How can I find the dimensions and area of an irregularly shaped region in an image?

I have an image with multiple irregularly shaped blobs. Is it possible to find the dimensions (ie max length, width etc) and area of each blob?

[Walter Roberson]

Given an irregular blob, how do you tell which axes is the "length" compared to which is the "width" ? Is this piece of wood 5 long and 3 wide, or is it 3 long and 5 wide?

[Tiffany Mao]

It doesn't matter as long as its consistent throughout the image, but I'd say go with the orientation of the image (ie in the example image, they are all "longer" than they are "Wide")

Answer 1

[Image Analyst]

See attached demo.

[Tiffany Mao]

Hi! I'm having trouble running this. When I replace the demos with my own image, nothing happens when it runs?

[Image Analyst]

Then attach your image. I can't help you with your image without it. Also attach the code you used to segment it (to produce the binary image).

[Tiffany Mao]

I tried with both a tiff and a JPG, the tiff I am unable to upload.

,

[Image Analyst]

Three problems. You left in the code that threw out blobs smaller than 10% of the image, which meant that you would have no blobs at all. Secondly the image has a white line running along the bottom and right side of the image - I got rid of that. Third, my code doesn't seem to handle blobs only a single pixel wide, like some of your blobs. Fixed code is below.

clc;    % Clear the command window.
close all;  % Close all figures (except those of imtool.)
imtool close all;  % Close all imtool figures if you have the Image Processing Toolbox.
clear;  % Erase all existing variables. Or clearvars if you want.
workspace;  % Make sure the workspace panel is showing.
format long g;
format compact;
fontSize = 18;

% Check that user has the Image Processing Toolbox installed.
hasIPT = license('test', 'image_toolbox');
if ~hasIPT
  % User does not have the toolbox installed.
  message = sprintf('Sorry, but you do not seem to have the Image Processing Toolbox.\nDo you want to try to continue anyway?');
  reply = questdlg(message, 'Toolbox missing', 'Yes', 'No', 'Yes');
  if strcmpi(reply, 'No')
    % User said No, so exit.
    return;
  end
end

% Read in a standard MATLAB gray scale demo image.
folder = pwd
baseFileName = 'ex.jpg'
% Get the full filename, with path prepended.
fullFileName = fullfile(folder, baseFileName);
% Check if file exists.
if ~exist(fullFileName, 'file')
  % File doesn't exist -- didn't find it there.  Check the search path for it.
  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
originalImage = imread(fullFileName);
% Display the original gray scale image.
hFig = figure;
subplot(2, 2, 1);
imshow(originalImage, []);
axis on;
title('Original Image', 'FontSize', fontSize);
% Enlarge figure to full screen.
set(gcf, 'Units', 'Normalized', 'OuterPosition', [0 0 1 1]);
% Give a name to the title bar.
set(gcf, 'Name', 'Demo by ImageAnalyst', 'NumberTitle', 'Off') 

% Get the dimensions of the image.  
% numberOfColorBands should be = 1.
[rows, columns, numberOfColorBands] = size(originalImage);
if numberOfColorBands > 1
  % It's not really gray scale like we expected - it's color.
  % Convert it to gray scale by taking only the green channel.
  grayImage = originalImage(:, :, 2); % Take green channel.
else
  % It's already grayscale.
  grayImage = originalImage;
end

% Binarize the image
level = graythresh(grayImage);
binaryImage = im2bw(grayImage, level);
% Display the image.
subplot(2, 2, 2);
imshow(binaryImage, []);
axis on;
title('Initial Binary Image', 'FontSize', fontSize);

% Fill holes
binaryImage = imfill(binaryImage, 'holes');
% Get rid of anything touching the edge of the image
binaryImage = imclearborder(binaryImage);
% Get rid of anything less than 20 pixels
binaryImage = bwareaopen(binaryImage, 20);
% Extract the largest blob only
% binaryImage = bwareafilt(binaryImage, 1);
% Display the image.
subplot(2, 2, 4);
imshow(binaryImage, []);
axis on;
hold on;
caption = sprintf('Filled, Cleaned Binary Image with\nBoundaries and Feret Diameters');
title(caption, 'FontSize', fontSize);

% Copy the gray scale image to the lower left.
subplot(2, 2, 3);
imshow(originalImage, []);
caption = sprintf('Original Image with\nBoundaries and Feret Diameters');
title(caption, 'FontSize', fontSize);
axis on;
hold on;

% Label the image so we can get the average perpendicular width.
labeledImage = bwlabel(binaryImage);
% Let's find the areas
props = regionprops(labeledImage, 'Area');
allAreas = sort([props.Area], 'descend')
% Measure the area
measurements = regionprops(labeledImage, 'Area');

% Make larger version so we can see it
% Copy the gray scale image to the lower left.
figure;
imshow(originalImage, []);
caption = sprintf('Original Image with\nBoundaries and Feret Diameters');
title(caption, 'FontSize', fontSize);
axis on;
hold on;
% Enlarge figure to full screen.
set(gcf, 'Units', 'Normalized', 'OuterPosition', [0 0 1 1]);
% Give a name to the title bar.
set(gcf, 'Name', 'Demo by ImageAnalyst', 'NumberTitle', 'Off') 

% bwboundaries() returns a cell array, where each cell contains the row/column coordinates for an object in the image.
% Plot the borders of all the coins on the original grayscale image using the coordinates returned by bwboundaries.
boundaries = bwboundaries(binaryImage);
numberOfBoundaries = size(boundaries, 1);
for blobIndex = 1 : numberOfBoundaries
  thisBoundary = boundaries{blobIndex};
  x = thisBoundary(:, 2); % x = columns.
  y = thisBoundary(:, 1); % y = rows.
  
  % Find which two boundary points are farthest from each other.
  maxDistance = -inf;
  for k = 1 : length(x)
    distances = sqrt( (x(k) - x) .^ 2 + (y(k) - y) .^ 2 );
    [thisMaxDistance, indexOfMaxDistance] = max(distances);
    if thisMaxDistance > maxDistance
      maxDistance = thisMaxDistance;
      index1 = k;
      index2 = indexOfMaxDistance;
    end
  end
  
  % Plot the boundary over this blob.
  plot(x, y, 'g-', 'LineWidth', 1);
  % For this blob, put a line between the points farthest away from each other.
  line([x(index1), x(index2)], [y(index1), y(index2)], 'Color', 'r', 'LineWidth', 1);
  % Put big red spots at the ends.
  plot([x(index1), x(index2)], [y(index1), y(index2)], 'r.', 'MarkerSize', 30);
  
  message = sprintf('The longest line is red.\nMax distance for blob #%d = %.2f\nArea = %d', ...
    blobIndex, maxDistance, measurements(blobIndex).Area);
  fprintf('%s\n', message);
  uiwait(helpdlg(message));
end
hold off;

[Tiffany Mao]

This is exactly what I was looking for! Thank you so much