Fix TypeError: can’t pickle cv2.KeyPoint objects Error in Python OpenCV

When we are using opencv SIFT algorithm to compute the similarity between two images, we will get two objects: keypoint, descriptor. In order to save processing time, we may save them to a file using python pickle. However, we may get this error: TypeError: can’t pickle cv2.KeyPoint objects. In this tutorial, we will introduce you how t fix it.

For example:

keypoint1, descriptor1 = computeSIFT(image_1_data)
print(type(keypoint1))
for k in keypoint1:
    print(type(k))

with open("test.txt", "wb") as f:
    pickle.dump(keypoint1, f)

Here keypoint1 is python list, it contains some <class ‘cv2.KeyPoint’>

Run this code, we will get this error:

TypeError: can’t pickle cv2.KeyPoint objects

How to fix this error?

In order to save KeyPoint object using pickle, we can wrap it by using a python dictionary.

For example:

    #-------------------------------------------------------------
    new_k = []
    k = keypoint1
    for i in range(len(k)):
        temp = {'pt': k[i].pt,
                'size': k[i].size,
                'angle': k[i].angle,
                'octave': k[i].octave,
                "response":k[i].response,
                "octave":k[i].octave,
                'class_id': k[i].class_id
                }  # 把这些信息写入到字典中
        new_k.append(temp)
    data = [new_k, descriptor1]

    with open("key.txt", 'wb') as fp:
        pickle.dump(data, fp)

Run this code, we will save keypoint1 and descriptor1 to a key.txt

How to load keypoint1 and descriptor1and from key.txt file?

We should rebuild keypoint1 from points saved in key.txt

For example:

    with open("key.txt", 'rb') as fp:
        data = pickle.load(fp)
        descriptor1 = data[1]
        new_k = data[0]
        keypoint1 = []
        for temp in new_k:
            point = cv2.KeyPoint(x=temp['pt'][0], y=temp['pt'][1],
                                 _size=temp['size'], _angle=temp['angle'],
                                 _response=temp['size'],
                                 _octave=temp['octave'], _class_id=temp['class_id'])
            keypoint1.append(point)
    #------------------------

Run this code, we can find keypoint1 and descriptor1 are restored.