How do I find the centre of an image?

Question

Kantachai Chamnarnkit 2023-2-8

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https://ww2.mathworks.cn/matlabcentral/answers/1908755-how-do-i-find-the-centre-of-an-image

评论： DGM 2023-3-15

I want to find the centre of the white area and measure the length of the x and y axes.

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Sarvesh Kale 2023-2-8

It would be nice if you upload the image as an attachment

DGM 2023-3-15

As I have a feeling that the question being asked isn't what's actually needed, see also:

https://www.mathworks.com/matlabcentral/answers/1906470-how-do-i-measure-distances-in-an-image

Jonas' answer gives the section widths intersecting the blob centroid -- what the question asks for.

The above link also covers similar size metrics:

height and width of bounding box
equivalent diameters
feret diameters

Sarvesh's answer below already touched on two of those.

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Answer 1

Jonas 2023-2-8

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在 MATLAB Online 中打开

you can use regionprops for that and let the function return the centroid. This may be the cetner you are searching for.

If you are then interested in the horizontal and vertical extent trough this point, you could just search for the first/last white pixel in the row/column given by the coordinates of the centroid. given centroidX,centroidY you could use something like

find(img(:,centroidX),1,'last')-find(img(:,centroidX),1,'last')
% and
find(img(centroidY,:),1,'last')-find(img(centroidY,:),1,'last')

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Kantachai Chamnarnkit 2023-2-9

I don't know how to search for the first/last white pixel in the row/column

DGM 2023-2-9

编辑：DGM 2023-2-9

在 MATLAB Online 中打开

tiltellipse.png

You use find() like Jonas mentioned.

img = imread('tiltellipse.png')>128;

% get the centroid

S = regionprops(img,'centroid');

centroidX = round(S.Centroid(1)); % round it for indexing purposes

centroidY = round(S.Centroid(2));

% i'm doing it this way so that i can plot the results

yrange = [find(img(:,centroidX),1,'first') find(img(:,centroidX),1,'last')];

xrange = [find(img(centroidY,:),1,'first') find(img(centroidY,:),1,'last')];

blobheight = diff(yrange)

blobheight = 128

blobwidth = diff(xrange)

blobwidth = 121

% plot the lines that are being measured

imshow(img); hold on

plot(xrange,[1 1]*centroidY,'r-o');

plot([1 1]*centroidX,yrange,'b-o');

Note that this gives the maximum width along each line. Consider how this would behave with an object like this:

It's up to you to decide what you want to measure.

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Answer 2

Sarvesh Kale 2023-2-8

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https://ww2.mathworks.cn/matlabcentral/answers/1908755-how-do-i-find-the-centre-of-an-image#answer_1166490

编辑：Sarvesh Kale 2023-2-8

在 MATLAB Online 中打开

Hi Kantachai Chamnarnkit,

the following code snippet might help you. Assuming the image in binary in nature

i = imread('image.jpeg'); % name of your image should be inside ' '
blobs=regionprops(i,"Centroid","MinorAxisLength","MajorAxisLength","BoundingBox");

for more information on regionprops

https://in.mathworks.com/help/images/ref/regionprops.html

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Jonas 2023-2-8

MinorAxisLength / MajorAxisLength does not give horizontal and vertical size of the blob. it assumes an ellipse and takes the axes of the ellipse, which can be rotated

the bounding box has the problem that the respective line will not necessarily go through the center

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