我正在尝试将使用 OpenCV 用 C++ 编写的现有计算机视觉代码移植到 Android NDK。我按照此处提供的信息成功导入了适用于 Java 和 NDK 的 OpenCV 库版本 3.4.0（使用官方预构建的 Android 包）：Stack Overflow Answer - Android 上 OpenCV 的 CMake 配置.
我能够使用 Java 和 C++ 中的 OpenCV 功能编译和运行一些代码。但是，我遇到了 2 个与某些 OpenCV 函数相关的“未定义引用”链接错误：持久性 JSON 读取器和特征 2D 描述符匹配器。
以下是我收到的错误消息：
Build command failed.
Error while executing process D:\Librairies\Android_SDK\cmake\3.6.4111459\bin\cmake.exe with arguments {--build D:\Dev\Android\PageDetector\app\.externalNativeBuild\cmake\debug\x86_64 --target page-recognition-lib}
[1/1] Linking CXX shared library ..\..\..\..\build\intermediates\cmake\debug\obj\x86_64\recognition-lib.so
FAILED: cmd.exe /C "cd . && D:\Librairies\Android_SDK\ndk-bundle\toolchains\llvm\prebuilt\windows-x86_64\bin\clang++.exe --target=x86_64-none-linux-android --gcc-toolchain=D:/Librairies/Android_SDK/ndk-bundle/toolchains/x86_64-4.9/prebuilt/windows-x86_64 --sysroot=D:/Librairies/Android_SDK/ndk-bundle/sysroot -fPIC -isystem D:/Librairies/Android_SDK/ndk-bundle/sysroot/usr/include/x86_64-linux-android -D__ANDROID_API__=21 -g -DANDROID -ffunction-sections -funwind-tables -fstack-protector-strong -no-canonical-prefixes -Wa,--noexecstack -Wformat -Werror=format-security -std=c++11 -std=c++11 -frtti -fexceptions -std=gnu++11 -O0 -fno-limit-debug-info -Wl,--exclude-libs,libgcc.a -Wl,--exclude-libs,libatomic.a -nostdlib++ --sysroot D:/Librairies/Android_SDK/ndk-bundle/platforms/android-21/arch-x86_64 -Wl,--build-id -Wl,--warn-shared-textrel -Wl,--fatal-warnings -LD:/Librairies/Android_SDK/ndk-bundle/sources/cxx-stl/llvm-libc++/libs/x86_64 -Wl,--no-undefined -Wl,-z,noexecstack -Qunused-arguments -Wl,-z,relro -Wl,-z,now -shared -Wl,-soname,libpage-recognition-lib.so -o ..\..\..\..\build\intermediates\cmake\debug\obj\x86_64\recognition-lib.so.so [...] -llog -llog ../../../../src/main/jniLibs/x86_64/libopencv_java3.so -latomic -lm "D:/Librairies/Android_SDK/ndk-bundle/sources/cxx-stl/llvm-libc++/libs/x86_64/libc++_static.a" "D:/Librairies/Android_SDK/ndk-bundle/sources/cxx-stl/llvm-libc++/libs/x86_64/libc++abi.a" && cd ."
D:/Librairies/OpenCV-android-sdk_340/sdk/native/jni/include\opencv2/core/persistence.hpp:1264: error: undefined reference to 'cv::read(cv::FileNode const&, std::__ndk1::vector<cv::KeyPoint, std::__ndk1::allocator<cv::KeyPoint> >&)'
D:\Dev\Android\PageDetector\app\src\main\cpp/PageMatcher.cpp:170: error: undefined reference to 'cv::DescriptorMatcher::radiusMatch(cv::_InputArray const&, cv::_InputArray const&, std::__ndk1::vector<std::__ndk1::vector<cv::DMatch, std::__ndk1::allocator<cv::DMatch> >, std::__ndk1::allocator<std::__ndk1::vector<cv::DMatch, std::__ndk1::allocator<cv::DMatch> > > >&, float, cv::_InputArray const&, bool) const'
clang++.exe: error: linker command failed with exit code 1 (use -v to see invocation)
ninja: build stopped: subcommand failed.
以下是编译器无法链接的代码片段：
//code that reads data from a JSON file
this->JSONFeatureFilePath = JSONFeatureFilePath;
cv::FileStorage reader(JSONFeatureFilePath, cv::FileStorage::READ);
this->bookTitle = (string) reader["book_title"];
this->pageNumber = (int) reader["page_num"];
string descType = (string)reader["desc_type"];
replace(descType.begin(), descType.end(), '_', '.');
this->descriptorType = descType;
reader["img_size"] >> this->imageSize;
//this instruction causes the linker error
reader["keypoints"] >> this->keyPoints;
reader["descriptors"] >> this->keyPointDescriptors;
reader["fsum2d"] >> this->fsum2DFeatureSummary;
reader.release();
and
//code performing key point descriptors matching
cv::Ptr<cv::DescriptorMatcher> matcher = cv::DescriptorMatcher::create(cv::DescriptorMatcher::BRUTEFORCE_HAMMING);
vector<vector<cv::DMatch>> matchesTmp;
//instruction responsible for the link error
matcher->radiusMatch(this->sortedMatches.at(refSortedIndex).refImage->getDescriptors(),
this->testImage->getDescriptors(), matchesTmp, matchThreshold);
我已经清楚地识别了导致链接器错误的行，如上面代码示例中的注释。如果我将它们注释掉，编译就会完成并且程序运行良好（当然没有我试图在 NDK 中实现的功能）。
我的猜测是，我调用的 OpenCV 函数在预构建库中缺失，或者与我用于 NDK 开发的编译器不兼容。我已经尝试过更改 OpenCV 版本（3.3.0 和 3.4.0）。
有谁知道什么会导致此问题以及我该如何解决它？这是 OpenCV 中的已知错误还是我的配置不受支持或只是错误的？
我使用的是带有 Android Studio 3.1.2、NDK r17、构建工具 27.0.3 和 OpenCV 3.4.0 预构建 android 包的 Windows 10 计算机（我自己没有从源代码编译它）。以下是 CMake 和 build.gradle 文件：
CMake:
cmake_minimum_required(VERSION 3.4.1)
set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -std=gnu++11")
set(opencv_340_dir D:/Librairies/OpenCV-android-sdk_340/sdk/native/jni)
set(app_dir D:/Dev/Android/PageDetector/app)
# native recognition library API
add_library(recognition-lib
SHARED
src/main/cpp/recognition-lib.h
src/main/cpp/recognition-lib.cpp
# + my classes' h and cpp files
)
# OpenCV lib linking and includes
include_directories(${opencv_340_dir}/include)
add_library(opencv-lib SHARED IMPORTED)
set_target_properties(opencv-lib PROPERTIES IMPORTED_LOCATION ${app_dir}/src/main/jniLibs/${ANDROID_ABI}/libopencv_java3.so)
find_library(log-lib log)
target_link_libraries(
recognition-lib
opencv-lib
${log-lib}
)
target_link_libraries(recognition-lib ${log-lib})
gradle:
apply plugin: 'com.android.application'
android {
compileSdkVersion 27
defaultConfig {
applicationId "com.companyname.detector"
minSdkVersion 21
targetSdkVersion 27
versionCode 1
versionName "1.0"
testInstrumentationRunner "android.support.test.runner.AndroidJUnitRunner"
externalNativeBuild {
cmake {
cppFlags "-std=c++11 -frtti -fexceptions"
}
}
}
buildTypes {
release {
minifyEnabled false
proguardFiles getDefaultProguardFile('proguard-android.txt'), 'proguard-rules.pro'
}
}
externalNativeBuild {
cmake {
path "CMakeLists.txt"
}
}
sourceSets {
main {
jniLibs.srcDirs = ['src/main/jniLibs/']
}
}
}
dependencies {
implementation fileTree(include: ['*.jar'], dir: 'libs')
implementation 'com.android.support:appcompat-v7:27.1.1'
implementation 'com.android.support.constraint:constraint-layout:1.1.0'
testImplementation 'junit:junit:4.12'
androidTestImplementation 'com.android.support.test:runner:1.0.2'
androidTestImplementation 'com.android.support.test.espresso:espresso-core:3.0.2'
implementation project(':openCVLibrary340')
}
