Cross-compiling OpenCV for OpenWRT (WR703N)

Introduction

OpenCV is currently not available through the package system for the WR703N. This guide shows how to compile OpenCV from source using the OpenWRT toolchain.

This was carried out for the Attitude Adjustment (12.09-rc1) release of OpenWRT and release 2.4.4 of OpenCV.

If you are looking for the libraries and don't want to have to compile them yourself I have placed them on github: http://github.com/mark4h/openwrt

Setting up working directory

Create a working directory

mkdir ~/WR703N_opencv

Download the OpenWRT toolchain (here) and the OpenCV source code (here) into your working directory.

Extract both tars

tar –xjf [filename]

Install make, cmake and cmake-gui.

Set environment variables.

export PATH=$PATH:/home/user/WR703N_opencv/OpenWrt-Toolchain-ar71xx-for-mips_r2-gcc-4.6-linaro_uClibc-0.9.33.2/toolchain-mips_r2_gcc-4.6-linaro_uClibc-0.9.33.2/bin/
export STAGING_DIR=/home/user/WR703N_opencv

Create a build directory and change to it.

mkdir ~/WR703N_opencv/opencv-2.4.4/build
cd ~/WR703N_opencv/opencv-2.4.4/build

Configure build

Create a toolchain.cmake file to use with cmake.

vim toolchain.cmake

set( CMAKE_SYSTEM_NAME Linux )
set( CMAKE_SYSTEM_PROCESSOR mips )
set( CMAKE_C_COMPILER mips-openwrt-linux-uclibc-gcc )
set( CMAKE_CXX_COMPILER mips-openwrt-linux-uclibc-g++ )
set( CMAKE_FIND_ROOT_PATH /home/user/WR703N_opencv/OpenWrt-Toolchain-ar71xx-for-mips_r2-gcc-4.6-linaro_uClibc-0.9.33.2/toolchain-mips_r2_gcc-4.6-linaro_uClibc-0.9.33.2/ )

Execute cmake-gui from within the build directory

cmake-gui ../

In the cmake-gui select configure. Cmake must now be told about the toolchain using the toolchain.cmake file created previously.

Select 'Specify toolchain file for cross-compiling' and then click 'Next'.

Select the toolchain.cmake file you just created and then select 'Finish'.
Cmake should now carry out its configuration.

Once it has finished you are ready to select the build options you require.

The following describes the options I used. They include building all the modules of OpenCV (apart from the python and gpu modules), with V4L and JPEG support. Other support (such as for other image formats) can be included by selecting the required options.

Deselect/select as necessary to include the following options:

• BUILD_opencv_apps
• BUILD_opencv_calib3d
• BUILD_opencv_contrib
• BUILD_opencv_core
• BUILD_opencv_features2d
• BUILD_opencv_flann
• BUILD_opencv_highgui
• BUILD_opencv_imgproc
• BUILD_opencv_legacy
• BUILD_opencv_ml
• BUILD_opencv_nonfree
• BUILD_opencv_objdetect
• BUILD_opencv_photo
• BUILD_opencv_stitching
• BUILD_opencv_ts
• BUILD_opencv_video
• BUILD_opencv_videostab
• BUILD_opencv_world
• ENABLE_OMIT_FRAME_POINTER
• WITH_JPEG
• WITH_V4L

To build shared libraries enable the following option, otherwise static libraries will be built:

• BUILD_SHARED_LIBS

The following are specific options that did not work:

• ENABLE_precompiled_headers
• BUILD_opencv_python
• BUILD_opencv_gpu

Once you have selected your options click 'configure' again.
Once the configuration has finished click 'generate'.
You are now ready to begin the compiling.
Close cmake.

Compile

On the terminal run make.

make

If no errors were given, you should now find the compiled libraries in the lib folder.