Why is my centroid giving only 1 element per blob using region props?

It's not clear exactly what you're calling an "element". I'll use more precise terminology. If your Iregion structure array is just one structure, then you have just one blob. The Centroid field of that structure will be a 1-by-2 array with the x and y locations of the centroid of that blob.

If you have multiple blobs, then you need to add an index after the structure variable name to get just one of the blobs, try this:

props = regionprops(bw2,'centroid');
hold on;
for k = 1 : length(props)
    % Get x and y centroid of the k'th blob.
    xCentroid = props(k).Centroid(1);
    yCentroid = props(k).Centroid(2);
    plot(xCentroid, yCentroid, 'r+', 'MarkerSize', 40, 'LineWidth', 2);
    % Let user know what they are
    message = sprintf('The largest blob has area %d and a centroid at (x,y) = (%f, %f)',...
  props.Area, xCentroid, yCentroid)
    uiwait(msgbox(message));
end

You can also get all the x and y centroids into arrays doing this:

centroids = [props.Centroid];
xCentroids = centroids(1:2:end);
yCentroids = centroids(2:2:end);

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Image Analyst 2017-7-22

在 MATLAB Online 中打开

Elias, there are 48 blobs, not 56 or 3. The largest blob is blob #28. See this code:

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 = 25;
%===============================================================================
% Get the name of the image the user wants to use.
baseFileName = 'binaryImage.png';
% Get the full filename, with path prepended.
folder = []; % Determine where demo folder is (works with all versions).
fullFileName = fullfile(folder, baseFileName);
%===============================================================================
% Read in a demo image.
grayImage = 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(grayImage)
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.
  % grayImage = grayImage(:, :, 2); % Take green channel.
end
% Display the image.
subplot(2, 1, 1);
imshow(grayImage, []);
axis on;
axis image;
caption = sprintf('Original Gray Scale Image');
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 1 1]);
% 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;
% Threshold the image.
binaryImage = grayImage >= 1;
% Display the image.
subplot(2, 1, 2);
imshow(binaryImage, []);
axis on;
axis image;
% Find connected components (blobs).
[labeledImage, numBlobs] = bwlabel(binaryImage);
caption = sprintf('Final binary image showing %d blob centroids', numBlobs);
title(caption, 'FontSize', fontSize, 'Interpreter', 'None');
% Make the measurements of the bounding boxes.
props = regionprops(labeledImage, 'Centroid', 'Area');
hold on;
for k = 1 : length(props)
  % Get x and y centroid of the k'th blob.
  xCentroid = props(k).Centroid(1);
  yCentroid = props(k).Centroid(2);
  plot(xCentroid, yCentroid, 'r*', 'MarkerSize', 20, 'LineWidth', 2);
  
  % Let user know what they are
  message = sprintf('Blob #%2d has area %3d and a centroid at (x,y) = (%6.2f, %6.2f)',...
    k, props(k).Area, xCentroid, yCentroid);
  fprintf('%s\n', message);
  promptMessage = sprintf('%s\n\nDo you want to Continue processing,\nor Quit processing?', message);
  titleBarCaption = 'Continue?';
  buttonText = questdlg(promptMessage, titleBarCaption, 'Continue', 'Quit', 'Continue');
  if strcmpi(buttonText, 'Quit')
    break;
  end
  
end
% Get all the centroids.
centroids = [props.Centroid];
allXCentroids = centroids(1:2:end);
allYCentroids = centroids(2:2:end);
% Find the largest blob
allAreas = [props.Area]
[maxArea, indexOfLargest] = max(allAreas)
% Plot a circle around the largest.
plot(allXCentroids(indexOfLargest), allYCentroids(indexOfLargest), 'go', 'MarkerSize', 40, 'LineWidth', 2);
plot(allXCentroids(indexOfLargest), allYCentroids(indexOfLargest), 'g*', 'MarkerSize', 40);
message = sprintf('The largest blob has an index of %d\nan area of %2d\nand a centroid at (x,y) = (%6.2f, %6.2f)',...
  indexOfLargest, maxArea, allXCentroids(indexOfLargest), allYCentroids(indexOfLargest))
uiwait(msgbox(message));