How to Rotate the Ellipses at calculated angle?

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Muhammad khan 2020-6-17

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/550128-how-to-rotate-the-ellipses-at-calculated-angle

编辑： Muhammad khan 2020-6-18

I am using the code as given below to calculate the ellipses and rotate by an angle. But it is not rotated at given angle. Can anyone please help me to resolve this issue?

%cov is 2x2 covariance matrix that is generated through a long process
t = 0:0:2*pi; %time axis
%Xest is center of ellipse and contain values of x and Y
%cov = 2x2 matrix
lambda = eig(cov);
	 [S, D] = eig(cov);
	 scale = 2.447;  %for 95% confidense ellipse from sigma1
	 [a, a_i] = max(lambda); %find maximum eigenvalue
	 [b, b_i] = min(lambda); %find minimum eigenvalue
	 a = scale*sqrt(a); %scale according to confidence interval
	 b = scale*sqrt(b);
	 
	 if cov(1,1) > cov(2,2) %resolve tilt of the ellipse
        theta = atan2(S(2,a_i),S(1,a_i))
         x_axis = a
        y_axis= b
     else
        theta = atan2(S(2,b_i),S(1,b_i))
        x_axis = b
        y_axis = a
     end
     
     x = Xest(1) + x_axis*cos(t)*cos(theta) - y_axis*sin(t)*sin(theta);
     y = Xest(2) + x_axis*cos(t)*sin(theta) + y_axis*sin(t)*cos(theta);
     r = patch(x,y,'b');

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Image Analyst 2020-6-18

在 MATLAB Online 中打开

test.m

Your code gives this:

Error using cov (line 46)
Not enough input arguments.
Error in test (line 6)
lambda = eig(cov); 

Muhammad khan 2020-6-18

编辑：Muhammad khan 2020-6-18

if you use the covariance matrix i posted earlier you definetly get some results.

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回答（1 个）

Rafael Hernandez-Walls 2020-6-18

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链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/550128-how-to-rotate-the-ellipses-at-calculated-angle#answer_453489

在 MATLAB Online 中打开

%cov is 2x2 covariance matrix that is generated through a long process
t = 0:0.1:2*pi; %time axis
%Xest is center of ellipse and contain values of x and Y
cov=[14.7813169401575                 18.14084502625
           18.140845026252            33.2623639805209];
       Xest=[0 0];
%cov = 2x2 matrix
lambda = eig(cov);
	 [S, D] = eig(cov);
	 scale = 2.447;  %for 95% confidense ellipse from sigma1
	 [a, a_i] = max(lambda); %find maximum eigenvalue
	 [b, b_i] = min(lambda); %find minimum eigenvalue
	 a = scale*sqrt(a); %scale according to confidence interval
	 b = scale*sqrt(b);
	 
	 if cov(1,1) > cov(2,2) %resolve tilt of the ellipse
        theta = atan2(S(2,a_i),S(1,a_i))
         x_axis = a
        y_axis= b
     else
        theta = atan2(S(2,b_i),S(1,b_i))
        x_axis = b
        y_axis = a
     end
     
     x = Xest(1) + x_axis*cos(t)*cos(theta) - y_axis*sin(t)*sin(theta);
     y = Xest(2) + x_axis*cos(t)*sin(theta) + y_axis*sin(t)*cos(theta);
     r = patch(x,y,'b');

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the cyclist 2020-6-18

OK, so this "answer" is just a recopying of OP's code, with the value of cov plugged in. Progress?

The following is the figure it produces. We continue to need more information from OP, about what the problem is. This ellipse looks nothing like the figure OP posted, so I find it impossible to give advice.