Android Opencv 交叉编译快速入门

opencv version: 3.4.3
NDK: 25.2
ubnutu: 20.04
ps: opencv 的版本最好是opencv3，我一开始使用opencv4.6.0的时候编译时候会报错

1 安装NDK

从下面的链接下载NDK，并解压：
https://developer.android.google.cn/ndk/downloads/
记住解药后的路径

如果你是在Android Studio里面下载的ndk，那么安装路径可以参加下面

/home/peter/Android/Sdk/ndk/25.2.9519653

2 安装Opencv

https://github.com/opencv/opencv/releases?page=4
下载压缩包，比如： opencv-3.4.3.zip
并解压到当前目录

3 交叉编译Opencv

cd opencv-3.4.3/

rm -rf build
mkdir build && cd build

export ANDROID_NDK=/home/peter/Android/Sdk/ndk/25.2.9519653

sudo cmake -DCMAKE_TOOLCHAIN_FILE=$ANDROID_NDK/build/cmake/android.toolchain.cmake \
-DCMAKE_ANDROID_NDK=$ANDROID_NDK \
-DANDROID_NATIVE_API_LEVEL=26 \
-DBUILD_ANDROID_PROJECTS=OFF \
-DBUILD_ANDROID_EXAMPLES=OFF \
-DCMAKE_BUILD_TYPE=Release  \
-DBUILD_JAVA=OFF  \
-DANDROID_ABI=arm64-v8a \
-DCMAKE_INSTALL_PREFIX=/home/temp/programs/opencv343_android ..

sudo make -j8

sudo make install

这里编译的是arm64的，其他可修改编译选项。
安装目录是 /home/temp/programs/opencv343_android ，可修改。

编译好之后，库文件在安装目录 /sdk/native 下
上面默认是生成静态库。
如果要生成动态库，加上以下cmake选项

-DANDROID_STL=c++_shared \
-DBUILD_SHARED_LIBS=ON \

4 交叉编译Android下的OpenCV demo

写一个使把图像test.jpg模糊成result.jpg的例子

4.1 源文件 boxfilter_arm.cpp

#include "opencv2/opencv.hpp"

int main()
{
    //read picture
    cv::Mat img = cv::imread("test.jpg");
    cv::Mat out;
    cv::boxFilter(img, out, -1, cv::Size(5, 5));
    cv::imwrite("result.jpg", out);

    return 0;
}

4.2 CMakeLists.txt

cmake_minimum_required(VERSION 2.8.3)
project(boxfilter_arm)

add_definitions("-Wall -g")

set(OpenCVHome /home/temp/programs/opencv343_android/sdk/native)

include_directories(${OpenCVHome}/jni/include)
link_directories(${OpenCVHome}/staticlibs/arm64-v8a
                 ${OpenCVHome}/3rdparty/libs/arm64-v8a)

link_libraries(
        opencv_imgcodecs
        opencv_imgproc
        opencv_highgui
        opencv_core
        opencv_video
        opencv_features2d
        opencv_videoio
        cpufeatures
        tegra_hal
        IlmImf
        libjasper
        libjpeg-turbo
        libpng
        libprotobuf
        libtiff
        libwebp
        # quirc
        log
        z
    )

add_executable(${PROJECT_NAME} boxfilter_arm.cpp)

install(TARGETS ${PROJECT_NAME}
  RUNTIME DESTINATION ${PROJECT_SOURCE_DIR})

4.3 cmake 编译


export ANDROID_NDK=/home/peter/Android/Sdk/ndk/25.2.9519653

rm -r build
mkdir build && cd build 

sudo cmake -DCMAKE_TOOLCHAIN_FILE=$ANDROID_NDK/build/cmake/android.toolchain.cmake \
    -DANDROID_ABI="arm64-v8a" \
    -DANDROID_NDK=$ANDROID_NDK \
    -DANDROID_PLATFORM=android-26 \
    ..

sudo make 
sudo make install

cd ..

5 放到Android 上执行

5.1 安装adb工具

sudo apt install android-tools-adb android-tools-fastboot

5.2 push 到手机上

adb push boxfilter_arm /data/local/tmp
adb push test.jpg /data/local/tmp

5.3 给boxfilter_arm执行权限

adb shell
cd /data/local/tmp
chmod +x boxfilter_arm

5.4 执行boxfilter_arm

./boxfilter_arm

如果没有报错，就会在当前目录下生成result.jpg
通过下面的命令取出来

adb pull /data/local/tmp/result.jpg .

6 注意事项

armv7和arm64-v8a的差别

差别在CMAKE_ANDROID_ARCH_ABI和ANDROID_ABI， CMAKE_ANDROID_ARCH_ABI有没有有用不知道，但是arm64-v8a要用-DANDROID_ABI=arm64-v8a，否则生成默认的armv7的

armv7的cmake选项如下：

cmake -DCMAKE_TOOLCHAIN_FILE=$ANDROID_NDK/build/cmake/android.toolchain.cmake \
-DCMAKE_ANDROID_NDK=$ANDROID_NDK \
-DANDROID_NATIVE_API_LEVEL=26 \
-DBUILD_ANDROID_PROJECTS=OFF \
-DBUILD_ANDROID_EXAMPLES=OFF \
-DCMAKE_BUILD_TYPE=Release  \
-DBUILD_JAVA=OFF  \
-DCMAKE_ANDROID_ARCH_ABI=armeabi-v7a \
-DCMAKE_INSTALL_PREFIX=/home/temp/programs/opencv343_android ..

arm64-v8a的cmake选项则如下：
要设置ANDROID_ABI=arm64-v8a，而不是ANDROID_ARCH_ABI=arm64-v8a

cmake -DCMAKE_TOOLCHAIN_FILE=$ANDROID_NDK/build/cmake/android.toolchain.cmake \
-DCMAKE_ANDROID_NDK=$ANDROID_NDK \
-DANDROID_NATIVE_API_LEVEL=26 \
-DBUILD_ANDROID_PROJECTS=OFF \
-DBUILD_ANDROID_EXAMPLES=OFF \
-DCMAKE_BUILD_TYPE=Release  \
-DBUILD_JAVA=OFF  \
-DANDROID_ABI=arm64-v8a \
-DCMAKE_INSTALL_PREFIX=/home/temp/programs/opencv343_android ..

7. 参考

https://www.jianshu.com/p/78875d220dbf
https://www.sisik.eu/blog/android/ndk/opencv-without-java
https://juejin.im/post/6844903793650302989
https://www.jianshu.com/p/925614d6e9e0