MATLAB: Extracting black pixels from a specific region of an image and overlaying it on another image

image editingimage overlayingimage processingImage Processing Toolboxmaskmaskingpixel extraction

I have a few images of the following nature (showing only 2 to exemplify; these are 2 separate images):
From these, I want to extract the central black image (the E or the Cross) and overlay it on other blank surfaces, like:
I cannot just extract the black pixels and overlay it, since the boundaries are also black. The size of all the images is same, 400x400x3, so, if I can get the exact pixels for the central black image (the E and the Cross), then I can just convert the corresponding pixels to black for the empty surfaces. So, is there an easy way of getting those pixels? Any help will be appreciated!

Best Answer

Here's the full demo for you:
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 short g;
format compact;
fontSize = 25;
%===============================================================================


% Get the name of the image the user wants to use.
baseFileName = 'o4s1_11.bmp';
% Get the full filename, with path prepended.
folder = pwd
fullFileName = fullfile(folder, baseFileName);
%===============================================================================
% Read in a first image.
grayImage1 = 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(grayImage1)
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(grayImage);

  % ALTERNATE METHOD: Convert it to gray scale by taking only the green channel,

  % which in a typical snapshot will be the least noisy channel.

  grayImage1 = grayImage1(:, :, 2); % Take green channel.

end
% Display the image.



subplot(2, 2, 1);
imshow(grayImage1, []);
axis on;
axis image;
caption = sprintf('Image1');
title(caption, 'FontSize', fontSize, 'Interpreter', 'None');
drawnow;
hp = impixelinfo();
% Set up figure properties:
% Enlarge figure to full screen.
set(gcf, 'Units', 'Normalized', 'OuterPosition', [0, 0.04, 1, .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;
%===============================================================================
% Read in a second image.
fullFileName = fullfile(pwd, 'blankv5.bmp');
grayImage2 = 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(grayImage2)
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(grayImage);
  % ALTERNATE METHOD: Convert it to gray scale by taking only the green channel,
  % which in a typical snapshot will be the least noisy channel.
  grayImage2 = grayImage2(:, :, 2); % Take green channel.
end
% Display the image.
subplot(2, 2, 2);
imshow(grayImage2, []);
axis on;
axis image;
caption = sprintf('Image2');
title(caption, 'FontSize', fontSize, 'Interpreter', 'None');
drawnow;
hp = impixelinfo();
% Display the histogram of the image so we can see what threshold to use.
subplot(2, 2, 3);
histogram(grayImage2);
grid on;
% Binarize the image.
threshold = 128;
binaryImage = grayImage1 < threshold;  % Determine number from histogram.
% Get rid of surround that is touching the border.
binaryImage = imclearborder(binaryImage);
% Display the image.
subplot(2, 2, 3);
imshow(binaryImage, []);
axis on;
axis image;
caption = sprintf('Binary Image');
title(caption, 'FontSize', fontSize, 'Interpreter', 'None');
drawnow;
hp = impixelinfo();
% Use this binary image to transfer the stuff in the second image.
grayImage2(binaryImage) = grayImage1(binaryImage);
% Display the image.
subplot(2, 2, 4);
imshow(grayImage2, []);
axis on;
axis image;
caption = sprintf('Final Image');
title(caption, 'FontSize', fontSize, 'Interpreter', 'None');
drawnow;
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