Code for finding center of circle

2015 11 10
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更新时间：2018 9 20
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Hello, Could someone please help me with a code for finding center of circle. I have known points (Xa,Xb, Ya,Yb) and Radius. I need to write a code in Matlab to display the center from a cloud data. Any help would be appreciated.
Thanks

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i used the code imfindcircle to find the radius .but i need edge detection code and then to calculate the center point.could you help me out.
Image Analyst
Image Analyst 2018-9-20
s., Are you SURE you looked at the documentation for imfindcircles(), which clearly states that it returns the radii:
[centers,radii] = imfindcircles(A,radiusRange) finds circles with radii in the range specified by radiusRange. The additional output argument, radii, contains the estimated radii corresponding to each circle center in centers.
Also see attached paper.

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Image Analyst
Image Analyst 2015-11-10

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avinash desh
avinash desh 2015-11-10
Thank You. To be clear th ebelow code will fid center from data pool. also can I ssign values to R, x, y and allow matlab to compute function [xc,yc,R,a] = circfit(x,y) %CIRCFIT Fits a circle in x,y plane % % [XC, YC, R, A] = CIRCFIT(X,Y) % Result is center point (yc,xc) and radius R. A is an optional % output describing the circle's equation: % % x^2+y^2+a(1)*x+a(2)*y+a(3)=0
Image Analyst
Image Analyst 2015-11-10
I'm not sure what you mean. Is that a question?
avinash desh
avinash desh 2015-11-10
I have attached the code, will this code get me center for circle. I know what R value, X1,X2, Y1,Y2 values,. How do I enter these values in code
Image Analyst
Image Analyst 2015-11-10
Where did you attach it? I don't see any circle fitting code attached, just the code in your comment, but there's no real code there, just a function definition and a bunch of comments.
avinash desh
avinash desh 2015-11-10
Hello, I need help in writing the code, with the formula's you had directed. Like I said I know R, X1,X2, Y1,Y2 values. I need to find the center from circle that is defined by 10000 point clouds from a laser scan. Hope this helps
Image Analyst
Image Analyst 2015-11-10
Like I said, the code was in the FAQ. Help me to understand why you say "I need help in writing the code". I assume you already have code to get your x and y values, and the code for the circfit() function is in the FAQ so I don't understand what code you need help writing. Just call the circfit() function after your code that gets the x and y values. If you have 10,000 x values and 10,000 y values, just pass them in to this function (from the FAQ):
function [xc,yc,R,a] = circfit(x,y)
%CIRCFIT Fits a circle in x,y plane
%
% [XC, YC, R, A] = CIRCFIT(X,Y)
% Result is center point (yc,xc) and radius R. A is an optional
% output describing the circle's equation:
%
% x^2+y^2+a(1)*x+a(2)*y+a(3)=0
% by Bucher izhak 25/oct/1991
n=length(x); xx=x.*x; yy=y.*y; xy=x.*y;
A=[sum(x) sum(y) n;sum(xy) sum(yy) sum(y);sum(xx) sum(xy) sum(x)];
B=[-sum(xx+yy) ; -sum(xx.*y+yy.*y) ; -sum(xx.*x+xy.*y)];
a=A\B;
xc = -.5*a(1);
yc = -.5*a(2);
R = sqrt((a(1)^2+a(2)^2)/4-a(3));
avinash desh
avinash desh 2015-11-10
Thank you for your prompt response. I should have been clearer in my question,I do not have a code to get x and y values, I need help in writing that code
Image Analyst
Image Analyst 2015-11-10
You must at least have the data file that your laser scanner produced. Please attach it.
avinash desh
avinash desh 2015-11-10
I have attached the file, it's only a portion of the data file as the full file size is 12MB. It is data for a part, attached snapshot. I am trying to find the center of the part as highlighted in picture using 3 sets of points
avinash desh
avinash desh 2015-11-11
Hello Sorry to bother you again, just wanted to follow up if you had a chance to look at the data file for code. Thank you again for your help.
Image Analyst
Image Analyst 2015-11-11
What does each column represent? There are 3 and I thought you would have 2: one column for x and one for y. If I plot the first 2 columns, it looks like this:
What is this? Where is the circle?
avinash desh
avinash desh 2015-11-11
Thank you for taking a look at this. The 3 columns are x, y,z. But y and z have been interchanged. so you only have to use 1 and 3 column The circle is in the center. I was able to make some progress I was able to get a contour to pick points on the perimeter of the circle. I now have 2 quadratic equations with 2 known and 2 unknown. Ho do I write a script to solve this equation. (x1-xc)^2+(y1-yc)^2=R^2 (x2-xc)^2+(y2-yc)^2=R^2 I know the values for x1,x2,y1,y2 and R, these can be input manually in the script. I need to find xc and yc. How do I solve it in Matlab
Image Analyst
Image Analyst 2015-11-11
Those x,y are over the whole part, not just the perimeter of the circle. You're going to have to find a way to get just the x,y around the perimeter of the circle. Maybe by converting to an image.

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