flann.knnMatch sometimes outputs a queryIdx that throws an out of bounds error

[manasmacherla]

Expected behaviour

we filter good matches from matches that are generated by flann.knnMatch using Lowe's ratio test.
usually we get the pixel location of the match in the queryImage or the trainImage by using queryIdx or trainIdx and the keypoint object.

Actual behaviour

sometimes (these are edge cases in my data) where there aren't sufficient matches in the image pair, when i try to get the pixel location of the match using keypoint objects generated by ORB using queryIdx or trainIdx, it generates an out of bound error meaning there aren't enough keypoints in the image and queryIdx is greater than the length of the keypoints.

Write here what went wrong.

Steps to reproduce

CODE:

# my initialization parameters for flann

    orb = cv2.ORB_create()

    FLANN_INDEX_LSH = 6
    index_params = dict(algorithm=FLANN_INDEX_LSH, table_number=6, key_size=12, multi_probe_level=1)
    search_params = dict(checks=50)

    flann = cv2.FlannBasedMatcher(index_params, search_params)

# working code 
matches = flann.knnMatch(prev_descriptors, descriptors, k=2)

good_matches = []
for match in matches:
    if len(match) == 2:
        m, n = match
    if m.distance < 0.7 * n.distance:
        good_matches.append(m)

for m in good_matches:
    # having to do these two below checks to ensure queryIdx is less than length of keypoints
    if m.queryIdx > len(keypoints): 
        continue
    if m.trainIdx > len(keypoints):
        continue
    
     # previous keypoints are keypoints saved from the previous image, 
     # running the feature matcher with current and previous frame of the video

    matched_points_prev.append(prev_keypoints[m.queryIdx].pt)
    matched_points_curr.append(keypoints[m.trainIdx].pt)

matched_points_prev = np.array(matched_points_prev, dtype=np.float32)
matched_points_curr = np.array(matched_points_curr, dtype=np.float32)

ERROR:

matched_points_prev.append(prev_keypoints[m.queryIdx].pt)
IndexError: tuple index out of range

• operating system: macOS
• architecture: ARM (apple silicon)
• opencv-python version: 4.10.0

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