Skip to main content

Swig with C++ and external libs

This is the kind of problem I work out how to solve, then I forget... and eventually have to do again... So more for my sake here is how to compile a python extension in c++ that has external libraries like opencv. Automatically wrapped with swig from distutils...

So first up I have a random library of C++ that I want to use from python. One such C file is doing some low level math on an opencv IPL image. Since I want to use this in python but require it to be fast, I need to create an extension.

// The header file I want to wrap.
#include <stdio.h>
#include <stdlib.h>
#include <math.h>

#include "cv.h"

void remove_corneal_reflection(IplImage *image, IplImage *threshold_image, int sx, int sy, int window_size,
     int biggest_crr, int &crx, int &cry, int& crr, int *valid_point_calc);


This has a few normal includes and "cv.h". This is refering to the opencv installed NOT in the default dir but in /usr/local/.

Next is the instructions for swig to use when creating the wrapper for the file. This uses the autodoc feature so in python the docstrings have some (almost) usefull information about what the parameters are.

/* Interface to the C file remove_corneal_reflection.c */
 
%module removeCornealReflection
%{
    /* Put header files here or function declarations like below */
    #include "opencv/cv.h"
    #include "remove_corneal_reflection.h"
 
%}
 
// Tell swig to put type information into the functions docstrings...
%feature("autodoc", "1");

// Tell swig to parse the header file.
%include "remove_corneal_reflection.h"

To get distutils to do all the compiling and linking we need a setup.py file:


import distutils
from distutils.core import setup, Extension
 
setup(
    name = "Brian's C++ Library wrapped up all nice for python",
    author = 'Brian Thorne',
    author_email = 'hardbyte@gmail.com',
    license='GPL v3 :: GNU General Public License',  
    version = "0.1",
    ext_modules = [
        Extension(
            "_helloWorld",
            sources = ["interfaceFile.i","Hello World.cpp"],
            swig_opts=["-c++"],
            ),
        Extension(
            "_removeCornealReflection",
            sources = ["remove_corneal_reflection.i","remove_corneal_reflection.cpp"],
            swig_opts=["-c++"],
            library_dirs=['/usr/local/lib/opencv'],
            libraries=['cv','highgui','cvaux','cvaux'],
            include_dirs = ['/usr/local/include/opencv']
            )
        ]
    )
And for the first time I also made a setup.cfg file with a whole two lines:

[build_ext]
inplace=1

And then the whole thing can be wrapped up with the familiar command:

brian@brian-hitlab:~/dev/swig/linkedLib$ python setup.py build
running build
running build_ext
building '_helloWorld' extension
swigging interfaceFile.i to interfaceFile_wrap.cpp
swig -python -c++ -o interfaceFile_wrap.cpp interfaceFile.i
creating build
creating build/temp.linux-x86_64-2.6
gcc -pthread -fno-strict-aliasing -DNDEBUG -g -fwrapv -O2 -Wall -Wstrict-prototypes -fPIC -I/usr/include/python2.6 -c interfaceFile_wrap.cpp -o build/temp.linux-x86_64-2.6/interfaceFile_wrap.o
cc1plus: warning: command line option "-Wstrict-prototypes" is valid for Ada/C/ObjC but not for C++
gcc -pthread -fno-strict-aliasing -DNDEBUG -g -fwrapv -O2 -Wall -Wstrict-prototypes -fPIC -I/usr/include/python2.6 -c Hello World.cpp -o build/temp.linux-x86_64-2.6/Hello World.o
cc1plus: warning: command line option "-Wstrict-prototypes" is valid for Ada/C/ObjC but not for C++
g++ -pthread -shared -Wl,-O1 -Wl,-Bsymbolic-functions build/temp.linux-x86_64-2.6/interfaceFile_wrap.o build/temp.linux-x86_64-2.6/Hello World.o -o _helloWorld.so
building '_removeCornealReflection' extension
swigging remove_corneal_reflection.i to remove_corneal_reflection_wrap.cpp
swig -python -c++ -o remove_corneal_reflection_wrap.cpp remove_corneal_reflection.i
gcc -pthread -fno-strict-aliasing -DNDEBUG -g -fwrapv -O2 -Wall -Wstrict-prototypes -fPIC -I/usr/local/include/opencv -I/usr/include/python2.6 -c remove_corneal_reflection_wrap.cpp -o build/temp.linux-x86_64-2.6/remove_corneal_reflection_wrap.o
cc1plus: warning: command line option "-Wstrict-prototypes" is valid for Ada/C/ObjC but not for C++
gcc -pthread -fno-strict-aliasing -DNDEBUG -g -fwrapv -O2 -Wall -Wstrict-prototypes -fPIC -I/usr/local/include/opencv -I/usr/include/python2.6 -c remove_corneal_reflection.cpp -o build/temp.linux-x86_64-2.6/remove_corneal_reflection.o
cc1plus: warning: command line option "-Wstrict-prototypes" is valid for Ada/C/ObjC but not for C++
g++ -pthread -shared -Wl,-O1 -Wl,-Bsymbolic-functions build/temp.linux-x86_64-2.6/remove_corneal_reflection_wrap.o build/temp.linux-x86_64-2.6/remove_corneal_reflection.o -L/usr/local/lib/opencv -lcv -lhighgui -lcvaux -lcvaux -o _removeCornealReflection.so


This creates two python extensions - one called helloWorld and one called removeCornealReflection. Ideal! Not sure why the warning message comes up, it looks like it is using gcc for the linking when surely g++ would be better... anyhow can't argue with positive results!

Popular posts from this blog

Python and Gmail with IMAP

Today I had to automatically access my Gmail inbox from Python. I needed the ability to get an unread email count, the subjects of those unread emails and then download them. I found a Gmail.py library on sourceforge, but it actually opened the normal gmail webpage and site scraped the info. I wanted something much faster, luckily gmail can now be accessed with both pop and imap. After a tiny amount of research I decided imap was the better albiet slightly more difficult protocol. Enabling imap in gmail is straight forward, it was under labs. The address for gmail's imap server is: imap.gmail.com:993 Python has a library module called imaplib , we will make heavy use of that to access our emails. I'm going to assume that we have already defined two globals - username and password. To connect and login to the gmail server and select the inbox we can do: import imaplib imap_server = imaplib . IMAP4_SSL ( "imap.gmail.com" , 993 ) imap_server . login ( use...
Read more

Bluetooth with Python 3.3

Since about version 3.3 Python supports Bluetooth sockets natively. To put this to the test I got hold of an iRacer from sparkfun . To send to New Zealand the cost was $60. The toy has an on-board Bluetooth radio that supports the RFCOMM transport protocol. The drive  protocol is dead easy, you send single byte instructions when a direction or speed change is required. The bytes are broken into two nibbles:  0xXY  where X is the direction and Y is the speed. For example the byte 0x16 means forwards at mid-speed. I was surprised to note the car continues carrying out the last given demand! I let pairing get dealt with by the operating system. The code to create a  Car object that is drivable over Bluetooth is very straight forward in pure Python: import socket import time class BluetoothCar : def __init__ ( self , mac_address = "00:12:05:09:98:36" ): self . socket = socket . socket ( socket . AF_BLUETO...
Read more

Matplotlib in Django

The official django tutorial is very good, it stops short of displaying data with matplotlib - which could be very handy for dsp or automated testing. This is an extension to the tutorial. So first you must do the official tutorial! Complete the tutorial (as of writing this up to part 4). Adding an image to a view To start with we will take a static image from the hard drive and display it on the polls index page. Usually if it really is a static image this would be managed by the webserver eg apache. For introduction purposes we will get django to serve the static image. To do this we first need to change the template. Change the template At the moment poll_list.html probably looks something like this: <h1>Django test app - Polls</h1> {% if object_list %} <ul> {% for object in object_list %} <li><a href="/polls/{{object.id}}">{{ object.question }}</a></li> {% endfor %} </ul> {% else %} <p>No polls...
Read more