How would you detect a certain color square in an image?

How would you detect the red square in the left middle section of the image?

Answer 1

[Image Analyst]

I'd get the index of the red square in the indexed image, and then mask it.

clc;    % Clear the command window.
close all;  % Close all figures (except those of imtool.)
clear;  % Erase all existing variables.
workspace;  % Make sure the workspace panel is showing.
format long g;
format compact;
fontSize = 20;

%===============================================================================
% Read in a indexed demo image.
folder = pwd;
baseFileName = 'squaresImage3.gif';
% Get the full filename, with path prepended.
fullFileName = fullfile(folder, baseFileName);
if ~exist(fullFileName, 'file')
  % Didn't find it there.  Check the search path for it.
  fullFileName = baseFileName; % No path this time.
  if ~exist(fullFileName, 'file')
    % Still didn't find it.  Alert user.
    errorMessage = sprintf('Error: %s does not exist.', fullFileName);
    uiwait(warndlg(errorMessage));
    return;
  end
end
[indexedImage, storedColorMap] = imread(fullFileName);
% Get the dimensions of the image.  numberOfColorChannels should be = 3.
[rows, columns, numberOfColorChannels] = size(indexedImage);
% Display the original color image.
subplot(2, 2, 1);
imshow(indexedImage, storedColorMap);
axis on;
title('Original Color Image', 'FontSize', fontSize, 'Interpreter', 'None');
% Enlarge figure to full screen.
set(gcf, 'Units', 'Normalized', 'Outerposition', [0, 0, 1, 1]);

% Get the index in the red region.
indexOfRed = indexedImage(200, 200)

% Create a mask
mask = indexedImage == indexOfRed;
% Display the mask image.
subplot(2, 2, 2);
imshow(mask);
axis on;
title('Mask Image', 'FontSize', fontSize, 'Interpreter', 'None');

% Mask the image using bsxfun() function
maskedIndexedImage = bsxfun(@times, indexedImage, cast(mask, 'like', indexedImage));

% Display the mask image.
subplot(2, 2, 3);
imshow(maskedIndexedImage, storedColorMap);
axis on;
title('Masked Image', 'FontSize', fontSize, 'Interpreter', 'None');