how to get size of holes in an object (image processing)

I have several images like this with a lot more noise though.

I can get outside boundaries and parameters following this example http://www.mathworks.in/help/images/examples/identifying-round-objects.html but how do I get the size (dia) of internal holes? Also when I reduce the noise using a structural element it considerably reduces the size of holes. How do I deal with this?

Answer 1

[Image Analyst]

I hope this wasn't your homework because it was so simple I did it all in about 3 minutes:

% Demo to find equivalent circular diameters.
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.
fontSize = 22;

% Read in a standard MATLAB color demo image.
folder = 'C:\Users\SG\Documents\Temporary';
baseFileName = 'nuts.jpg';
% 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
rgbImage = imread(fullFileName);
% Get the dimensions of the image.  numberOfColorBands should be = 3.
[rows, columns, numberOfColorBands] = size(rgbImage);
% Display the original color image.
subplot(2, 2, 1);
imshow(rgbImage);
axis on;
title('Original Color Image', 'FontSize', fontSize);
% Enlarge figure to full screen.
set(gcf, 'units','normalized','outerposition',[0 0 1 1]);

% Get a binary image
binaryImage = rgbImage(:,:,2) < 100;
% Get rid of background.
binaryImage = imclearborder(binaryImage);
% Get rid of small noise regions.
binaryImage = bwareaopen(binaryImage, 500);
subplot(2, 2, 2);
imshow(binaryImage);
title('Binary Image', 'FontSize', fontSize);

% Label the parts
labeledImage = bwlabel(binaryImage);
% Measure the equivalent circular diameter
measurements = regionprops(labeledImage, 'EquivDiameter');
allDiameters = [measurements.EquivDiameter]

The answer is in the command window and gives the diameters in pixels:

allDiameters =
   66.8416   67.5050   54.6070   67.4012

Adapt as needed.