using the attached color image and ROI mask and how can I remove the background from the image and retain only the approximate ROI.

the color image must have the dark background in the output image.

Answer 1

[Matt Gaidica]

IM = imread('ROI.png');
mask = logical(imread('mask.png'));
rmMask = bsxfun(@times, IM, cast(mask, 'like', IM));

close all
figure;
imshow(rmMask);

See also: Masking out Image area using Binary Mask

[Nosipho Majola]

Thank you Matt. Would you please help me out with a code that does not use built-in functon but still does the same thing? i.e. not using bsxfun()

Answer 2

[Image Analyst]

Looks like you've accepted Matt's answer so you probably don't need this but

rgbImage = imread('ROI.png');
mask = logical(imread('mask.png'));
% Make mask 3-D
mask = cat(3, mask, mask, mask);
rgbImage(~mask) = 0;

I haven't tested it but that will probably work. If it's not your homework, try that.

If it's your homework then you may not be able to use imread(), cat(), logical(), imshow(), etc. so see this link:

What can be programmed "without any built-in functions" ? - MATLAB Answers - MATLAB Central

and of course you wouldn't be able to use our code either - that would be even worse than using built-in functions and you could face disciplinary action.

How do I get help on homework questions on MATLAB Answers? - MATLAB Answers - MATLAB Central

[Nosipho Majola]

@Image Analyst the code does not really answer my question. About the built-in function I wanted to find if it possible to do ROI without using the bsxfun

[Image Analyst]

@Nosipho Majola I know you want it done without that function but you didn't say why. And it appears you didn't even try my code or notice that it does not use bsxfun() like the accepted answer does. So I'm puzzled as to why you say "the code does not really answer my question". Please explain why it does not work. Here is a full demo, using your images, that proves the code does work. But again, don't use this code or you risk getting into trouble with your instructor if it's homework.

% Demo by Image Analyst, December, 2021.
clc;    % Clear the command window.
close all;  % Close all figures (except those of imtool.)
clearvars;
workspace;  % Make sure the workspace panel is showing.
format long g;
format compact;
fontSize = 16;
fprintf('Beginning to run %s.m ...\n', mfilename);
%-----------------------------------------------------------------------------------------------------------------------------------
% Read in image.
folder = [];
baseFileName = 'image1.png';
fullFileName = fullfile(folder, baseFileName);
% Check if file exists.
if ~isfile(fullFileName)
    % 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
    fullFileName = fullFileNameOnSearchPath;
end
image1 = imread(fullFileName);
[rows, columns, numberOfColorChannels] = size(image1)
% Display the image.
subplot(2, 2, 1);
imshow(image1, []);
axis('on', 'image');
caption = sprintf('RGB Image : "%s"', baseFileName);
title(caption, 'FontSize', fontSize, 'Interpreter', 'None');
drawnow;
hp = impixelinfo(); % Set up status line to see values when you mouse over the image.
% Read in image.
folder = [];
baseFileName = 'image2.png';
fullFileName = fullfile(folder, baseFileName);
% Check if file exists.
if ~isfile(fullFileName)
    % 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
    fullFileName = fullFileNameOnSearchPath;
end
image2 = imread(fullFileName);
[rows, columns, numberOfColorChannels] = size(image2)
% Display the image.
subplot(2, 2, 2);
imshow(image2, []);
axis('on', 'image');
caption = sprintf('Mask Image : "%s"', baseFileName);
title(caption, 'FontSize', fontSize, 'Interpreter', 'None');
drawnow;
hp = impixelinfo(); % Set up status line to see values when you mouse over the image.
% Set up figure properties:
% Enlarge figure to full screen.
hFig1 = gcf;
hFig1.Units = 'Normalized';
hFig1.WindowState = 'maximized';
% 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.
hFig1.Name = 'Demo by Image Analyst';
%--------------------------------------------------------------------------------------------------------
% Segment (mask) the image.
% Make mask 3-D
mask = cat(3, image2, image2, image2);
image1(~mask) = 0;% Display the mask image.
subplot(2, 2, 3);
imshow(image1, []);
axis('on', 'image');
caption = sprintf('Masked Image');
title(caption, 'FontSize', fontSize, 'Interpreter', 'None');
drawnow;
hp = impixelinfo(); % Set up status line to see values when you mouse over the image.

[Nosipho Majola]

@Image Analyst I did run the previous code you had given and the output image was not the one I was expecting, which is the one written "masked image" in this new code.

about the function, I did see that yours does not have it. it just that the output image was different from the one required.

i am about to try the edited one now, thank you.

[Image Analyst]

OK, well if you run my demo with images called image1 and image2, it should look identical to the screenshot I posted in my last comment. If you alter it (like to handle your image names, or display things somewhat differently) and it doesn't work, then post your screenshot, two original images with the paperclip icon, and your code and I'll fix it.