How can I remap a point after an image rotation?

Question

I have a mathematical question: let\'s suppose I rotate an image around its center by an angle of 30°, using the opencv with the following commands:

M = cv2.         


        

Answer 1

You can use transform() function to apply given transformation to arrays of points.

cv2.transform(pointsToTransform, M)

Answer 2

Just use matrix operation as described in Affine Transformations and inverse matrix.

# inverse matrix of simple rotation is reversed rotation.
M_inv = cv2.getRotationMatrix2D((100/2, 300/2),-30,1)


# points
points = np.array([[35.,  0.],
                   [175., 0.],
                   [105., 200.],
                   [105., 215.],
                  ])
# add ones
ones = np.ones(shape=(len(points), 1))

points_ones = np.hstack([points, ones])

# transform points
transformed_points = M_inv.dot(points_ones.T).T