Exposing the LBP descriptors from OpenCV in Python

Question

I want to be able to calculate the LBP discriptor in python using OpenCV. According to this I need to compile openCV again.

I changed the elbp() functio

Answer 1

This is actually the same problem, if not the same question, as how to call a dll in python - But you can use either ctypes or swig however since there is already a python interface into OpenCV your best bet is to take a look to see how the existing one is done.

It may also be worth taking a look at pyopencv which provides a Boost based interface.

Update:

To see how the current system does things take a look at CMakeLists.txt in opencv/modules/python and you will find that a number of generated headers are created by opencv/modules/python/src2/gen2.py - you will need to spend some time looking at these two files.

Answer 2

So, it worked.

To make the function accessible: .

1. I made the following change to facerec.cpp, from:

   static void elbp(InputArray src, OutputArray dst, int radius, int neighbors)
   {
   ...
   }    
   
   static Mat elbp(InputArray src, int radius, int neighbors) {
       Mat dst;
       elbp(src, dst, radius, neighbors);
       return dst;
   }
   
   static Mat spatial_histogram(InputArray _src, int numPatterns,
                            int grid_x, int grid_y, bool /*normed*/)
   {
   ...
   }
   

   to:

   void elbp(InputArray src, OutputArray dst, int radius, int neighbors)
   {
   ...
   }    
   
   Mat elbp(InputArray src, int radius, int neighbors) {
       Mat dst;
       elbp(src, dst, radius, neighbors);
       return dst;
   }
   
   Mat spatial_histogram(InputArray _src, int numPatterns,
                            int grid_x, int grid_y, bool /*normed*/)
   {
   ...
   }
   

2. I added the following into /modules/contrib/include/opencv2/include/contrib.hpp under the cv namespace:

   CV_EXPORTS_W void elbp(InputArray src, OutputArray dst, int radius, int neighbors);
   

3. I then creted a release folder in the opencv root.

4. From within that folder, I ran:

   cmake -D CMAKE_BUILD_TYPE=RELEASE -D CMAKE_INSTALL_PREFIX=/usr/local -D BUILD_PYTHON_SUPPORT=ON -D WITH_TBB=ON -D BUILD_EXAMPLES=ON ..
   

5. Next, I ran make
6. take the cv2.so shared object created under /lib and put it where python looks for the packages. For me, it was /usr/local/lib/python2.7/dist-packages/

7. run python

   from cv2 import spatial_histogram
   

Voila!