MATLAB: Regionprops – finding the weighted centroid of one half of the image frame.

Question

I am trying to find the weightedcentroid of an image that is mirrored. As in, a frame will have an image twice, split down the middle. How can I set the region coordinates to find the weighted centroid of just one side of the frame and capture a single instance of the image?

Thank you.

Answer 1

Try this demo. Adapt as needed, like to change the input filename:

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 = 15;
%===============================================================================
% Read in gray scale demo image.
% Let's let the user select from a list of all the demo images that ship with the Image Processing Toolbox.
folder = fileparts(which('cameraman.tif')); % Determine where demo folder is (works with all versions).
baseFileName = 'cameraman.tif';
% 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, 2, 1);
imshow(rgbImage, []);
title('Original Grayscale Image', 'FontSize', fontSize, 'Interpreter', 'None');
axis('on', 'image');
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(:, :, 2); % Take green channel.
else
	grayImage = rgbImage; % It's already gray scale.
end
% Now it's gray scale with range of 0 to 255.
%------------------------------------------------------------------------------

% 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;
% Make a mask for the top half of the image
binaryImage = false(rows, columns);
binaryImage(1 : round(rows/2), 1 : columns) = true;
% Display the image.
subplot(2, 2, 2);
imshow(binaryImage, []);
title('Upper half mask', 'FontSize', fontSize, 'Interpreter', 'None');
axis('on', 'image');
% Use it to zero out the other parts of the image.
grayImage(~binaryImage) = 0;
% Display the image.
subplot(2, 2, 3);
imshow(grayImage, []);
title('Masked gray image', 'FontSize', fontSize, 'Interpreter', 'None');
axis('on', 'image');
hp = impixelinfo();
%------------------------------------------------------------------------------
% Measure blobs
props = regionprops(binaryImage, grayImage, 'WeightedCentroid');
xy = props.WeightedCentroid
% Put a red cross at the centroid.
hold on;
plot(xy(1), xy(2), 'r+', 'MarkerSize', 100, 'LineWidth', 3);
% Update title
caption = sprintf('Centroid at x (column) = %.1f, y (row) = %.1f', xy(1), xy(2));
title(caption, 'FontSize', fontSize, 'Interpreter', 'None');