ndk-build giving me "error: undefined reference" to cv::CascadeClassifier::load(std::string const&) even after I link the libraries

Question

I am using openCV in my Android native program with C++.

My ndk-build version is 14 and I am using Android Studio 3.3 RC3 and OpenCV 4.0.

While I was trying to add a simple face detection program for camera and try to build it with ndk-build, it gives me the following error:

error: undefined reference to 'cv::CascadeClassifier::load(std::string const&)'

Here are some of my code:

void *frameRetriever(void *p){
 ...

 const std::string face_cascade_name = "/data/local/tmp/haarcascade_frontalface_alt.xml";
 cv::CascadeClassifier face_cascade;
 face_cascade.load(face_cascade_name);
 ...
}

I have tried using

cv::CascadeClassifier face_cascade = cv::CascadeClassifier(face_cascade_name)

instead of the load function, or even try

void loadCascade(std::string filepath){

  cv::CascadeClassifier face_cascade;
  if ( !face_cascade.load(filepath) ){
    LOGE("--Error Loading face cascade\n");
  }

}

these still gives me the undefined reference to error on ndk-build. (The file in face_cascade_name is rwx for all users)

I have linked all the libraries in my Android.mk file:

...

LOCAL_LDLIBS += -L$(OPENCV_ANDROID_SDK)/libs/armeabi-v7a/
LOCAL_LDLIBS += -lopencv_core
LOCAL_LDLIBS += -lopencv_calib3d
LOCAL_LDLIBS += -lopencv_imgproc
LOCAL_LDLIBS += -lopencv_objdetect
LOCAL_LDLIBS += -lopencv_features2d
LOCAL_LDLIBS += -lopencv_flann
LOCAL_LDLIBS += -lopencv_stitching
LOCAL_LDLIBS += -lopencv_video
LOCAL_LDLIBS += -lopencv_dnn
LOCAL_LDLIBS += -lopencv_highgui
LOCAL_LDLIBS += -lopencv_imgcodecs
LOCAL_LDLIBS += -lopencv_ml
LOCAL_LDLIBS += -lopencv_photo
LOCAL_LDLIBS += -lopencv_videoio

...

and I do have all these OpenCV libraries as .so file under <myProject>/app/src/main/libs/armeabi-v7a/

As for building the OpenCV libraries, I use the following bash script:

#!/bin/bash

if [ ! -d "./android_build/" ]; then
  mkdir android_build
fi
if [ ! -d "./build/" ]; then
  mkdir build
fi

rm -rf android_build/out/

cd build

cmake .. \
    -DCMAKE_TOOLCHAIN_FILE=../platforms/android/android.toolchain.cmake \
    -DCMAKE_BUILD=Release \
    -DBUILD_DOCS=OFF \
    -DBUILD_JAVA=OFF \
    -DBUILD_ANDROID_EXAMPLES=OFF \
    -DBUILD_ANDROID_PROJECTS=OFF \
    -DBUILD_SHARED_LIBS=ON \
    -DANDROID_NDK=${ANDROID_NDK} \
    -DANDROID_NATIVE_API_LEVEL=android-19 \
    -DANDROID_SDK_TARGET=19 \
    -DANDROID_ABI="armeabi-v7a with NEON" \
    -DANDROID_TOOLCHAIN=clang \
    -DANDROID_STL=c++_shared \
    -DCV_TRACE=OFF \
    -DWITH_IPP=OFF \
    -DCMAKE_TOOLCHAIN_FILE=${ANDROID_NDK}/build/cmake/android.toolchain.cmake \
    -DBUILD_opencv_core=ON \
    -DBUILD_opencv_imgproc=ON \
    -DBUILD_opencv_calib3d=ON \
    -DBUILD_opencv_imgcodecs=ON \
    -DBUILD_opencv_dnn=ON \
    -DBUILD_opencv_flann=ON \
    -DBUILD_opencv_ml=ON \
    -DBUILD_opencv_objdetect=ON \
    -DBUILD_opencv_photo=ON \
    -DBUILD_opencv_stiching=ON \
    -DBUILD_opencv_videoio=ON \
    -DBUILD_opencv_highgui=ON \
    -DCMAKE_INSTALL_PREFIX:PATH=$(pwd)/../android_build/out \
    -DWITH_CUDA=OFF \
    -DWITH_TBB=OFF \
    -DHAVE_OPENCL=ON \
    -DOPENCL_FOUND=ON \
    -DWITH_OPENCL=ON \
    -DHAVE_OPENCL_STATIC=ON \
    -DOPENCL_LIBRARY=$(pwd)/../libopencl-stub/libOpenCL.so \
    -DOPENCL_INCLUDE_DIRS=$(pwd)/../libopencl-stub/include \

cmake -LA
make -j4
make install

After testing out few things, I noticed that other OpenCV functions that has "const String &" in its parameter gives me the same error on ndk-build. (other OpenCV functions such as cvtColor works perfectly).

Any help would be appreciated.

Answer 1

Apparently, changing some values in makefile fixes the problem.

In Application.mk, chaning APP_STL := gnustl_shared to c++_shared and comment out NDK_TOOLCHAIN_VERSION. In Android.mk, comment out some LOCAL_CPPFLAG.

Built it and it solved the problem.