Integrating OpenCV with Unity Using Android NDK: Issues Loading Native Libraries in Unity

Question

I am trying to integrate C++ code into Unity, which should use OpenCV 4.8.0 for Android (OpenCV-android-sdk). To create the .so file, I am using the Android NDK (android-ndk-r26b).

My issue as I suspect, lies in linking the OpenCV library to my code and in the creation of the '.so' file.

Here's what I have tried so far:

• "So far, I have successfully managed to create a .so file from a .cpp file using the Android ndk-build an got in to work in Unity. However, things get complicated when I try to integrate OpenCV into the process."
• I have a setup and an Android.mk file which runs without errors, but I'm not sure if it's functioning correctly. When calling the functions in Unity, I encounter an error: Error Unity at TestCppPlugin.plus (System.Single a, System.Single b) [0x00000] in <00000000000000000000000000000000>:0 and Error Unity at TestCppPlugin.Start () [0x00000] in <00000000000000000000000000000000>:0."

folder structure

├── android-ndk-r26b
│   ├── build
│   ├── ndk-build
│   ├── ...
│   ├── ...
└── cpp-library
    ├── jni
    │   ├── Android.mk
    │   ├── Application.mk
    │   ├── CppToUnity_plugin.cpp
    │   └── CppToUnity_plugin.h
    │    
    ├── libs
    ├── obj
    └── OpenCV-android-sdk

CppToUnity_plgin.cpp

#include "CppToUnity_plugin.h"
#include <opencv2/core.hpp>
#include <opencv2/imgproc/imgproc.hpp>

extern "C" float plus(float x, float y) {
        return x+y;
}

extern "C" const char* hello_from_cpp() {
        return "Hello form C++";
}



extern "C" int test_opencv() {
    // Erstellen Sie eine einfache Matrix (Bild)
    cv::Mat img(100, 100, CV_8UC3, cv::Scalar(0, 255, 0)); 

    // Führen Sie eine einfache Bildoperation durch (konvertieren Sie es zu Graustufen)
    cv::Mat grayImg;
    cv::cvtColor(img, grayImg, cv::COLOR_BGR2GRAY);

    // Überprüfen Sie, ob die Konvertierung erfolgreich war
    if (grayImg.empty()) {
        return -1; // Fehler
    }

    return 0; //Erfolg
}

CppToUnity_plugin.h

extern "C" {
        float plus(float x, float y);
        const char* hello_from_cpp();
        int test_opencv();
}

Android.mk

LOCAL_PATH := $(call my-dir)

CVROOT := /path/to/OpenCV-android-sdk/sdk/native
#CVROOT := $(LOCAL_PATH)/../OpenCV-android-sdk/sdk/native/

include $(CLEAR_VARS)
LOCAL_MODULE := add_prebuilt
LOCAL_SRC_FILES += $(CVROOT)/libs/arm64-v8a/libopencv_java4.so
LOCAL_LDLIBS := -llog
LOCAL_EXPORT_C_INCLUDES := CppToUnity_plugin.h
include $(PREBUILT_SHARED_LIBRARY)

include $(CLEAR_VARS)
LOCAL_C_INCLUDES :=$(LOCAL_PATH) $(CVROOT)/jni/include
LOCAL_MODULE := native
LOCAL_SHARED_LIBRARIES := add_prebuilt
LOCAL_SRC_FILES := CppToUnity_plugin.cpp
include $(BUILD_SHARED_LIBRARY)

TestCppPlugin.cs

using System.Collections;
using System.Collections.Generic;
using UnityEngine;
using System.Runtime.InteropServices;
using System;
using UnityEngine.UI; 

public class TestCppPlugin : MonoBehaviour
{
    public UnityEngine.UI.Text mText; 

    [DllImport("native")]
    private static extern float plus(float a, float b);

    [DllImport("native")]
    private static extern System.IntPtr hello_from_cpp();



    [DllImport("native")]
    private static extern int test_opencv();


    void Start() {
        mText.text = "Testergebnisse:\n";

        // Test plus
        float summe = plus(2.0f, 3.0f);
        Debug.Log("Das Ergebnis von plus_rechnen ist: " + summe);
        mText.text += "Das Ergebnis von plus_rechnen(2.0f, 3.0f) ist: " + summe + "\n"; 

        // Test hello_from_cpp
        System.IntPtr ptr = hello_from_cpp();
        string message = Marshal.PtrToStringAnsi(ptr);
        Debug.Log("Nachricht von C++: " + message);
        mText.text += "Nachricht von C++: " + message + "\n";


        // Test test_opencv
        int test = test_opencv();
        Debug.Log("test_opencv return value: " + test);
        mText.text += "Ergebnis von test_opencv(): " + test + "\n";

  
    }
}

Does anyone have suggestions on how to resolve this issue or what I might be doing wrong?

I've tried many different approaches, but all ended in errors. At least with this current setup, I can see two lib_.so files in the libs directory