OpenCV - undefined reference to 'cv::CascadeClassifier::detectMultiScale()' after NDK update

Question

Yesterday I updated my Android Studio included NDK to version 17.0.4754217and since then I can't run my app anymore. When I tried to rerun the code after the update it gave me the error ABIs [mips64, armeabi, mips] are not supported for platform. Supported ABIs are [armeabi-v7a, arm64-v8a, x86, x86_64] so I excluded them from the project in my app.gradle file the following way: abiFilters 'x86', 'x86_64', /*'armeabi',*/ 'armeabi-v7a', 'arm64-v8a'/*, 'mips', 'mips64'*/.

However, since then I'm having a problem with the C++-file where I use the OpenCV-function cv::CascadeClassifier::detectMultiScale.

It always displays the error: CMakeFiles/native-lib.dir/src/main/cpp/native-lib.cpp.o: In function detectAndDisplay(cv::Mat, double, int, std::__ndk1::basic_string<char, std::__ndk1::char_traits<char>, std::__ndk1::allocator<char> >, cv::CascadeClassifier&)': D:\Schule\OpenCV\ARcpp\app\src\main\cpp/native-lib.cpp:158: undefined reference to cv::CascadeClassifier::detectMultiScale(cv::_InputArray const&, std::__ndk1::vector<cv::Rect_<int>, std::__ndk1::allocator<cv::Rect_<int> > >&, double, int, int, cv::Size_<int>, cv::Size_<int>)'.

I call the function like this: cascadeClassifier.detectMultiScale(frame_gray, sights, scaleFactor, minNeighbours, 0 | CV_HAAR_SCALE_IMAGE, Size(30, 30));. The rest of the code is pretty much like shown in the OpenCV-tutorial https://docs.opencv.org/2.4/doc/tutorials/objdetect/cascade_classifier/cascade_classifier.html.

Additionally to the NDK I use CMake and LLDB and my included OpenCV-library is openCVLibrary320. Again, all of this worked until I downloaded the mentioned NDK update.

The rest of the error that always appears on building or executing the app is: Build command failed. Error while executing process C:\Users\chris\AppData\Local\Android\Sdk\cmake\3.6.4111459\bin\cmake.exe with arguments {--build D:\Schule\OpenCV\ARcpp\app\.externalNativeBuild\cmake\debug\arm64-v8a --target native-lib} [1/1] Linking CXX shared library ..\..\..\..\build\intermediates\cmake\debug\obj\arm64-v8a\libnative-lib.so FAILED: cmd.exe /C "cd . && C:\Users\chris\AppData\Local\Android\Sdk\ndk-bundle\toolchains\llvm\prebuilt\windows-x86_64\bin\clang++.exe --target=aarch64-none-linux-android --gcc-toolchain=C:/Users/chris/AppData/Local/Android/Sdk/ndk-bundle/toolchains/aarch64-linux-android-4.9/prebuilt/windows-x86_64 --sysroot=C:/Users/chris/AppData/Local/Android/Sdk/ndk-bundle/sysroot -fPIC -isystem C:/Users/chris/AppData/Local/Android/Sdk/ndk-bundle/sysroot/usr/include/aarch64-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 -O0 -fno-limit-debug-info -Wl,--exclude-libs,libgcc.a -Wl,--exclude-libs,libatomic.a -nostdlib++ --sysroot C:/Users/chris/AppData/Local/Android/Sdk/ndk-bundle/platforms/android-21/arch-arm64 -Wl,--build-id -Wl,--warn-shared-textrel -Wl,--fatal-warnings -LC:/Users/chris/AppData/Local/Android/Sdk/ndk-bundle/sources/cxx-stl/llvm-libc++/libs/arm64-v8a -Wl,--no-undefined -Wl,-z,noexecstack -Qunused-arguments -Wl,-z,relro -Wl,-z,now -shared -Wl,-soname,libnative-lib.so -o ..\..\..\..\build\intermediates\cmake\debug\obj\arm64-v8a\libnative-lib.so CMakeFiles/native-lib.dir/src/main/cpp/native-lib.cpp.o -llog ../../../../src/main/jniLibs/arm64-v8a/libopencv_java3.so -latomic -lm "C:/Users/chris/AppData/Local/Android/Sdk/ndk-bundle/sources/cxx-stl/llvm-libc++/libs/arm64-v8a/libc++_static.a" "C:/Users/chris/AppData/Local/Android/Sdk/ndk-bundle/sources/cxx-stl/llvm-libc++/libs/arm64-v8a/libc++abi.a" && cd ." clang++.exe: error: linker command failed with exit code 1 (use -v to see invocation) ninja: build stopped: subcommand failed.

I`m really lost, hopefully someone knows an answer.

Answer 1

OpenCV is built with ANDROID_STL=gnustl_static. After upgrade, your NDK uses the default libc++ instead. You can set ANDROID_STL explicitly in your app/build.gradle:

android { defaultConfig { externalNativeBuild { cmake {
    arguments '-DANDROID_STL=gnustl_static'
} } } }

(see an example here).

Answer 2

When linkin opencv with your project executables, you always have to link with the general library -lopencv_core. But some packages require additional link flags. For example, if you use highgui as in

#include <opencv2/highgui/highgui.hpp>

you must add opencv_highgui link flag for : -lopencv_highgui.

In you case, CascadeClassifiers are defined in

#include "opencv2/objdetect.hpp"

and thus requires a link with opencv_objdetect -lopencv_objdetect.

The solution is to add the link flag -lopencv_objdetect when compiling.