
        if img1(i,j)>=img2(i,j)
            image_result(i,j)= img1(i,j)-img2(i,j);
        else
            image_result(i,j)= img2(i,j)-img1(i,j);
        end
    end
end

%Finding the coordinates of the white zone (difference) 
[x,y]=find(image_result>128); %0:black 255:white
x1=(sum(x)/length(x)); %Trying here to figure out the center of my white area
y1=(sum(y)/length(y)); %Trying here to figure out the center of my white area

% Display
figure(1)
subplot(221)
imshow(img1)
subplot(222)
imshow(img2)
subplot(223)
imshow(image_result)
hold on;

plot(x1,y1,'r.','MarkerSize',20)

Top Right: Reference Image

Top Left: Image including difference

Bottom: Result image by subtraction. The Red dot corresponds to the result of my white zone determination. As you can see there is a sightly error in the calculus...

Answer 1

[Hamoon]

That's because x1 refers to columns of the image and y1 refers to the rows of the image, so for plot you should write:

 plot(y1,x1,'r.','MarkerSize',20)

in plot we first write rows and then columns: plot(rows,columns)

[Charles P]

Thank you very much.

Answer 2

[Image Analyst]

find() returns [y, x], (which is rows, columns). find() does not return [x, y] like you have. Switch that and you'll have fewer problems. Of course you don't need a double for loop, and can just use regionprops() like in my image segmentation tutorial: http://www.mathworks.com/matlabcentral/fileexchange/?term=authorid%3A31862

[Sriram Badri]

Hi Image analyst I have a different problem and I need your help. Can you please tell me how I can contact you?

[Image Analyst]

You just did.

[Sriram Badri]

http://www.mathworks.com/matlabcentral/answers/234157-how-to-combine-a-set-of-2d-images-into-a-3d-image

This is the link for the question.

[Sriram Badri]

If you could suggest a way to solve it, it would be really helpful. N thanks a lot