General conic curve fit with constrained coefficients

Question

Scott Larwood 2021-2-26

1
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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/757189-general-conic-curve-fit-with-constrained-coefficients

移动：Matt J 2023-2-21

采纳的回答： Matt J

Hello,

I would like to fit a set of xy points to a general conic equation of the form:

ax^2 + bxy + cy^2 +dx + ey + f = 0

where I need to constrain the coefficient "c" to zero. So I would like to fit:

ax^2 + bxy +dx + ey + f = 0

to determine the values of the coefficients.

I have tried:

https://www.mathworks.com/matlabcentral/fileexchange/22683-ellipse-fit-taubin-method

but I cannot determine how to contrain the coefficient. It gives me a good fit, but the coefficient "c" is non-zero. The fit is not good if I plot it with the "c" coefficient set to zero.

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Matt J 2021-3-1

It would be recommendable to attach a single.mat file containing your x,y data, so we can play with it.

tarek hussein 2023-2-21

移动：Matt J 2023-2-21

hello dear

how can i fit an ellipse from experimenta data(x and y vectors).. i really need to find the diameter of that ellipse.

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Answer 1

Matt J 2021-3-1

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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/757189-general-conic-curve-fit-with-constrained-coefficients#answer_636284

编辑：Matt J 2021-3-1

在 MATLAB Online 中打开

Here's a basic analytical fit, using the tools in this File Exchange package:

[xy,T]=conicFit.homogNorm([x(:),y(:)].');
x=xy(1,:).'; y=xy(2,:).';
q = conicFit.mostnull([x.^2, x.*y, x, y, x.^0]);
C=[q(1), q(2), q(3);
    0      0   q(4);
    0      0   q(5)];
C= T.'*(C/2+C.'/2)*T;
[a, b, d, e, f] = deal(C(1), 2*C(4), 2*C(7), 2*C(8), C(9));