How do you wrap C++ OpenCV code with Boost::Python? - c++

Question

I want to wrap my C++ OpenCV code with boost::python, and to learn how to do it, I tried a toy example, in which
I use the Boost.Numpy project to provide me with boost::numpy::ndarray.
The C++ function to be wrapped, square() takes a boost::numpy::ndarray and modifies it in place by squaring each element in it.
The exported Python module name is called test.
The square() C++ function is exported as the square name in the exported module.
I am not using bjam because IMO it is too complicated and just doesn't work for me no matter what. I'm using good old make.
Now, here's the code:
// test.cpp
#include <boost/python.hpp>
#include <boost/numpy.hpp>
#include <boost/scoped_array.hpp>
#include <opencv2/imgproc/imgproc.hpp>
#include <iostream>
namespace py = boost::python;
namespace np = boost::numpy;
void square(np::ndarray& array)
{
if (array.get_dtype() != np::dtype::get_builtin<int>())
{
PyErr_SetString(PyExc_TypeError, "Incorrect array data type.");
py::throw_error_already_set();
}
size_t rows = array.shape(0), cols = array.shape(1);
size_t stride_row = array.strides(0) / sizeof(int),
stride_col = array.strides(1) / sizeof(int);
cv::Mat mat(rows, cols, CV_32S);
int *row_iter = reinterpret_cast<int*>(array.get_data());
for (int i = 0; i < rows; i++, row_iter += stride_row)
{
int *col_iter = row_iter;
int *mat_row = (int*)mat.ptr(i);
for (int j = 0; j < cols; j++, col_iter += stride_col)
{
*(mat_row + j) = (*col_iter) * (*col_iter);
}
}
for (int i = 0; i < rows; i++, row_iter += stride_row)
{
int *col_iter = row_iter;
int *mat_row = (int*)mat.ptr(i);
for (int j = 0; j < cols; j++, col_iter += stride_col)
{
*col_iter = *(mat_row + j);
}
}
}
BOOST_PYTHON_MODULE(test)
{
using namespace boost::python;
def("square", square);
}
And here's the Makefile:
PYTHON_VERSION = 2.7
PYTHON_INCLUDE = /usr/include/python$(PYTHON_VERSION)
BOOST_INC = /usr/local/include
BOOST_LIB = /usr/local/lib
OPENCV_LIB = $$(pkg-config --libs opencv)
OPENCV_INC = $$(pkg-config --cflags opencv)
TARGET = test
$(TARGET).so: $(TARGET).o
g++ -shared -Wl,--export-dynamic \
$(TARGET).o -L$(BOOST_LIB) -lboost_python \
$(OPENCV_LIB) \
-L/usr/lib/python$(PYTHON_VERSION)/config -lpython$(PYTHON_VERSION) \
-o $(TARGET).so
$(TARGET).o: $(TARGET).cpp
g++ -I$(PYTHON_INCLUDE) $(OPENCV_INC) -I$(BOOST_INC) -fPIC -c $(TARGET).cpp
With this scheme, I can type make and test.so gets created. But when I try to import it,
In [1]: import test
---------------------------------------------------------------------------
ImportError Traceback (most recent call last)
<ipython-input-1-73ae3ffe1045> in <module>()
----> 1 import test
ImportError: ./test.so: undefined symbol: _ZN5boost6python9converter21object_manager_traitsINS_5numpy7ndarrayEE10get_pytypeEv
In [2]:
This is a linker error which I can't seem to fix. Can anyone please help me with what's going on? Do you have (links to) code that already does integrate OpenCV, numpy and Boost.Python without things like Py++ or the likes?.

Answer 1

Okay I fixed this. It was a simple issue, but a sleepy brain and servings of bjam had made me ignore it. In the Makefile, I'd forgotten to put -lboost_numpy that links the Boost.Numpy libs to my lib. So, the modified Makefile looks like this:
PYTHON_VERSION = 2.7
PYTHON_INCLUDE = /usr/include/python$(PYTHON_VERSION)
BOOST_INC = /usr/local/include
BOOST_LIB = /usr/local/lib
OPENCV_LIB = $$(pkg-config --libs opencv)
OPENCV_INC = $$(pkg-config --cflags opencv)
TARGET = test
$(TARGET).so: $(TARGET).o
g++ -shared -Wl,--export-dynamic \
$(TARGET).o -L$(BOOST_LIB) -lboost_python -lboost_numpy \
$(OPENCV_LIB) \
-L/usr/lib/python$(PYTHON_VERSION)/config -lpython$(PYTHON_VERSION) \
-o $(TARGET).so
$(TARGET).o: $(TARGET).cpp
g++ -I$(PYTHON_INCLUDE) $(OPENCV_INC) -I$(BOOST_INC) -fPIC -c $(TARGET).cpp