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How to remove connected components inside polygon in binary image
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I have a set of images which I have already segmented and obtained theirs extern perimeter(the cornermask we can say). When the segmentation is perfect I get a perfect circle(or square, or another polygon, it's not always a circle); however when it's not perfect I get the extern perimeter and also some connected components inside, like in the image below.
My question is how can I remove those connected components inside?
I tried with erosion but with no successfull result because too thin and I lose the extern perimeter.
I need them to be removed because after I use regionfill and I need to pass it an exact cornermask
![](https://www.mathworks.com/matlabcentral/answers/uploaded_files/190680/image.jpeg)
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Accepted Answer
Image Analyst
on 19 May 2018
Try this:
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;
% Check that user has the specified Toolbox installed and licensed.
hasLicenseForToolbox = license('test', 'image_toolbox'); % license('test','Statistics_toolbox'), license('test','Signal_toolbox')
if ~hasLicenseForToolbox
% User does not have the toolbox installed, or if it is, there is no available license for it.
% For example, there is a pool of 10 licenses and all 10 have been checked out by other people already.
ver % List what toolboxes the user has licenses available for.
message = sprintf('Sorry, but you do not seem to have the Image Processing Toolbox.\nDo you want to try to continue anyway?');
reply = questdlg(message, 'Toolbox missing', 'Yes', 'No', 'Yes');
if strcmpi(reply, 'No')
% User said No, so exit.
return;
end
end
%===============================================================================
% Read in gray scale demo image.
folder = pwd; % Determine where demo folder is (works with all versions).
baseFileName = 'cornermask.jpg';
% Get the full filename, with path prepended.
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, 3, 1);
imshow(rgbImage, []);
title('Original Image', 'FontSize', fontSize, 'Interpreter', 'None');
axis on;
hp = impixelinfo();
% 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(rgbImage);
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 = rgb2gray(rgbImage);
% ALTERNATE METHOD: Convert it to gray scale by taking only the green channel,
% which in a typical snapshot will be the least noisy channel.
grayImage = rgbImage(:, :, 1); % Take blue channel.
else
grayImage = rgbImage; % It's already gray scale.
end
% Now it's gray scale with range of 0 to 255.
% Display the image.
subplot(2, 2, 1);
imshow(grayImage, []);
title('Original Gray Scale Image', 'FontSize', fontSize, 'Interpreter', 'None');
axis on;
%------------------------------------------------------------------------------
% Set up figure properties:
% Enlarge figure to full screen.
set(gcf, 'Units', 'Normalized', 'OuterPosition', [0, 0.04, 1, 0.96]);
% Get rid of tool bar and pulldown menus that are along top of figure.
% set(gcf, 'Toolbar', 'none', 'Menu', 'none');
% Give a name to the title bar.
set(gcf, 'Name', 'Demo by ImageAnalyst', 'NumberTitle', 'Off')
drawnow;
%------------------------------------------------------------------------------
% Erode the image
binaryImage = grayImage > 127;
% Figure out how much we need to erode it to get just the thick outer ring.
windowWidth = 19; % 19 seems good.
erodedImage = imerode(binaryImage, true(windowWidth)); % 19 seems good.
% Display the image.
subplot(2, 2, 2);
imshow(erodedImage, []);
title('Eroded Image', 'FontSize', fontSize, 'Interpreter', 'None');
axis on;
hp = impixelinfo();
% Fill the image.
filledImage = imfill(binaryImage, 'holes');
% Display the image.
subplot(2, 2, 3);
imshow(filledImage, []);
title('Filled Image', 'FontSize', fontSize, 'Interpreter', 'None');
axis on;
% Erode it by the determined amount to get just the thick outer ring.
erodedImage = imerode(filledImage, true(windowWidth));
% XOR them to get the ring
perimeterMask = xor(filledImage, erodedImage);
% Display the image.
subplot(2, 2, 4);
imshow(perimeterMask, []);
title('Perimeter Mask Image', 'FontSize', fontSize, 'Interpreter', 'None');
axis on;
hp = impixelinfo();
![](https://www.mathworks.com/matlabcentral/answers/uploaded_files/188626/image.png)
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