How should I use the function Circfit ?

Hi All

How do I use the function Circfit ? the size and class of inputs is not clear

Answer 1

[Adam Danz]

From the help section of that file,

%  x,y are column vector where (x(i),y(i)) is a measured point

Their class are double.

They are column vectors of any length but must have the same length between x and y.

Demo

Here's a demo; see the inline comments to understand each step.

First, compute noisy circle coordinates for the demo.

% Create noisy circle coordinates
radius=3;
theta=linspace(0,2*pi,200); 
theta(randi(numel(theta),1,190)) = []; % remove some data
x=radius*cos(theta);
y=radius*sin(theta);

% add noise
x = x + (rand(size(x))-0.5)*radius/4; 
y = y + (rand(size(y))-0.5)*radius/4; 

% Plot it
plot(x,y,'o')
axis equal

Do the fitting but you have to fill in the inputs. The reason I'm not telling you what the inputs should be is not to cause frustration but to get you to think about this. Again, the two inputs are column vectors of your x and y coordinates of each point you're fitting.

% Fit it
[xc,yc,R,a] = circfit(___, ___); % FILL IN THE INPUTS HERE
% [update: here's the solution: [xc,yc,R,a] = circfit(x, y);

Now you can use the outputs to compute the fitted circle and plot it.

% compute circle based on fit params
theta=linspace(0,2*pi,200); 
xFit=R*cos(theta)  + xc;
yFit=R*sin(theta)  + yc;

% add fit
hold on
plot(xc,yc,'rx') % plot circle center
plot(xFit,yFit, 'r-') 

[farzad]

What length ? if I have to find the circumcircle of 3 points with their x and y , how do I write the command ?

[Adam Danz]

x and y would be the x and y values of the 3 points. So, they would have length 3 if you only have 3 points.

Try it. If it doesn't work, show me what inputs you tried and I can help straighten it out.

[farzad]

I'm asking if I have 3 coordinates what should I put instead of circLocs

[centerLoc, circleNormal, radius] = CircFit3D(circLocs)

should I do

[centerLoc, circleNormal, radius] = CircFit3D(x1,y1,x2,y2,x3,y3) ?

if not , please write the line, I don't get it from the incomplete documentation, and nothing on internet

[Adam Danz]

The syntax is

[xc,yc,R,a] = circfit(x,y)

where x and y are column vectors describing the x and y points along the circumference.

You said you have 3 (x,y) coordinates. So x and y each have 3 elements.

I've gotten that from the documentation which I would not call 'incomplete'.

[farzad]

what do you mean ? shall you please write it , how you understood ? are you sure it works ?

I don't understand your definitions, please write the line, why are you making me wait for longer time ?

I have already exhausted my guesses, if you know, why are you refraining ?

[Adam Danz]

I've updated my answer, see the demo I've provided.

[farzad]

why a vector? and how can the vector length be the same for different points?

I justice don't get it

[Adam Danz]

Run my example. The x and y variables are vectors and together they are the coordinates of each noisy point along the cirlce. They are the variables you are fitting.

If you are only fitting 3 points, then your x vector will have 3 values (the x value for each point) and your y vector will have 3 values (the y value for each point).

[Adam Danz]

@farzad, in case you still haven't figured it out, I've updated my answer to show what the two inputs should be to circfit().