Why can't a circle be detected in such an obvious image?

Why can't a circle be detected in such an obvious image?

img = imread('circle.png');
[centers1,radii1] = imfindcircles(img,100,...
'Sensitivity',0.85,...
'ObjectPolarity','bright')

No matter how you adjust the value of 'sensitivity' or the radius or 'ObjectPolarity', the circle is not detected, why?

os: win10

matlab R2021a

Answer 1

[Image Analyst]

Try increasing the sensitivity to 0.95

clc;    % Clear the command window.
close all;  % Close all figures (except those of imtool.)
clear;  % Erase all existing variables. Or clearvars if you want.
workspace;  % Make sure the workspace panel is showing.
format long g;
format compact;
fontSize = 20;

%--------------------------------------------------------------------------------------------------------
%    READ IN IMAGE
folder = pwd;
baseFileName = 'circle1.png';
grayImage = imread(baseFileName);
% Get the dimensions of the image.
% numberOfColorChannels should be = 1 for a gray scale image, and 3 for an RGB color image.
[rows, columns, numberOfColorChannels] = size(grayImage)
if numberOfColorChannels > 1
	% It's not really gray scale like we expected - it's color.
	% Use weighted sum of ALL channels to create a gray scale image.
	grayImage = min(grayImage, [], 3);
end

%--------------------------------------------------------------------------------------------------------
% Display the image.
imshow(grayImage, []);
axis('on', 'image');
title('Binary Image', 'FontSize', fontSize, 'Interpreter', 'None');
impixelinfo;
hFig = gcf;
hFig.WindowState = 'maximized'; % May not work in earlier versions of MATLAB.
drawnow;

[centers1, radii1] = imfindcircles(grayImage,[75, 125],'Sensitivity',0.95,'ObjectPolarity','bright')
viscircles(centers1, radii1, 'LineWidth', 4);

Answer 2

[Stephan]

Use a range for radius input argument such as:

[centers1,radii1] = imfindcircles(img,[50 200],'Sensitivity',0.85,'ObjectPolarity','bright')

[xingxingcui]

Hi, your solution is not working and still not detecting any results!

you can try code