Getting area of tilted ellipse with regionprops

Hi and thank you for thank you for taking your time.

I am using the following code, made by Akira Agata, to fit an ellipse to an image:

https://de.mathworks.com/matlabcentral/answers/495720-how-to-fit-an-ellipse-to-an-image-in-matlab; 03.03.2022; 2:11 pm

% The multiplication with (7.5/1000) has simply to do with the camera's
% calibration. The mewasures are in mm^2 and mm^3 (therefore µL).

%% The code I got from Akira Agata:

s = regionprops(BW, 'Orientation', 'MajorAxisLength','MinorAxisLength','Centroid', 'Circularity');
    % Calculate the ellipse line

    theta = linspace(0,2*pi);
    col = (s.MajorAxisLength/2)*cos(theta);
    row = (s.MinorAxisLength/2)*sin(theta);
    M = makehgtform('translate',[s.Centroid, 0],'zrotate',deg2rad(-1*s.Orientation));
    D = M*[col;row;zeros(1,numel(row));ones(1,numel(row))];

The images I am using are pretty similar with the one used in the question so unless the exact image is necessary to understand the situation, I'd rather not post it. I am not sure wether I am allowed to do so or not.

Now the following part of my task is to get the volume Vm of the ellipsoid. We assume we have a symmetry around the z-axis so we use the following code to get the volume (nothing special):

% Calculate the volume
z =4/3;
a = (s.MajorAxisLength)./2; 
el_a = (a*7.5)./1000;  % wideness      
b = (s.MinorAxisLength)./2;
el_b = (b*7.5)./1000;  % height       
Area = el_a*el_b*pi;
Vm = (Area*el_b*z); % The volume of the elipsoid.

I have trouble to understand the code completely, so I'd like to ask if anybody knows wether this code also makes sure to adjust it's variable to the ellipse's angle? I mean we have the 'Orientation' with regionpros but I am not sure if we use it here to adjust the Major- and MinorAxisLength. Otherwise it might use different points of the image to set as axis-lengths and I'd get a different volume.

If you need any more Information just ask me! I'd be glad if someone could help me with an answer. Thank you for your time anyways, wether you can help me or not!

Answer 1

[Image Analyst]

Did you email him back, or call him?

I don't think you need the Orientation (angle) to compute volume. The area of an ellipse is pi*a*b and the volume is pi*a*b^2.

The stuff about 7.5 you probably don't need to do if you have square pixels or voxels. In fact I don't think you need most of that. You can simply do

s = regionprops(BW, 'Orientation', 'MajorAxisLength','MinorAxisLength','Centroid', 'Circularity');
lengths = [s.MajorAxisLength];
widths = [s.MinorAxisLength];
areas = pi * lengths .* widths;
volumes = areas .* widths;

[Sokratis Panagiotidis]

Hi thank you for your answer.

The 7.5 is because we know that every pixel of the images we took with our camera is exactly 7.5 µm, hence why I multiplied it by it. Otherwise the unit would be square-pixel or pixels^3.

You're right, I did not write Mr. Agata. I'll do that now to ask him about it, as I am pretty sure the orientation is being needed.

Otherwise it would mean the Minor- & MajorAxisLength are already taking the agle of the tilted ellipse into consideration, which would make me question the need of the 'Orientation'.

I'll ask Agata-san about his code. Please know, although you did help me a bit with your answer, I won't be labeling this question as 'answered' untill I figured it out so it might take a while.

[Image Analyst]

The area of an ellipse is pi*a*b regardless of the tilt/angle/orientation. The axis lengths are not the length along the x or y axis so they're not multiplied by the sine of cosine of the angle. They're the full length just like you'd draw them by hand. You just use them directly in the formula.

The only reason why you'd need to know the angle is if you want to draw a line segment along the major and minor axes. In that case you'd need two endpoints (x1, y1) and (x2, y2)

s = regionprops(BW, 'Orientation', 'MajorAxisLength','MinorAxisLength','Centroid', 'Circularity');
lengths = [s.MajorAxisLength];
widths = [s.MinorAxisLength];
areas = pi * lengths .* widths;
volumes = areas .* widths;
allAngles = [s.Orientation]; % In degrees so need sind() not sin().
% Get centroids.
xy = vertcat(s.Centroid);
xc = xy(:, 1);
yc = xy(:, 2);
% Get line segments along major axes.
x1 = xc - lengths .* cosd(allAngles);
y1 = yc - lengths .* sind(allAngles);
x2 = xc + lengths .* cosd(allAngles);
y2 = yc + lengths .* sind(allAngles);
% Plot line segments along the major axes of each blob.
for k = 1 : length(x1)
    plot([x1,x2], [y1,y2], 'r.-', 'LineWidth', 2, 'MarkerSize', 15);
    hold on;
end
hold off;

[Sokratis Panagiotidis]

Dear @Image Analyst, I just marked your answer as helpful. Thank you!