openCV 3.1.0 optimized for Raspberry Pi, with libjpeg-turbo 1.5.0 and NEON SIMD support

This is a small log for myself on building openCV 3.1.0 on a Raspberry Pi 2. This should work on Raspberry Pi 3 too (but not on RPi 1 as it does not support NEON).

1. Getting required libraries and stuff:
• I am not mentioning much here, just get the normal required dependencies for building openCV 3.1.0
2. Getting our core packages:
• openCV 3.1.0:
  

  wget https://github.com/Itseez/opencv/archive/3.1.0.tar.gz -O opencv.tar.gz

• openCV 3.1.0 Extras:
  

  wget https://github.com/Itseez/opencv_contrib/archive/3.1.0.tar.gz -O opencv_contrib.tar.gz

• libjpeg-turbo 1.5.0:
  

  wget https://github.com/libjpeg-turbo/libjpeg-turbo/archive/1.5.0.tar.gz -O libjpeg-turbo.tar.gz

3. Decompress everything:

• tar xvf opencv.tar.gz
  tar xvf opencv_contrib.tar.gz
  tar xvf libjpeg-turbo.tar.gz

4. Compiling all packages:
• libjpeg-turbo 1.5.0
  

  cd libjpeg-turbo-1.5.0/
  mkdir build
  autoreconf -fiv
  cd build
  export CFLAGS="-mcpu=cortex-a7 -mfpu=neon-vfpv4 -ftree-vectorize -mfloat-abi=hard -fPIC -O3"
  export CXXFLAGS="-mcpu=cortex-a7 -mfpu=neon-vfpv4 -ftree-vectorize -mfloat-abi=hard -fPIC -O3"
  sh <path_to_the_source_code>/configure

  The magic is at the C/CXX FLAGS, which optimize our generated binaries to tune with our CPU and use NEON Hardware FPU.
  Moreover, the libjpeg-turbo itself also contains NEON SIMD instructions which speed up the JPEG en/decode process.
  Now, compile libjpeg-turbo with:
  

  make -j4

  "-j4" means running the process with 4 threads. In case of any errors (such as gcc or its child processes got killed due to insufficient memory), reduce the number, or omit this flag.
  Make sure you have also do a "make clean" after something went wrong and you wish to recompile.
  After the process is done, install libjepg-turbo with:
  

  sudo make install

  The result binaries will be reside in /opt/libjpeg-turbo/
• openCV 3.1.0 with extras
  I have modified the default CMake scripts to modfiy the default NEON C/CXX FLAGS:
  

  cd opencv-3.1.0/cmake/
  nano OpenCVCompilerOptions.cmake

  Go to line 27 and 28, add:

  -mcpu=cortex-a7 -mfpu=neon-vfpv4 -ftree-vectorize -mfloat-abi=hard

  to "OPENCV_EXTRA_C_FLAGS" and "OPENCV_EXTRA_CXX_FLAGS"
  Also, go to line 150, under the section of "if (ENABLE_NEON)", modify the parameter of "add_extra_compiler_option" from

  -mfpu=neon

  To:

  -mfpu=neon-vfpv4

  Now, save the file (Ctrl+O), and:

  cd ../
  mkdir build
  cd build
  export CFLAGS="-mcpu=cortex-a7 -mfpu=neon-vfpv4 -ftree-vectorize -mfloat-abi=hard" # Notice here does not have -fPIC and -O3
  export CXXFLAGS="-mcpu=cortex-a7 -mfpu=neon-vfpv4 -ftree-vectorize -mfloat-abi=hard" # Notice here does not have -fPIC and -O3
  cmake -DCMAKE_BUILD_TYPE=RELEASE -DCMAKE_INSTALL_PREFIX=/usr/local -DINSTALL_C_EXAMPLES=OFF -DINSTALL_PYTHON_EXAMPLES=OFF -DOPENCV_EXTRA_MODULES_PATH=<path_to_opencv_contrib-3.1.0>/modules -DBUILD_EXAMPLES=ON -DWITH_FFMPEG=OFF -DWITH_V4L=OFF -DWITH_LIBV4L=OFF -DENABLE_NEON=ON -DEXTRA_C_FLAGS=-mcpu=cortex-a7 -mfpu=neon-vfpv4 -ftree-vectorize -mfloat-abi=hard -DEXTRA_CXX_FLAGS=-mcpu=cortex-a7 -mfpu=neon-vfpv4 -ftree-vectorize -mfloat-abi=hard -DWITH_JPEG=ON -DBUILD_JPEG=OFF -DJPEG_INCLUDE_DIR=/opt/libjpeg-turbo/include/ -DJPEG_LIBRARY=/opt/libjpeg-turbo/lib32/libjpeg.a ..
  make -j2 # You can use -j4, but my gcc crashed with -j4, you can test by yourself
  

  I am building the openCV for C++ (and no python wrappers), no FFMPEG and V4L included in the build. You may adjust the build flags according to your needs.
  After 3 to 4 hours, the build should be done, and you can install openCV with:

  sudo make install
  sudo ldconfig

And that's it! You have successfully built and install openCV 3.1.0, optimized for Raspberry Pi with NEON FPU, and with libjpeg-turbo 1.5.0 with NEON SIMD instructions and NEON FPU support!