Drawing the major and minor axis of an elliptical object in Matlab

This program currently inputs an image of a coin, thresholds it, binarizes it, and finds the major and minor axis lengths of the segmented elliptical using the regionprops function. What I want to do is output a subplot where I draw the axes used to calculate the 'MajorAxisLength' and 'MinorAxisLength' over the original image. Would really appreciate help with this.

I have appended the code for your perusal.

    rgbImage = imread(coin2.jpg);
  subplot(2, 3, 1);
  imshow(rgbImage, []);
  title('Original Image');

redChannel = rgbImage(:, :, 1);
binaryImage = redChannel < 100;
subplot(2, 3, 3);
imshow(binaryImage, []);
title('Binarized Image');

labeledImage = bwlabel(binaryImage);
area_measurements = regionprops(labeledImage,'Area');
allAreas = [area_measurements.Area];
biggestBlobIndex = find(allAreas == max(allAreas));
keeperBlobsImage = ismember(labeledImage, biggestBlobIndex);
measurements = regionprops(keeperBlobsImage,'MajorAxisLength','MinorAxisLength')

% Display the original color image with outline.
subplot(2, 3, 4);
imshow(rgbImage);
hold on; 
title('Original Color Image with Outline', 'FontSize',fontSize);
boundaries = bwboundaries(keeperBlobsImage);
blobBoundary = boundaries{1};
plot(blobBoundary(:,2), blobBoundary(:,1), 'g-', 'LineWidth', 1);
hold off;

Answer 1

[Rick Rosson]

Here's a start:

   ctr = regionprops(keeperBlobsImage,'centroid');
   theta = regionprops(keeperBlobsImage,'orientation');

   xMajor = ctr(1)  +  [ -1 +1 ] * measurements(1)*cosd(theta)/2;
   yMajor = ctr(2)  +  [ -1 +1 ] * measurements(1)*sind(theta)/2;

   line(xMajor,yMajor);

[LoserG G]

thanks for the feedback, it was very useful.

[Lingaraj Hadimani]

Dear Sir,

After running the above code, I am getting the following error:

Undefined function 'mtimes' for input arguments of type 'struct'.

Error in axisMajMin (line 15) xMajor = ctr(1) + [ -1 +1 ] * measurements(1)*cosd(theta)/2;

I am naive to the Matlab. I would be thankful, if you could help me to solve this.

[ahmet ardahanli]

it was very useful. thanks but how to minor axis?

[DGM]

% a logical image with multiple blobs
mask = imread('tiltellipses.png')>128;
imshow(mask); hold on

% get the properties of all blobs
S = regionprops(mask,'centroid','orientation','MajorAxisLength','MinorAxisLength');

% annotate each blob
for k = 1:numel(S)
    % major
    xj = S(k).Centroid(1)  +  [-1 1] * S(k).MajorAxisLength*cosd(S(k).Orientation)/2;
    yj = S(k).Centroid(2)  -  [-1 1] * S(k).MajorAxisLength*sind(S(k).Orientation)/2;
    
    % minor
    xn = S(k).Centroid(1)  +  [-1 1] * S(k).MinorAxisLength*cosd(S(k).Orientation + 90)/2;
    yn = S(k).Centroid(2)  -  [-1 1] * S(k).MinorAxisLength*sind(S(k).Orientation + 90)/2;
    
    % each pair of lines is one object
    plot([xj NaN xn],[yj NaN yn],'-o','linewidth',2)
end

Answer 2

[Image Analyst]

This is discussed in this blog entry of Steve Eddins:

https://blogs.mathworks.com/steve/2010/07/30/visualizing-regionprops-ellipse-measurements/