---
tags: tutorial, cross_compile
---

# Cross Compile OpenCV using Pi-cross

## Prepare pi-cross

Pull the arch you need from pi-cross. In this example, `aarch64`,
`bullseye` and `g++ 10.2.0` are uesd for rpi 4 bullseye 64 bit os.

docker pull wesleych3n/pi-cross:arm64-bullseye-10.2.0

Create helper script and grant it execution privilege

docker run --rm wesleych3n/pi-cross:arm64-bullseye-10.2.0 > ./arm64-10.2.0
chmod +x ./arm64-10.2.0

Place script to the `$PATH`. For me, `~/.local/bin`

mv ./arm64-10.2.0 ~/.local/bin

## Compiling OpencCV

Goto your working directory and create pi-cross shell.

arm64-10.2.0 bash

Note: The working directory are *MOUNTED* into `/work` inside pi-cross container.

Start Compiling,

Getting source code,

# In pi-cross

# if you need to apt install something change arch first like below
# sudo dpkg --add-architecture arm64
# sudo dpkg --print-foreign-architectures

# get opencv source code
wget -O opencv.zip https://github.com/opencv/opencv/archive/4.6.0.zip
wget -O opencv_contrib.zip https://github.com/opencv/opencv_contrib/archive/4.6.0.zip

unzip opencv.zip
unzip opencv_contrib.zip
mv opencv-4.5.0 opencv
mv opencv_contrib-4.5.0 opencv_contrib

Start Cmake with the option you need. Basically I need static
lib with minimum requirements of image format. Also, I specify the
installation path to `/opt` (default will be `/usr/local/`)

mkdir -p opencv/build/ && cd opencv/build

cmake -D CMAKE_INSTALL_PREFIX=/opt/opencv \
-D OPENCV_EXTRA_MODULES_PATH=../../opencv_contrib/modules \
-D CMAKE_BUILD_TYPE=RELEASE \
-D BUILD_SHARED_LIBS=OFF \
-D BUILD_DOCS=OFF \
-D BUILD_EXAMPLES=OFF \
-D INSTALL_C_EXAMPLES=OFF \
-D BUILD_opencv_apps=OFF \
-D WITH_CAROTENE=OFF \
-D BUILD_opencv_python2=OFF \
-D BUILD_opencv_python3=OFF \
-D BUILD_PERF_TESTS=OFF \
-D BUILD_TESTS=OFF \
-D FORCE_VTK=OFF \
-D WITH_FFMPEG=OFF \
-D WITH_GDAL=OFF \
-D WITH_IPP=OFF \
-D WITH_OPENEXR=OFF \
-D WITH_OPENGL=OFF \
-D WITH_QT=OFF \
-D WITH_TBB=OFF \
-D WITH_XINE=OFF \
-D BUILD_JPEG=ON \
-D BUILD_ZLIB=ON \
-D BUILD_PNG=OFF \
-D BUILD_TIFF=OFF \
-D BUILD_BUILD_JASPER=OFF \
-D WITH_ITT=OFF \
-D WITH_LAPACK=OFF \
-D WITH_OPENCL=OFF \
-D WITH_TIFF=OFF \
-D WITH_PNG=OFF \
-D WITH_OPENCLAMDFFT=OFF \
-D WITH_OPENCLAMDBLAS=OFF \
-D WITH_VA_INTEL=OFF \
-D WITH_WEBP=OFF \
-D WITH_JASPER=OFF \
-D OPENCV_GENERATE_PKGCONFIG=ON ..

# on 32 bit, better to open these two
# -D ENABLE_NEON=ON \
# -D ENABLE_VFPV3=ON \

Make sure cmake indicate the right cross compiler at the final output as below:

...
--     C++ Compiler: /usr/local/aarch64-linux-gnu/bin/aarch64-linux-gnu-g++  (ver 10.2.0)
...

Start compiling. Enjoy the speed from cross-compiling :heart:.

make -j 8
mkdir install_aarch64
make DESTDIR=./install_aarch64 install

Because we don't want to install opencv to the container `/opt`, it will
disappear as you exit pi-cross. So just create a folder inside `/work` and
install in it.

make -j 8
mkdir install_aarch64
make DESTDIR=./install_aarch64 install

The lib and header file are all in `./install_aarch64`.

Exit pi-cross. Finished :confetti_ball: .

## How to use the lib to compile your program by Pi-cross??

The fastest way to show you is the example.

Create following files in working directory.

`main.cc`

#include <iostream>
#include "opencv2/opencv.hpp"

int main(int argc, char *argv[]) {
  cv::Mat img = cv::imread("../test.jpg");
  std::cout << img.size << '\n';
  return 0;
}

`CMakeLists.txt`

cmake_minimum_required(VERSION 3.10)

project(opencv_example)

find_package(PkgConfig REQUIRED)
pkg_check_modules(OPENCV REQUIRED opencv4)
include_directories(${OPENCV_INCLUDE_DIRS})

add_executable(main main.cc)
target_link_libraries(main ${OPENCV_STATIC_LDFLAGS})

Create pi-cross container.
There are two things need to mount into container.
First, `opencv4.pc` is for `pkg-config` to find the right place of opencv
lib. Second of course is to mount opencv lib to `/opt` we set earlier.
The way we mount is to use `-a` to pass docker mount arguments.

arm64-10.2.0 -a \
"-v $PWD/opencv/aarch64/opt/opencv/lib/pkgconfig/opencv4.pc:/usr/lib/pkgconfig/opencv4.pc:ro \
 -v $PWD/opencv/aarch64/opt:/opt/" bash

create build directory and build the executable

mkdir build && cd build
cmake ..
make

then the `main` executable can run on your aarch64 system.