MATLAB: How to isolate certain part of image

circleimage processingImage Processing Toolboxmasking

Suppose i have a eye image, in that image i draw a circle. Now i want to isolate the circle part from the rest of image.

Best Answer

All right, I have one expecially for circles that's simpler than the other one (which perhaps was ignored because its length scared you off). Don't worry - the main code in this is only one line long. Mostly it's long because of all the comments and fancy things like displaying images and giving titles to them. The main line of code is this:
maskedImage(~circleImage) = 0; % Zero image outside the circle mask.

Which you can change to
maskedImage(~circleImage) = 255; % Whiten image outside the circle mask.
if you want white instead of black.
Try this demo (also attached):
% Demo to mask an image with a circle.
% Requires the Image Processing Toolbox.
%
clc;    % Clear the command window.
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 = 20;
% Change the current folder to the folder of this m-file.
if(~isdeployed)
  cd(fileparts(which(mfilename)));
end
  
% Read in a standard MATLAB gray scale demo image.
folder = fullfile(matlabroot, '\toolbox\images\imdemos');
% Comment out whichever demo image you don not want to use.
baseFileName = 'cameraman.tif'; % Grayscale demo image.
baseFileName = 'peppers.png'; % Color demo image.
fullFileName = fullfile(folder, baseFileName);
% See if the image exists.
if ~exist(fullFileName, 'file')
  % Doesn't exist in that folder. See if it exists anywhere on the path.
  fullFileName = baseFileName;
  if ~exist(fullFileName, 'file')
    errorMessage = sprintf('Error: file %s not found', fullFileName);
    uiwait(warndlg(errorMessage));
    return;
  end
end
% Read in the image from disk.
originalImage = imread(fullFileName);
% Get the dimensions of the image.  numberOfColorBands should be = 1.
[rows, columns, numberOfColorBands] = size(originalImage);
% Display the original gray scale image.
subplot(2, 2, 1);
imshow(originalImage, []);
% Change imshow to image() if you don't have the Image Processing Toolbox.


title('Original Image', 'FontSize', fontSize);
% Enlarge figure to full screen.
set(gcf, 'Position', get(0,'Screensize')); 
set(gcf,'name','Image Analysis Demo','numbertitle','off') 
% Initialize parameters for the circle,
% such as it's location and radius.
circleCenterX = 130; 
circleCenterY =  75; % square area 0f 500*500 
circleRadius = 50;    % big circle radius 
% Initialize an image to a logical image of the circle. 
circleImage = false(rows, columns); 
[x, y] = meshgrid(1:columns, 1:rows); 
circleImage((x - circleCenterX).^2 + (y - circleCenterY).^2 <= circleRadius.^2) = true; 
% Display it in the upper right plot. 
subplot(2,2,2); 
imshow(circleImage, []); 
% Change imshow to image() if you don't have the Image Processing Toolbox.
title('Circle Mask', 'FontSize', fontSize); 
set(gcf, 'Position', get(0,'Screensize')); % Maximize figure.
drawnow;
% Mask the image with the circle.
if numberOfColorBands == 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(circleImage,class(originalImage)));
end
% Display it in the lower right plot. 
subplot(2, 3, 5); 
imshow(maskedImage, []); 
% Change imshow to image() if you don't have the Image Processing Toolbox.
title('Image masked with the circle.', 'FontSize', fontSize);
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