How to crop circle from an image?

Here I have have defined a circle using viscircles. Now i want to get the image inside the circle. How can I do that? I have tried it with impolygon but I have to crop the image for all the image in folder and turns out its really slow. Is there any way to do that?

im = imread('flower.jpg');
imshow(im);
[x,y] = getpts;
j = viscircles([x(1),y(2)], sqrt((x(1)-x(2))^2+(y(1)-y(2))^2));

Answer 1

[Image Analyst]

To process a sequence of images, see the code in the FAQ:

https://matlab.fandom.com/wiki/FAQ#How_can_I_process_a_sequence_of_files.3F

To crop the circle from the image, use drawcircle():

% Demo to have the user click and draw a circle over an image, then blacken outside the circle and crop out the circular portion into a new image.
clc;    % Clear the command window.
fprintf('Beginning to run %s.m ...\n', mfilename);
close all;  % Close all figures (except those of imtool.)
imtool close all;  % Close all imtool figures.
clear;  % Erase all existing variables.
workspace;  % Make sure the workspace panel is showing
fontSize = 15;

% Get image.
originalImage = imread('peppers.png');
[rows, columns, numberOfColorChannels] = size(originalImage)
subplot(2, 2, 1);
imshow(originalImage);
axis('on', 'image');
title('Original Image', 'FontSize', fontSize);
% Maximize the window to make it easier to draw.
g = gcf;
g.WindowState = 'maximized'
% Ask user to draw a circle:
uiwait(helpdlg('Please click and drag out a circle.'));
h.Radius = 0;
while h.Radius == 0
	h = drawcircle('Color','k','FaceAlpha',0.4)
	if h.Radius == 0
		uiwait(helpdlg('You double-clicked.  You need to single click, then drag, then single click again.'));
	end
end
% Get coordinates of the circle.
angles = linspace(0, 2*pi, 10000);
x = cos(angles) * h.Radius + h.Center(1);
y = sin(angles) * h.Radius + h.Center(2);
% Show circle over image.
subplot(2, 2, 2);
imshow(originalImage);
axis('on', 'image');
hold on;
plot(x, y, 'r-', 'LineWidth', 2);
title('Original image with circle mask overlaid', 'FontSize', fontSize);
% Get a mask of the circle
mask = poly2mask(x, y, rows, columns);
subplot(2, 2, 3);
imshow(mask);
axis('on', 'image');
title('Circle Mask', 'FontSize', fontSize);

% Mask the image with the circle.
if numberOfColorChannels == 1
	maskedImage = originalImage; % Initialize with the entire image.
	maskedImage(~circleImage) = 0; % Zero image outside the circle mask.
else
	% Mask the image.
	maskedImage = bsxfun(@times, originalImage, cast(mask, class(originalImage)));
end

% Crop the image to the bounding box.
props = regionprops(mask, 'BoundingBox');
maskedImage = imcrop(maskedImage, props.BoundingBox);

% Display it in the lower right plot.
subplot(2, 2, 4);
imshow(maskedImage, []);
% Change imshow to image() if you don't have the Image Processing Toolbox.
title('Image masked with the circle', 'FontSize', fontSize);
fprintf('Done running %s.m ...\n', mfilename);

[Santiago Acosta]

Thank you very much! This has been super useful to me as well. There is however a minor error in the code. When masking with just one color channel:

maskedImage(~circleImage) = 0

But it should be:

maskedImage(~mask) = 0

[rman]

Hi @Image Analyst, is there a way to crop an already made plot into a circle so that the all the points on the plot are only within the area of the circle?

This explains my question better:

https://uk.mathworks.com/matlabcentral/answers/1751295-how-to-only-plot-a-certain-area-of-an-existing-plot

[Krishna Sarkar]

draw circle is not working

[DGM]

"not working" in what way? Did it produce an error? Do you have Image Processing Toolbox?

[Svenja Jetzinger]

Is it possible to crop a part of the image automatically without drawing the circle or any other shape around it?

I already have the special shape (nearly a circle) and black backround color around it, but would like to only have the special shape part of it, which is mostly not black. (picture attached) Because I have to load different images, I can not only "draw" one shape and it fits every image, right?

At the moment I cut the picture to a "square" which has black spots at the corners of the picture which I want to cut too.

In my nex step I would like to delete all black spots inside the special shape and work with those and all the other pixels. But before I want to "cut" and show the special shape of the image. Is there any way for doing that?

Many thanks for your help!

[Image Analyst]

@Svenja Jetzinger Can you start a new question on this and I'll tell you how to do it, once I understand what you want. Can you attach a desired output image there?

Also explain why you want to crop the image to the bounding box. Is it just to show it bigger when you display the image? Because I don't see any reason why the image would need to be cropped. Why can't you do what you need to do with the excess black surround still there?

% Demo by Image Analyst
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 = 16;
markerSize = 20;
%--------------------------------------------------------------------------------------------------------
%    READ IN TEST IMAGE
folder = [];
baseFileName = 'komplett_S1.jpg';
fullFileName = fullfile(folder, baseFileName);
% Check if file exists.
if ~exist(fullFileName, 'file')
    % The file doesn't exist -- didn't find it there in that folder.
    % Check the entire search path (other folders) for the file by stripping off the folder.
    fullFileNameOnSearchPath = baseFileName; % No path this time.
    if ~exist(fullFileNameOnSearchPath, 'file')
        % Still didn't find it.  Alert user.
        errorMessage = sprintf('Error: %s does not exist in the search path folders.', fullFileName);
        uiwait(warndlg(errorMessage));
        return;
    end
end
rgbImage = imread(fullFileName);
%--------------------------------------------------------------------------------------------------------
% Display the image.
subplot(2, 2, 1);
imshow(rgbImage, []);
impixelinfo;
axis('on', 'image');
title('Original RGB Image', 'FontSize', fontSize, 'Interpreter', 'None');
% Update 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(rgbImage)
% Maximize window.
g = gcf;
g.WindowState = 'maximized';
g.Name = 'Demo by Image Analyst';
g.NumberTitle = 'off';
drawnow;
%--------------------------------------------------------------------------------------------
% Take the green channel
greenImage = rgbImage(:, :, 2);
subplot(2, 2, 2);
imshow(greenImage)
impixelinfo;
axis('on', 'image');
title('Green Channel Image', 'FontSize', fontSize, 'Interpreter', 'None');
drawnow;
%--------------------------------------------------------------------------------------------
% Threshold image
% imhist(greenImage)
mask = greenImage >= 10;
% Fill holes.
mask = imfill(mask, 'holes');
% Take largest blob only.
mask = bwareafilt(mask, 1);
subplot(2, 2, 3);
imshow(mask)
impixelinfo;
axis('on', 'image');
title('Mask Image', 'FontSize', fontSize, 'Interpreter', 'None');
drawnow;
%--------------------------------------------------------------------------------------------
% Find bounding box
props = regionprops(mask, 'BoundingBox')
bb = props.BoundingBox;
% Crop original image
rgbImage2 = imcrop(rgbImage, bb);
subplot(2, 2, 4);
imshow(rgbImage2)
impixelinfo;
axis('on', 'image');
title('Cropped RGBImage', 'FontSize', fontSize, 'Interpreter', 'None');
drawnow;

[Svenja Jetzinger]

Hello Image Analyst,

thank you for your answer, but that's what I already found by myself. I started, as you asked for, a new question of it and attach some more pictures to explain what I mean.

That's the headline of my question:

I want to change the backgroung color of image into transparent and then work with the pixels of it

Greetings

[Image Analyst]

First, why?

Second, what does that mean? Does that mean if you display the image with something like imshow() you want the background to be transparent, like to show your computer desktop and other applications where the background is?

[DGM]

This is the question:

https://www.mathworks.com/matlabcentral/answers/2105491-i-want-to-change-the-backgroung-color-of-image-into-transparent-and-then-work-with-the-pixels-of-it

Answer 2

[Thi Ng]

Hi,

Thanks for the solution. Just a general question, how would I cite this if I am using the code to do data analysis for a paper in academia?

Thanks,

Thi

[Image Analyst]

I'd just say that it came from the MATLAB Central Answers forum and give the link.

[Thi Ng]

Thanks!

Answer 3

[Pauline Audurier]

Hi,

Thank you for hte code, it's working well.

I wonder if there is a way to chose the outside color of the mask.

For exemple, in my case, I'ld like the outside color in grey and not in black (my images are colored).

Thank you,

Pauline

[Image Analyst]

@Pauline Audurier, try this:

% Demo to have the user click and draw a circle over an image, then blacken outside the circle and crop out the circular portion into a new image.
clc;    % Clear the command window.
fprintf('Beginning to run %s.m ...\n', mfilename);
close all;  % Close all figures (except those of imtool.)
imtool close all;  % Close all imtool figures.
clear;  % Erase all existing variables.
workspace;  % Make sure the workspace panel is showing
fontSize = 15;
% Get image.
originalImage = imread('peppers.png');
[rows, columns, numberOfColorChannels] = size(originalImage)
subplot(2, 2, 1);
imshow(originalImage);
impixelinfo; % let user mouse around and see (x,y) and RGB value.
axis('on', 'image');
title('Original Image', 'FontSize', fontSize);
% Maximize the window to make it easier to draw.
g = gcf;
g.WindowState = 'maximized'
% Ask user to draw a circle:
uiwait(helpdlg('Please click and drag out a circle.'));
h.Radius = 0;
while h.Radius == 0
    h = drawcircle('Color','k','FaceAlpha',0.4)
    if h.Radius == 0
        uiwait(helpdlg('You double-clicked.  You need to single click, then drag, then single click again.'));
    end
end
% Get coordinates of the circle.
angles = linspace(0, 2*pi, 10000);
x = cos(angles) * h.Radius + h.Center(1);
y = sin(angles) * h.Radius + h.Center(2);
% Show circle over image.
subplot(2, 2, 2);
imshow(originalImage);
axis('on', 'image');
hold on;
plot(x, y, 'r-', 'LineWidth', 2);
title('Original image with circle mask overlaid', 'FontSize', fontSize);
% Get a mask of the circle
mask = poly2mask(x, y, rows, columns);
subplot(2, 2, 3);
imshow(mask);
axis('on', 'image');
title('Circle Mask', 'FontSize', fontSize);
% Mask the image with the circle.
if numberOfColorChannels == 1
    maskedImage = originalImage; % Initialize with the entire image.
    maskedImage(~circleImage) = 150; % Zero image outside the circle mask.
else
    % Mask the image.
    darkGray = [100, 100, 100];
    [r, g, b] = imsplit(originalImage);
    r(~mask) = darkGray(1);
    g(~mask) = darkGray(2);
    b(~mask) = darkGray(3);
    maskedImage =  cat(3, r, g, b);
end
% Crop the image to the bounding box.
props = regionprops(mask, 'BoundingBox');
maskedImage = imcrop(maskedImage, props.BoundingBox);
% Display it in the lower right plot.
subplot(2, 2, 4);
imshow(maskedImage, []);
impixelinfo; % let user mouse around and see (x,y) and RGB value.
% Change imshow to image() if you don't have the Image Processing Toolbox.
title('Image masked with the circle', 'FontSize', fontSize);
fprintf('Done running %s.m ...\n', mfilename);