How to serialize and deserialize SURF descriptor in OpenCV?

Question

I'm working around SURF using OpenCV. I want to put the SURF descriptor into cache, so I have to serialize the descriptor and then deserialize it back to descriptor.

What I did is as follows:

[serialize]
    kp, des = surf.detectAndCompute(img, None)
    jm = json.dumps(des.tolist())

[deserialize]
    du = json.loads(jm)
    dn = np.asarray(du)

I printed the type of des , dn , which both are type of np.ndarray . Howerver, after the deserialization, I can't use the dn to perform the knnMatch , while using the original des is ok.

There is an error:

OpenCV Error: Assertion failed (_queryDescriptors.type() == trainDescType) in knnMatchImpl, file /home/zibo/opencv/modules/features2d/src/matchers.cpp, line 722

It seems that, the original descriptor des is not just an type of numpy.ndarray . I searched the OpenCV codes but don't find the answer yet.

So, can anyone help. How to serialize and deserialize SURF descriptor?

Answer 1

When you convert the loaded data to array, set data type to float32.

dn = np.asarray(du, dtype = np.float32)

I assume that 'np' here is the alias of 'numpy'.