how to extract x,y coordinates from OpenCV "cv2.keypoint" object?

Question

I tried to use the following code:

    xCoordinate=point.x

(point is type of cv2.keyPoint) It gives me error saying cv2.keyPoint has no attribute 'x'

Answer 1

point.pt is a tuple(x,y)`.

So,

x = point.pt[0]
y = point.pt[1]

or,

(x,y) = point.pt

Answer 2

OpenCV provides a function for this. You can run:

pts = cv2.KeyPoint_convert(kp)

Answer 3

Read the docs.

class KeyPoint

Data structure for salient point detectors.

• Point2f pt -- coordinates of the keypoint

• float size -- diameter of the meaningful keypoint neighborhood

• float angle ...¶

So point.pt is a Point2f.

Try x,y= point.pt

Answer 4

Here is my take (runable code):

import cv2, os
import numpy as np
import matplotlib.pyplot as plt

# INITIALISATION
filename = os.path.join('foo', 'bar.jpg')
img0 = cv2.imread(filename) # original image
gray = cv2.cvtColor(img0, cv2.COLOR_BGR2GRAY) # convert to grayscale
sift = cv2.xfeatures2d.SIFT_create() # initialize SIFT 
f, (ax1, ax2) = plt.subplots(1, 2) # create subplots

# DETECT AND DRAW KEYPOINTS
# sift.detect() returns a list of keypoints
# keypoint is a standard class of opencv (not just SIFT-related)
kp = sift.detect(gray,None) # calculates SIFT points
img1=cv2.drawKeypoints(gray,kp, None) # mae new image with keypoints drawn
ax1.imshow(img1) # plot 

# RETREIVE KEYPOINTS COORDINATES AND DRAW MANUALLY
# Reade these and make numpy array
pts = np.asarray([[p.pt[0], p.pt[1]] for p in kp])
cols = pts[:,0]
rows = pts[:,1]
ax2.imshow(cv2.cvtColor(img0, cv2.COLOR_BGR2RGB))
ax2.scatter(cols, rows)

plt.show()

Answer 5

I solved your problem like this.

kp,des = surf.detectAndCompute(img,None) 
pts = [p.pt for p in kp]

Now you get a list of x,y co-ordinates for all keypoints in your image.

Answer 6

to get the correct shape for optical flow input I used a combination of the above

kp = sift.detect(old_frame, None)
pts0 = cv2.KeyPoint.convert(kp).reshape(-1, 1, 2)