How does the GIL release happen for drawing functions exposed to Python?

edit

not sure, if it answers your question, but here's the generated code for cv2.circle():

(if you built your cv2 locally, you will find it opencv/build/modules/python_bindings_generator/pyopencv_generated_funcs.h)

actually, every c++ opencv call is wrapped in this ERRWRAP2 macro, so the answer is probably: yes !

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

    {
    PyObject* pyobj_img = NULL;
    Mat img;
    PyObject* pyobj_center = NULL;
    Point center;
    int radius=0;
    PyObject* pyobj_color = NULL;
    Scalar color;
    int thickness=1;
    int lineType=LINE_8;
    int shift=0;

    const char* keywords[] = { "img", "center", "radius", "color", "thickness", "lineType", "shift", NULL };
    if( PyArg_ParseTupleAndKeywords(args, kw, "OOiO|iii:circle", (char**)keywords, &pyobj_img, &pyobj_center, &radius, &pyobj_color, &thickness, &lineType, &shift) &&
        pyopencv_to(pyobj_img, img, ArgInfo("img", 1)) &&
        pyopencv_to(pyobj_center, center, ArgInfo("center", 0)) &&
        pyopencv_to(pyobj_color, color, ArgInfo("color", 0)) )
    {
        ERRWRAP2(cv::circle(img, center, radius, color, thickness, lineType, shift));
        return pyopencv_from(img);
    }
    }
    PyErr_Clear();

    {
    PyObject* pyobj_img = NULL;
    UMat img;
    PyObject* pyobj_center = NULL;
    Point center;
    int radius=0;
    PyObject* pyobj_color = NULL;
    Scalar color;
    int thickness=1;
    int lineType=LINE_8;
    int shift=0;

    const char* keywords[] = { "img", "center", "radius", "color", "thickness", "lineType", "shift", NULL };
    if( PyArg_ParseTupleAndKeywords(args, kw, "OOiO|iii:circle", (char**)keywords, &pyobj_img, &pyobj_center, &radius, &pyobj_color, &thickness, &lineType, &shift) &&
        pyopencv_to(pyobj_img, img, ArgInfo("img", 1)) &&
        pyopencv_to(pyobj_center, center, ArgInfo("center", 0)) &&
        pyopencv_to(pyobj_color, color, ArgInfo("color", 0)) )
    {
        ERRWRAP2(cv::circle(img, center, radius, color, thickness, lineType, shift));
        return pyopencv_from(img);
    }
    }

    return NULL;
}