# How to calculate the rotation of this egg?

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Ashfaq Ahmed on 9 Aug 2022
Commented: Matt J on 9 Aug 2022
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 ^_^

John D'Errico on 9 Aug 2022
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.
##### 2 CommentsShowHide 1 older comment
John D'Errico on 9 Aug 2022
Edited: John D'Errico on 9 Aug 2022
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.
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.

### More Answers (2)

Matt J on 9 Aug 2022
Edited: Matt J on 9 Aug 2022
First, crop the black corders from your image fiels. Then, use ellipticalFit() from,
[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)
end
Ashfaq Ahmed on 9 Aug 2022
This code works perfectly! Thank you so much!!

Matt J on 9 Aug 2022
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)
end
##### 2 CommentsShowHide 1 older comment
Matt J on 9 Aug 2022
The difference is 0.007 degrees. I'm surprised that it would be considered significant.