Setup CMakeLists.txt for Android Studio and OpenCV C++ coding

If you want to develop C++ code making use of Android and OpenCV classes, then you have to configure Android Studio's CMakeLists.txt build file accordingly.

Please follow the previous post https://dominoc925.blogspot.com/2018/10/how-i-setup-opencv4android-java-sdk.html to download and setup the OpenCV4Android Java SDK with and Android project before doing the following steps.

1. In Android Studio, open up the app's build.gradle file.
   
   Note: the OpenCV native libraries are under the jniLibs directory relative to the app's build.gradle file according to the source set jniLibs srcDirs parameter.
    
   
   
2. Enter the cmake cpp flags and platform abi filters  in the android defaultConfig externalNativeBuild section, as shown below.
   
   

   externalNativeBuild {
       cmake {
         cppFlags "-frtti -fexceptions"        
   
   abiFilters 'x86', 'x86_64', 'armeabi-v7a', 'arm64-v8a'    
   
   
   }
   }

   
   
3. Next, open up the CMakeLists.txt file in the editor.
   
    
4. Add in the cmake macros to add in the path to the OpenCV C++ include files e.g. /path/to/OpenCV-android-sdk/sdk/native/jni/include/
   
    
   
5.  Then, use the macro add_library to add the OpenCV library.
   
   
6. Next, use the macro set_target_properties to tell CMake the location of the OpenCV library.
   
   

   set_target_properties(
        lib_opencv        
   
   PROPERTIES        
   
   IMPORTED_LOCATION        
   
   ${CMAKE_CURRENT_SOURCE_DIR}/jniLibs/${ANDROID_ABI}/libopencv_java3.so
   
   )
   

   
   
   
7. Finally, link the OpenCV library with your code with the target_link_libraries macro.
   
   

The full sample of the CMakeLists.txt file is shown below.

# For more information about using CMake with Android Studio, read the
# documentation: https://d.android.com/studio/projects/add-native-code.html

# Sets the minimum version of CMake required to build the native library.

cmake_minimum_required(VERSION 3.4.1)

set (OPENCV4ANDSDK_INCLUDE
        /path/to/OpenCV-android-sdk/sdk/native/jni/include)
include_directories(
    ${OPENCV4ANDSDK_INCLUDE}
)
# Creates and names a library, sets it as either STATIC
# or SHARED, and provides the relative paths to its source code.
# You can define multiple libraries, and CMake builds them for you.
# Gradle automatically packages shared libraries with your APK.

add_library( # Sets the name of the library.
        native-lib

        # Sets the library as a shared library.
        SHARED

        # Provides a relative path to your source file(s).
        src/main/cpp/native-lib.cpp)
add_library(
        lib_opencv
        SHARED
        IMPORTED
)
set_target_properties(
        lib_opencv
        PROPERTIES
        IMPORTED_LOCATION
        ${CMAKE_CURRENT_SOURCE_DIR}/jniLibs/${ANDROID_ABI}/libopencv_java3.so
)

# Searches for a specified prebuilt library and stores the path as a
# variable. Because CMake includes system libraries in the search path by
# default, you only need to specify the name of the public NDK library
# you want to add. CMake verifies that the library exists before
# completing its build.

find_library( # Sets the name of the path variable.
        log-lib

        # Specifies the name of the NDK library that
        # you want CMake to locate.
        log)


# Specifies libraries CMake should link to your target library. You
# can link multiple libraries, such as libraries you define in this
# build script, prebuilt third-party libraries, or system libraries.

target_link_libraries( # Specifies the target library.
        native-lib

        # Links the target library to the log library
        # included in the NDK.
        ${log-lib}
        lib_opencv
        )