MATLAB Answers

Finding center of a line

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samir
samir on 30 Jan 2012
Hi i need to find the center of a line.A robot needs to foolow this line in real time but my problem is how to find the center of a line.I used bwmorph(image,'thin',inf); and i get the that my thick line thins to a line with 1 pixel size.
so now the only thing that i need is to calculate the distance between the center of my image (blue +) and the line so i could say to my robot move to left so that the center of the line is equal to the image center. .

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Accepted Answer

Kevin Moerman
Kevin Moerman on 2 Feb 2012
If your edge/line is the only segmented object (only the line consists of ones) in your image then if M is your binary image (containing ones for white) then: [I,J]=find(M); %equivalent to find(M==1) gives you the I and J indices or image coordinates of the whole line. If your line is straight then the mean of these coordinates is the same as the mean of the first and last coordinate and thus gives you the middle.
If you have multiple objects in the image with ones, not just your line. Then you could filter those out based on size/shape criteria and do the above. Alternatively could create an adjacency matrix whereby you check whether pixels are one and also have neighbours in the line direction that are one. For the end points of the line the latter will be false which allows you to find them. But this depends on the quality and thickness of your line depictions. So not sure if this would work. Also you could develop a type of mask (filter) that will highlight the ends of lines. But I would go for the simple mean of the object approach.
Kevin

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samir
samir on 2 Feb 2012
well i used find for finding all x and y and then i used mean x and mean y so i got the mean values of x and y where every pixel is 1.

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More Answers (2)

Image Analyst
Image Analyst on 30 Jan 2012
How about using the Pythagorean Theorem? Look up MATLAB's hypot() function.

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samir
samir on 30 Jan 2012
well the harder part is to find the center of the line when i have the centers i can find the distance between them
Image Analyst
Image Analyst on 30 Jan 2012
You're right, if the "line" is not straight. You can't just find the centroid because the centroid may not line on or in the line. You could try bwboundaries() and play around with what that returns.
samir
samir on 31 Jan 2012
i will try it with bwboundaries and see what i get

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Kevin Moerman
Kevin Moerman on 31 Jan 2012
If its a straight line then you could simply take the mean of your line coordinates (or even centre of gravity of the image object).
If the line is curved then sum up line segments to get total curve length (calculate point-to-point distances and use cumsum for instance), and then solve for (e.g. interpolate) what coordinate describes half of the curve length.

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samir
samir on 31 Jan 2012
ok thank you for your help.
I did some modifications i zoomed with the camera to the line so the line is now more or less straight.I used edge() function and i get the edges .So now i have to find the coordinates of first white and the last white pixel pixel in middle row
[x,y]=size(image);
middle_x=x/2;
With the funciton find() i get every white pixel.So my question is how to get only the twho white pixels where x=middle_x

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