How do you find a centroid of an image?

I am trying to find the centroid of a binary image using region props but I keep receiving the error "Field reference for multiple structure elements that is followed by more reference blocks is an error". Here is my code for finding the centroid:

 measurements = regionprops(bw, 'Centroid');
  hold on;
  plot(measurements.Centroid(1), measurements.Centroid(2), 'r+', 'MarkerSize', 120, 'LineWidth', 1);
hold off;
imshow(bw)

Any help would be appreciated thank you

[Image Analyst]

Do you want the centroid of the whole, entire image, or of individual blobs in the image? You asked about the whole image but your code seems to indicate you maybe want centroids of individual blobs. Of course the centroid of the whole image is at the center, and the "Weighted centroid" depends on the gray levels in the image.

Answer 1

[Image Analyst]

Maybe try

centroids = [measurements.Centroid];
xCentroids = centroids(1:2:end);
yCentroids = centroids(2:2:end);
plot(xCentroids, yCentroids, .............

[Rish]

Thank you for your help. When I tried this a line graph showed up without the image. I am not sure why though.

[Image Analyst]

If you want the markers to be over the image, put "hold on" before plot(). You must have removed it for some reason.

hold on; % Prevent plot() from blowing away image.
plot(..........., 'r+'........

It looks like you used 'b-' in your call to plot because I see blue lines instead of red plus signs.

Answer 2

[Nade Sritanyaratana]

The example at the end of the regionprops documentation has an elegant use of the cat function, which might be able to fix the error you are getting.

measurements = regionprops(bw, 'Centroid');
imshow(bw); 
centroids = cat(1,measurements.Centroid);
hold on;
plot(centroids(:,1), centroids(:,2), 'r+', 'MarkerSize', 120, 'LineWidth', 1);
hold off;

[Rish]

Thank you for your help. I tried this code and it plots lines but it doesn't just plot the centroid line, instead it plots multiple different lines. Any way to isolate just the centroid line?

[Image Analyst]

Your markers are too big - 120. Try something like 10 or 15 instead.

[Rujal]

I have used this code

but I want to find mid point between two centroids. for that I have used

mid= mean(centroids); plot(mid,'r*');

and the result is:

but I want to find midpoint between two centroids of objects. I would be thankful if someone helps me.

[Image Analyst]

You can't do that because centroids is [x1,y1,x2,y2,x3,y3,x4,y4,.....] You need to extract x and y separately:

xCenters = centroids(1:2:end);
yCenters = centroids(2:2:end);

Then you can pick out which two blobs you want to find the midpoint between and average them. For example

xMid = (xCenters(1)+xCenters(2))/2;
yMid = (yCenters(1)+yCenters(2))/2;
hold on;
plot(xMid, yMid, 'r+', 'LineWidth', 2, 'MarkerSize', 20);

Answer 3

[Rish]

Thank you very much guys. I really appreciate your help and I figured it out.

Answer 4

[hallouma amari]

help me too please

this is my error

Field reference for multiple structure elements that is followed by more reference blocks is an error.

img2=imread('56.png');
se=strel('disk',3);
J=imclose(img2,se);
J=rgb2gray(J);
M=imadjust(J);
B=0; %initialisation
[R C]=size(J);

measurements = regionprops(J, 'Centroid');
x2=measurements.Centroid(1);
y2=measurements.Centroid(2);
bw = bwboundaries(J);
boundary = bw{1};
x1=boundary(:,1);
y1=boundary(:,2);
x1 = x1-x2;
y1 = y1-y2;

[Image Analyst]

Try like what I showed:

centroids = [measurements.Centroid];
xCentroids = centroids(1:2:end);
yCentroids = centroids(2:2:end);
% Compute distances of perimeter to centroid of blob #1
bw = bwboundaries(J);
boundary = bw{1};
x1=boundary(:,1);
y1=boundary(:,2);
deltaXs = x1 - xCentroids(1);
deltaYs = y1 - yCentroids(1);

Answer 5

[somia gupta]

i want to find the centroid of an image in matlab ??? need help for the code for head pose detection

[Image Analyst]

See my Image Segmentation Tutorial in my File Exchange http://www.mathworks.com/matlabcentral/fileexchange/?term=authorid%3A31862 It goes over that plus more.

Answer 6

[vinoba]

How to find centroid of each and every blob(objects) in a binary image?

[Image Analyst]

See my Image Segmentation Tutorial in my File Exchange: http://www.mathworks.com/matlabcentral/fileexchange/?term=authorid%3A31862

Basically

[labledImage, numBlobs] = bwlabel(binaryImage);
props = regionprops(labeledImage, 'Centroid', 'WeightedCentroid');

You get a structure array with both types of centroids for every region.

Answer 7

[tanuwyot]

How to locate the nucleus centroid and extract the seven HU moments value ? Please help me

[Image Analyst]

Use regionprops(). See http://www.mathworks.com/matlabcentral/fileexchange/25157-image-segmentation-tutorial

I don't have code for Hu's moments.