How to calculate the rotation of this egg?

Hi all!

Suppose I have this egg in figure1, standing still. But when it rotates a bit in figure 2, how can I calculate the rotation angle using MATLAB coding? Can anyone please give an idea on it?

I have attached the images in the question. (egg1, and egg2). Any suggestion from you will be much appreciated ^_^

Answer 1

[John D'Errico]

1. Find a bounding box, where the box can be rotated, not a box that is aligned with the axes.
2. Find the orientation (angle of inclination) of the long sides of the bounding box. atan2 will help you there.
3. Nothing more needed, since atan2 did all the work.

[Ashfaq Ahmed]

Hi! Thank you for your reply. How can I find a bounding box? I don't understand it when you say, "where the box can be rotated, not a box that is aligned with the axes". Can you please explain?

[John D'Errico]

I'm sure the Image processing toolbox has a bounding box utility. (I'll find it in a minute or so, and post the link too.) But you can also extract the points as a set of pixel locations, and then use my minboundrect utility.

https://www.mathworks.com/matlabcentral/fileexchange/34767-a-suite-of-minimal-bounding-objects?s_tid=srchtitle

Ok. A quick search tells me the tool is regionprops. It looks like you need to do something like:

regionprops(egg2,'boundingbox')

Lacking the IPT, I've never used it, but one of those tools will get you there.

Answer 2

[Matt J]

First, crop the black corders from your image fiels. Then, use ellipticalFit() from,

https://www.mathworks.com/matlabcentral/fileexchange/87584-object-oriented-tools-for-fitting-conics-and-quadrics

getEgg=@(file) imbinarize(im2gray(imread(file)));

[Y1,X1]=find(edge(getEgg('egg1.png')));
[Y2,X2]=find(edge(getEgg('egg2.png')));

fobj1=ellipticalFit([X1(:),Y1(:)]');
fobj2=ellipticalFit([X2(:),Y2(:)]');


rotation = fobj2.angle-fobj1.angle

rotation =

137.9371

function out=getEgg(file)
out=imbinarize(im2gray(imread(file)));
end

[Ashfaq Ahmed]

Hi! Thank you for sharing the link. I went through it but it does not seem like it can actually measure the rotation.

[Matt J]

Yes, it can. But be sure to clean the boundaries of the image box.

[Ashfaq Ahmed]

This code works perfectly! Thank you so much!!

Answer 3

[Matt J]

BW1=~getEgg('egg1.png');
BW2=~getEgg('egg2.png');

S1=regionprops(BW1,'Orientation');
S2=regionprops(BW2,'Orientation');

rotation = S2.Orientation-S1.Orientation

rotation = -137.9439

function out=getEgg(file)
out=imbinarize(im2gray(imread(file)));
end

[Ashfaq Ahmed]

Your previous code gives the perfect angle! But this code (where you are using regionprops), is not giving the proper angle.

[Matt J]

The difference is 0.007 degrees. I'm surprised that it would be considered significant.