how to calculate the major and minor axis of an ellipse in contour plot

I have a contour plot which is more of a ellipse.I would like to know its major and minor axis...a and b respectively.How do i go about.I am naive user please guide me.

Answer 1

[Image Analyst]

You need the Image Processing Toolbox. Type ver to see if you have it.

It looks like an indexed image with a jet colormap being applied. Get one "ring" by thresholding

binaryImage = grayImage == theValue;  % Different rings will have different intensity values.
% Now get the ellipse parameters with regionprops.
measurements = regionprops(binaryImage, 'MajorAxisLength', 'MinorAxisLength', 'Orientation');
majorAxisLength = measurements.MajorAxisLength;
minorAxisLength = measurements.MinorAxisLength;
% Repeat for other rings.

[dEEPAK MAT]

Thanks A lot !!!

Answer 2

[Mohammad Bhat]

How can I pass ellipse as an object to regionprops

[Image Analyst]

Make sure the ellipse is a binary image, then pass it as the first input argument to regionprops().

[Rahul Verma]

I am having a similar contour image which I converted to grayscale as attached...

a=imread('test1.png');

grayImage = rgb2gray(a);

binaryImage = grayImage == 0.40;

measurements = regionprops(binaryImage, 'MajorAxisLength', 'MinorAxisLength', 'Orientation');

However, on calling regionprops, I get an empty matrix

0×1 empty struct array with fields:

MajorAxisLength

MinorAxisLength

Orientation

As a result, I cannot get the major and minor axis lengths...

Any suggestions :)

[Image Analyst]

You probably don't have anything that is EXACTLY 0.4. Try

binaryImage = grayImage >= 0.40;

Do it again for a different value and AND the images if you just want the ring.

binaryImage = grayImage >= 0.40 & grayImage <= someOtherValue;

Answer 3

[Bhavesh Garg]

VHVHV

[Image Analyst]

How is this an "Answer" to dEEPAK's question? What does it mean?