MATLAB: Extract the largest object in the RGB image

Question

Hi,

I have an image with multipe objects as seen in the first image below and I want just extract part of that image (the largest object in the image) as seen in the second image. How can I do that?

* It is better to keep the image in RGB

Answer 1

OK, if you really need to it work for a full color image, here is code for that:

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 = 22;
%--------------------------------------------------------------------------------------------------------

%    READ IN IMAGE
folder = pwd;
baseFileName = 'image.jpeg';
% 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);
% 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);
% Display the image.

hFig = figure;
subplot(2, 2, 1);
imshow(rgbImage, []);
impixelinfo;
title('Original RGB Image', 'FontSize', fontSize, 'Interpreter', 'None');
impixelinfo;
hFig.WindowState = 'maximized'; % May not work in earlier versions of MATLAB.

drawnow;
%--------------------------------------------------------------------------------------------------------
%    EXTRACTION OF IMAGE
% Get the max image
grayImage = min(rgbImage, [], 3);
subplot(2, 2, 2);
imshow(grayImage, []);
impixelinfo;
title('Gray Scale Image', 'FontSize', fontSize, 'Interpreter', 'None');
impixelinfo;
hFig.WindowState = 'maximized'; % May not work in earlier versions of MATLAB.
drawnow;
threshold = 240; % Whatever....
mask = grayImage < threshold; % Create a binary image (a mask)
% Extract the largest blob only.
mask = bwareafilt(mask, 1);
% Fill holes
mask = imfill(mask, 'holes');
% Display the image.
subplot(2, 2, 3);
imshow(mask, []);
title('Mask Image', 'FontSize', fontSize, 'Interpreter', 'None');
axis('on', 'image');
% Extract the individual red, green, and blue color channels.
redChannel = rgbImage(:, :, 1);
greenChannel = rgbImage(:, :, 2);
blueChannel = rgbImage(:, :, 3);
% Get the mean gray levels in R, G, and B.
meanGLR = mean(rgbImage(redChannel > threshold))
meanGLG = mean(rgbImage(greenChannel > threshold))
meanGLB = mean(rgbImage(blueChannel > threshold))
% Mask the individual color channel images.
% Make outside the mask the mean of what's above the threshold.
redChannel(~mask) = meanGLR;
greenChannel(~mask) = meanGLG;
blueChannel(~mask) = meanGLB;
% Recombine separate color channels into a single, true color RGB image.
rgbImage2 = cat(3, redChannel, greenChannel, blueChannel);
% Display the cropped image.
subplot(2, 2, 4);
imshow(rgbImage2, []);
title('Final Image', 'FontSize', fontSize, 'Interpreter', 'None');
axis('on', 'image');