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# Center of mass in binary image

Asked by Felix on 18 Sep 2013

Hello, I have a binary image with one object. How can I calculate the center of this object, to mark it in my image and to write the coordinates in the corner of this image, like this: object: X, Y

I used this code to calculate the center of mass:

s = regionprops(bw, 'centroid');

centroids = cat(1, s.Centroid);

imshow('MY_IMAGE.jpg')

hold on

plot(centroids(:,1), centroids(:,2), 'b*')

hold off

but how can i plot this coordinates on my image in the left corner?

Thank you for help

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Answer by David Sanchez on 18 Sep 2013

use text command:

```your_string = strcat( num2str(centroids(:,1)),'-', num2str(centroids(:,2)));
text(10,10,your_string)
```

change the position by changing the "10,10" values

Felix on 18 Sep 2013

Thank you it's works, another question, how can i write in this way:

Object: X,Y(coordinates)

now it gives me only a coordinates and can i also write a coordinates without a numbers after '.'? only integers

Image Analyst on 18 Sep 2013

Use sprintf to create your string, and cast your numbers to integers or use %.0f:

```textString = sprintf('Object: %d, %d (coordinates)', int32(X), int32(Y));
text(x,y,textString);
```
Felix on 18 Sep 2013

thnx

Answer by Walter Roberson on 18 Sep 2013

ndgrid() of 1:width and 1:height, do a logical indexing of those according to the binary image, calculate the mean() of what is left in each of the two coordinate matrices. Result will be centre of mass along each of the two coordinates.