Fitting data from file
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Hello,
I am trying to fit data from a file. For that I have created data with simply a sine wave function. Here is my code:
load test_fit
x = test_fit(:,1);
y = test_fit(:,2);
fo=fit(y,x,'a*sin(b*x+c)+d');
plot(fo,y,x)
But the results I got are clearly not good:
I'm just wondering if I should specify somwhere the number of itterations or the fitting constrains?
Thank you in advance for any help or comments!
采纳的回答
You can specify those parameters, but what is much more effective is providing good initial guesses (and not mixing up x and y):
test_fit=load(websave('test_fit.txt','https://www.mathworks.com/matlabcentral/answers/uploaded_files/1137860/test_fit.txt'));
x = test_fit(:,1);
y = test_fit(:,2);
plot(x,y,'*')
fo=fit(x,y,'a*sin(b*x+c)+d','StartPoint',[1 4 0 2])
hold on,plot(fo)
10 个评论
Piotr
2022-9-29
Rik
2022-9-29
If the fit function requires column vectors, why don't you transpose x and y to be column vectors?
But there is a fundamental problem here: you have 2 points, to which you are fitting an equation with 4 unknowns. That is not going to be a reliable fit. It seems like you want to use the profile as your data, but you don't.
Piotr
2022-9-29
Okay I transformed to column vector with:
fo=fit(x(:),y(:),'m*sin(n*x+o)+p','StartPoint',[125 100 0 125])
and also got the error about insuficient point number which I don't understand becaouse there are many points:
Rik
2022-9-29
Where exactly are you telling the fit function that you want to use all those points? (Hint: you don't)
Piotr
2022-9-29
You provide x and y as input arguments to the fit function, but that only contains the coordinates of the line you're using for improfile, not the data you're plotting. You probably want to use fit(x_adapted,c,___). That first variable should contain the x position for each value in c. Currently when you're plotting your data, you are implicitly using the index as the x-value. That's fine for getting a first impression of your data, but not for actual analysis.
You probably want this:
x_adapted = linspace(y(1),y(2),numel(c));
I would also suggest doing a basic Matlab tutorial. The Onramp tutorial is provided for free by Mathworks.
Okay, thank you now that is clear but still got an error with fit weights. Even if I define wieghts I still get it
Error using fit>iFit
X, Y and WEIGHTS cannot have NaN values.
Error in fit (line 116)
[fitobj, goodness, output, convmsg] = iFit( xdatain, ydatain, fittypeobj, ...
Error in testfitimage (line 20)
fo=fit(transpose(x_adapted),c(:,1,1),ft)
websave('testf5.png','https://www.mathworks.com/matlabcentral/answers/uploaded_files/1139785/testf5.png');
I=imread('testf5.png');
x = [size(I,2)/2 size(I,2)/2];
y = [0 size(I,1)];
c = improfile(I,x,y);
figure
subplot(2,1,1)
imshow(I)
hold on
plot(x,y,'-o')
subplot(2,1,2)
plot(c(:,1,1),'-o')
x_adapted = linspace(y(1),y(2),numel(c(:,1,1)));
ft = fittype('m*sin(n*x+o)+p');
options=fitoptions(ft);
options.StartPoint = [125 100 0 125];
options.Weights = [1,1];
plot(x_adapted(:),c(:,1,1))
fo=fit(transpose(x_adapted),c(:,1,1),ft)
Walter Roberson
2022-9-30
x = [size(I,2)/2 size(I,2)/2];
y = [0 size(I,1)];
c = improfile(I,x,y);
y = 0 is outside the image coordinates, so the profile gives NaN at that location.
If you look at the code below, you see that c starts with a NaN. So we should remove it.
I = imread(websave('testf5.png','https://www.mathworks.com/matlabcentral/answers/uploaded_files/1139785/testf5.png'));
x = [size(I,2)/2 size(I,2)/2];
y = [0 size(I,1)];
c = improfile(I,x,y);
c = c(:,1,1); % select red chanel
x_adapted = linspace(y(1),y(2),numel(c));
x_adapted = x_adapted.'; % transpose 1x1491 to 1491x1
subplot(2,1,1)
imshow(I)
hold on
plot(x,y,'-o')
hold off
subplot(2,1,2)
plot(x_adapted,c)
ft = fittype('m*sin(n*x+o)+p');
options=fitoptions(ft);
options.StartPoint = [125 100 0 125];
options.Weights = [1,1];
find(isnan(x_adapted)),find(isnan(c))
Here is the problem, so we'll have to remove that point.
L = isnan(x_adapted) | isnan(c); % the first part is just good habit
x_adapted(L) = [];
c(L) = [];
% Now we can safely to the fit:
fo=fit(x_adapted,c,ft,StartPoint=[125 100 0 125])
figure,plot(fo)
So the fit is not great, you will have to adjust the start point.
Rik
2022-9-30
If my answer helped you, please consider marking it as accepted answer.
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