OpenCV, How to pass parameters into cv2.TrackerMedianFlow_create function?

Question

I'm trying to create MEDIANFLOW tracker with OpenCV3.3 using opencv-python with Python3.6. I need to pass some arguments into the constructor according to this page of OpenCV docs.

The problem is I don't know how to pass available arguments into this function correctly? I wasn't able to find any information about it.

What I do (and it works):

tracker = cv2.TrackerMedianFlow_create()

What I want to do:

tracker = cv2.TrackerMedianFlow_create(maxLevel=3)

But it doesn't work and gives me an error:

SystemError: <built-in function TrackerMedianFlow_create> returned NULL without setting an error

Could you help?

Note that the documentation you linked is the constructor *for the params* structure. I don't have 3.3 to test how to do it, but you need to instantiate a `TrackeMedianFlowr::Params()` object, and then pass *that* object into `TrackerMedianFlow_create()`. Check the [docs for `TrackerMedianFlow()`](https://docs.opencv.org/3.3.0/d7/d86/classcv_1_1TrackerMedianFlow.html) to see that the constructor accepts only a `TrackerMedianFlow::Params()` object. — alkasm, Dec 09 '17 at 00:07

score 4 · Accepted Answer · answered Dec 09 '17 at 05:53

I search the intermediate code generated by cmake/make when compiling the OpenCV 3.3 for Python 3.5, just cannot find the methods to set the parameters for cv2.TrackerXXX.

In modules/python3/pyopencv_generated_funcs.h, I find this function:

static PyObject* pyopencv_cv_TrackerMedianFlow_create(PyObject* , PyObject* args, PyObject* kw)
{
    using namespace cv;

    Ptr<TrackerMedianFlow> retval;

    if(PyObject_Size(args) == 0 && (kw == NULL || PyObject_Size(kw) == 0))
    {
        ERRWRAP2(retval = cv::TrackerMedianFlow::create());
        return pyopencv_from(retval);
    }

    return NULL;
}

This is to say, you cann't pass any parameter to cv::TrackerMedianFlow_create().

In modules/python3/pyopencv_generated_types.h, I find this one:

static PyGetSetDef pyopencv_TrackerMedianFlow_getseters[] =
{
    {NULL}  /* Sentinel */
};

This to say, you have no way to change the parameters for Python wrapper by default, Unless you modified the source code and recompile it.

Thanks. Unfortunately, it seems like you say. I'll find other ways. — Alexandr Abramov, Dec 09 '17 at 12:54

score 4 · Answer 2 · answered Feb 11 '19 at 11:35

You can customize Tracker parameters via FileStorage interface.

import cv2

# write
tracker = cv2.TrackerMedianFlow_create()
tracker.save('params.json')

# write (another way)
fs = cv2.FileStorage("params.json", cv2.FileStorage_WRITE)
tracker.write(fs)
fs.release()

# read
tracker2 = cv2.TrackerMedianFlow_create()
fs = cv2.FileStorage("params.json", cv2.FileStorage_READ)
tracker2.read(fs.getFirstTopLevelNode())

OpenCV, How to pass parameters into cv2.TrackerMedianFlow_create function?

2 Answers2

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