Calculating rotation between two images

Question

I want to measure the rotation between two objects in two images. The second image is created by rotating the first one. I tried techniques like fitting a line but like you can see they get the same slope/angle.

Here is my code:


import cv2
import numpy as np
import matplotlib.pyplot as plt
#import imutils

img = cv2.imread("4.jpg",0)
indices = np.where(img!= [255])
coordinates = zip(indices[0], indices[1])

from scipy.optimize import curve_fit

def f(x, A, B): # this is your 'straight line' y=f(x)
    return A*x + B

A,B = curve_fit(f, indices[1], indices[0])[0] # your data x, y to fit
x=np.arange(0,1000,1)
y=A*x+B
#%%
plt.plot(y)
plt.scatter(indices[1],indices[0])

plt.show()

Original image:

Original image rotated by 180°:

It is the slope and angle. I fit a line to the points in the image — Alsen57, Jul 03 '19 at 20:45
What do you mean the slope? Please just include in your post exactly what the expected output is. — Akaisteph7, Jul 03 '19 at 21:17
i edited the post. So for both pictures the resulting fitted line is the same, but its obvious that the second image is rotated by 180°. How can i measure these 180°? — Alsen57, Jul 04 '19 at 08:18
Will the image always be exactly the same, with the only modifications being the rotation? — Akaisteph7, Jul 04 '19 at 16:23

Piotr Siekański · Answer 1 · 2019-07-08T09:06:22.960

You can calculate the center of mass and find the point that is the farthest point from the center. Based on this two points you can calculate the line. You can find the third point that is the farthest point from this line and belong to this object. This set of 3 points will give you the unique solution unless your object is symmetric. Based on these 3 points you can calculate the rotation between original and rotated image:

Calculate the matrix using getAffineTransform https://docs.opencv.org/2.4/modules/imgproc/doc/geometric_transformations.html#Mat%20getAffineTransform(InputArray%20src,%20InputArray%20dst) function
Calculate angle from rotation this matrix using equations from this link: opencv estimateRigidTransform: How to get global scale?
Additionally, you can also get the scale factor and translation vector from this matrix.

But i guess it will find the same center of maß and points at all. I dont see what makes the difference — Alsen57, Jul 04 '19 at 15:56
@AliSentürk, The points will be the same in both images, I updated the answer. Hope that it helps you. — Piotr Siekański, Jul 08 '19 at 09:07

Calculating rotation between two images

1 Answers1