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pythonopencvsift

Compute descriptor with Sift compute python

I am trying to compute descriptors given a set of Keypoint that i have computed with cv2.KeyPoint ! But when i try to call sift.compute() like this, just to see if it works:

sift= cv2.SIFT()
sift.compute(img, Keypoint)

I get the following error that i can't manage to understand: <method 'compute' of 'cv2.Feature2D' objects> returned NULL without setting an error

I am using Opencv 4.4

Solution

  • If you look at the documentation

    You should declare SIFT as:

    sift = cv.SIFT_create()

    Therefore the correct code will be:

    sift = cv.SIFT_create()
kp, des = sift.detectAndCompute(gray,None)

    Update

    If you have already calculated the key-points (kp) then draw it on the current image:

    img=cv.drawKeypoints(gray,kp,img)
cv.imwrite('sift_keypoints.jpg',img)