Draw line and Cut off Circuler area

Question

I have got the below Image after running the below code.

file=\'grayscale.png\';
I=imread(file);
bw = im2bw(I);
bw = bwareaopen(bw,870);
imwrite(bw,\'n         


        

Answer 1

Alternatively, you can optimize/fit the circle with least r that contains all the points:

bw = imread('http://i.stack.imgur.com/il0Va.png');
[yy xx]=find(bw);

Now, let p be a three vector parameterizing a circle: p(1), p(2) are the x-y coordinates of the center and p(3) its radii. Then we want to minimize r (i.e., p(3)):

obj = @(p) p(3);

Subject to all points inside the circle

con = @(p) deal((xx-p(1)).^2+(yy-p(2)).^2-p(3).^2, []);

Optimizing with fmincon:

[p, fval] = fmincon(obj, [mean(xx), mean(yy), size(bw,1)/4], [],[],[],[],[],[],con);

Yields

p =
471.6397  484.4164  373.2125

Drawing the result

imshow(bw,'border','tight');
colormap gray;hold on;
t=linspace(-pi,pi,1000);
plot(p(3)*cos(t)+p(1),p(3)*sin(t)+p(2),'r', 'LineWidth',1);

You can generate a binary mask of the same size as bw with true in the circle and false outside

msk = bsxfun(@plus, ((1:size(bw,2))-p(1)).^2, ((1:size(bw,1)).'-p(2)).^2 ) <= p(3).^2;

The mask looks like: