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OK. I got it. The following does the job:

'''Takes 2 vectors and returns the rotation matrix between these 2 vectors'''
def get_camera_rotation(homographyMatrix):
# Points in the camera frame
camera_pts = np.float32([[round(CAM_WIDTH / 2), round(CAM_HEIGHT / 2)],
[round(10 + CAM_WIDTH / 2), round(10 + CAM_HEIGHT / 2)]]).reshape(-1, 1, 2)
# Find these points in the projector image
proj_pts = cv2.perspectiveTransform(camera_pts, homographyMatrix)

# Find the vectors between the sets of points
camera_vector = (camera_pts[0][0][0] - camera_pts[1][0][0], camera_pts[0][0][1] - camera_pts[1][0][1])
proj_vector = (proj_pts[0][0][0] - proj_pts[1][0][0], proj_pts[0][0][1] - proj_pts[1][0][1])

# change the vectors to unit vectors
camera_vector = camera_vector / np.absolute(np.linalg.norm(camera_vector))
proj_vector = proj_vector / np.absolute(np.linalg.norm(proj_vector))

# calculate the angle between the 2 vectors
# Change the sign of the angle if the rocket is turning the opposite way to desired
#sine of the angle
sinAngle = camera_vector[0] * proj_vector[1] - camera_vector[1] * proj_vector[0]
#angle between the vectors
angle = np.arcsin(np.clip(sinAngle, -1.0, 1.0))

#print("The angle between the camera and the projector is:")
#print(angle)
# calculate the 2D rotation matrix from this angle
rotation_matrix = np.matrix([[np.cos(angle), -1 * np.sin(angle)], [np.sin(angle), np.cos(angle)]])
return angle


to use:

 # rotate the shuttle as the camera does
# first though, get a copy
toBeRotatedShuttle = shuttleIcon.copy()
rows, cols, w = toBeRotatedShuttle.shape
angle = get_camera_rotation(smoothenedMatrix)
angleInDegrees = round(math.degrees(math.asin(angle)),2) # convert radian to degrees
rotationMatrix = cv2.getRotationMatrix2D((cols / 2, rows / 2), angleInDegrees, 1)
shuttleIcon = cv2.warpAffine(toBeRotatedShuttle, rotationMatrix, (cols, rows), cv2.INTER_AREA)