How to measure radius of circle in big image?

(image above is copydrawing of original image. Sorry for impossible of uploading original image)

Because its size is over 2000x2000 pixels, Some problem has been found.

I tried 3 ways, and failed with each following reasons.

%%%%%%%%%%%%%%%fail 1 %%%%%%%%%%%%%%%%%%%%%%%%%

   stats = regionprops('table',K,'Centroid',...
     'MajorAxisLength','MinorAxisLength');

   centers = stats.Centroid;
   diameters = mean([stats.MajorAxisLength stats.MinorAxisLength],2);
   radii = diameters/2;

   viscircles(centers,radii);

% Tracked wrong circle.

%%%%%%%%%%%%%%%fail 1 %%%%%%%%%%%%%%%%%%%%%%%%%

%%%%%%%%%%%%%%%fail 2 %%%%%%%%%%%%%%%%%%%%%%%%%

   [centers, radii] = imfindcircles(out,[1000 1100], 'Sensitivity', 0.92);
   h = viscircles(centers, radii);

% Too big size to track it
% One radius per multiple circles tracked.

%%%%%%%%%%%%%%%fail 2 %%%%%%%%%%%%%%%%%%%%%%%%%

%%%%%%%%%%%%%%%fail 3 %%%%%%%%%%%%%%%%%%%%%%%%%

   r = sqrt(sum(out(:))/pi);
  % Code for full circle

%%%%%%%%%%%%%%%fail 3 %%%%%%%%%%%%%%%%%%%%%%%%%

And trying one experiment. seeing code from http://matlab.wikia.com/wiki/FAQ#How_can_I_fit_a_circle_to_a_set_of_XY_data.3F

%%%%%%%%%%%%%%%exp 1 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

[B,L,N] = bwboundaries(out,4,'holes');

figure; imshow(out); hold on;
for k = 1 : length(B),
    boundary = B{k};
    if(k > N)
        plot(boundary(:,2), boundary(:,1), 'g', 'LineWidth', 2);
    else
        plot(boundary(:,2), boundary(:,1), 'r', 'LineWidth', 2);
    end
end

x = boundary(:,2);
y = boundary(:,1);

   n=length(x);  xx=x.*x; yy=y.*y; xy=x.*y;
   A=[sum(x) sum(y) n;sum(xy) sum(yy) sum(y);sum(xx) sum(xy) sum(x)];
   B=[-sum(xx+yy) ; -sum(xx.*y+yy.*y) ; -sum(xx.*x+xy.*y)];
   a=A\B;
   xc = -.5*a(1);
   yc = -.5*a(2);
   R  =  sqrt((a(1)^2+a(2)^2)/4-a(3));
  %

%%%%%%%%%%%%%%%exp 1 %%%%%%%%%%%%%%%%%%%%%%%%%

and still finding which elements should be selected as x,y component in circfit function.

• Is there anything I missed?
• Or did I choose wrong way?

Any small kind answer would be a huge help!

Answer 1

[hyong suk choo]

Solved with calc_circle code

https://kr.mathworks.com/matlabcentral/newsreader/view_thread/165185

Select 3 point on edge (2 Lowest point with extreme right and left side and 1 highest point on right side) and use this code

Of course it is not precise method, but error is negligible for me. (error in 10 pixel scale.)

I = imread(image);

%Detect Entire Cell

[~, threshold] = edge(I,'sobel');
fudgeFactor = .8;
BWs = edge(I,'sobel',threshold*fudgeFactor);

%Dilate the image

se90 = strel('line',3,90);
se0 = strel('line',3,0);
BWsdil = imdilate(BWs,[se90 se0]);

 [B,L,N] = bwboundaries(BWsdil,4,'holes');
 figure; 
 imshow(BWsdil); 
 axis equal; 
 hold on;

 for k = 1 : length(B),
     boundary = B{k};
     if(k > N)
         plot(boundary(:,2), boundary(:,1), 'g', 'LineWidth', 2);
     else
         plot(boundary(:,2), boundary(:,1), 'r', 'LineWidth', 2);
     end
 end

 [y,x] = find(BWsdil);

 table(:,1) = x;                    % making table array
 table(:,2) = y;

 yyy = max(y);
 exp1 = table(table(:,2)==yyy);    % lowest position on image
 pp1 = min(exp1);                   % end of left side
 pp3 = max(exp1);                   % end of right side

 tablefor3rd = table(:,1) > 2000;   % X coordinate over 2000
 extracted = table(tablefor3rd,:);  % rows with X coordinates over 2000
 maxiy = min(extracted(:,2));       % highest position
 list = extracted(:,2) == maxiy;    % point list with highest point
 list2 = extracted(list,:);
 maxix = max(list2(:,1));

 p1 = [pp1,yyy];
 p2 = [maxix,maxiy];
 p3 = [pp3,yyy];

 [c r] = calc_circle(p1, p2, p3);

 axis equal
 hold on
 if r == -1
     disp('COLLINEAR')
 else
     rectangle('Position', [c(1)-r,c(2)-r,2*r,2*r],...
 'Curvature', [1,1], 'EdgeColor', 'g')
 end
 plot(p1(1), p1(2), '*')
 plot(p2(1), p2(2), '*')
 plot(p3(1), p3(2), '*')