How to crop a circle from an image?

hey, asking for the second time.

I need to find a circle in an image and crop that region of the circle to a new image, I already found a way to find the circle using "imfindcircle" fuction, but when I use that function it only marks the circle and i need to crop it somehow but I dont know what info i can use... thanks in advance

[Ameer Hamza]

Can you post the image and code you have developed until now? It will be easier for us to suggest a solution if we know what you have already done.

[Shoval Matzner]

A = imread('Speed.jpg');


% Resize the image
rowSZ = size(A, 1);
colSZ = size(A, 2);
rows = 250*(colSZ/rowSZ);
J = imresize(A, [rows 250]);
% Show image
imshow(J)

% Set circle radius to find
Rmin = 50;
Rmax = 100;
% Find circles within the raidus
[centers, radii, metric] = imfindcircles(J,[Rmin Rmax]);

% Show circle on the image
viscircles(centers, radii,'EdgeColor','b');

this is the circle finding code, and the pic i used

Answer 1

[Turlough Hughes]

Try the following tutorial:

% Setup
clear
close all
clc
inputImg = imread('cameraman.tif');

% Show demo Image
hf = figure();
set(hf,'Units', 'Normalized', 'OuterPosition', [0 0 1 1])
subplot(1,2,1)
imshow(inputImg)

% Example of data output from findcircles with two circle outputs
centers = [50 50; 150 200];
radii = [20; 30];

% Draw circles on image (just for demo)
drawcircle('Center',centers(1,:),'Radius',radii(1))
drawcircle('Center',centers(2,:),'Radius',radii(2))

% Generate binary image (mask) for representing cropped regions
mask = false(size(inputImg));
for c = 1:numel(radii)
    [colsInImg,rowsInImg] = meshgrid(1:size(inputImg,1),1:size(inputImg,2));
    circlePixels = (rowsInImg - centers(c,2)).^2 ...
        + (colsInImg - centers(c,1)).^2 <= radii(c).^2;
    mask(circlePixels) = true;
end

% Use mask to get cropped image
cropped = inputImg;
cropped(~mask) = false;

% Show results
subplot(1,2,2)
imshow(cropped)

[Turlough Hughes]

Using your code to find the region of interest that you wish to crop:

clear
close all
clc
A = imread('Speed.jpg');
% Resize the image
rowSZ = size(A, 1);
colSZ = size(A, 2);
rows = 250*(colSZ/rowSZ);
J = imresize(A, [rows 250]);
% Show image
imshow(J)
% Set circle radius to find
Rmin = 50;
Rmax = 100;
% Find circles within the raidus
[centers, radii, metric] = imfindcircles(J,[Rmin Rmax]);
% Show circle on the image
viscircles(centers, radii,'EdgeColor','b');

You could then do the following to crop the region inside the circle. Note that I reduced the radius by 2 pixels because otherwise some of the red gets included, and that I set the background colour to white.

[colsInImg,rowsInImg] = meshgrid(1:size(J,1),1:size(J,2));
circlePixels = (colsInImg - centers(1,1)).^2 + ...
    (rowsInImg - centers(1,2)).^2 < (radii(1)-2).^2; 

Jcrop = J;
Jcrop(repmat(~circlePixels,1,1,3)) = 255;
imshow(Jcrop)
figure(), imshow(Jcrop)

[Shoval Matzner]

thank you so much!!