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OpenCV and C++: "Can't resolve variable 'Mat`"

I'm trying to integrate OpenCV in Android Java native interface using C++ language. I have placed the OpenCV inside the jni folder, where I'm writing my C++ code. I have included the Opencv header files in my HelloNative.c file. But I'm still getting this error while trying to access Mat object.
"Can't resolve variable Mat".
I have tried using namespace cv, but it gives an error to predeclare using and namespace; which is not a solution. I'm posting my code below, please somebody have a look at it and let me know what I'm doing wrong.
#include <string.h>
#include <jni.h>
#include <stdio.h>
#include <stdlib.h>
#include <android/asset_manager.h>
#include <android/asset_manager_jni.h>
#include <android/log.h>
#include <malloc.h>
#include <sys/stat.h>
#include <sys/types.h>
#include <OpenCV/include/opencv2/opencv.hpp>
#include <OpenCV/include/opencv/cv.h>
#include <OpenCV/modules/core/include/opencv2/core.hpp>
#include <OpenCV/modules/core/include/opencv2/core/core_c.h>
#include <OpenCV/modules/core/include/opencv2/core/mat.hpp>
#include <OpenCV/modules/imgproc/include/opencv2/imgproc/imgproc.hpp>
#include <OpenCV/modules/imgproc/include/opencv2/imgproc/imgproc_c.h>
#include <OpenCV/modules/legacy/include/opencv2/legacy/legacy.hpp>
#include <OpenCV/modules/videostab/include/opencv2/videostab/motion_stabilizing.hpp>
#include <OpenCV/modules/videostab/include/opencv2/videostab/global_motion.hpp>
JNIEXPORT jint JNICALL
Java_com_example_soimporttest_MainActivity_getStringFromJNI(JNIEnv *env, jobject thisObj,
jstring imagepath) {
const char *jnamestr = (*env)->GetStringUTFChars(env, imagepath, 0);
int width, height, gray;
FILE *fp = fopen(jnamestr, "rb");
if (fp) {
//Read .pgm file
int scan = fscanf(fp, "P5%d%d%d", &width, &height, &gray);
if (scan != 3 || &gray > 256 || &width > 0xfffff || &height > 0xfffff || &gray <= 1 ||
&width < 32 || &height < 32) {
fclose(fp);
return 9;
} else {
return 0;
}
} else {
return -1;
}
}
JNIEXPORT void JNICALL
Java_com_example_soimporttest_MainActivity_displayImage(JNIEnv *env, jobject thisObj, jclass type, jlong inpAddr, jlong outAddr) {
::Mat &src = *(Mat*)inpAddr;
Mat &dst = *(Mat*)outAddr;
applyFilter(src , dst);
}
There are two functions in this file, the first function getStringFromJNI is working perfectly fine.
Thank you.

After a lot of research, I finally managed to find out a good solution for my question. Please follow below mentioned tutorial:
https://github.com/leadrien/opencv_native_androidstudio
You'll be facing some issues while integrating it. You can post those solutions in the comments and I will guide you through each.
Thanks.

Error base operand of '->' has non-pointer type 'JNIEnv {aka _JNIEnv}' when create NDK project

I create my first NDK project, that show some text on screen.
I use native method in class NativeLib in java and implement in a class in C.
But I received an error base operand of '->' has non-pointer type 'JNIEnv {aka _JNIEnv}
#include <jni.h>
#include <string.h>
#include <stdio.h>
#include <android/log.h>
#define DEBUG_TAG "MY_NDK_DEMO"
JNIEXPORT jstring JNICALL Java_com_example_helloworld_NativeLib_helloWorld
(JNIEnv * env, jobject obj) {
return (*env)->NewStringUTF("Hello World JNI!");
}

try return env->NewStringUTF("Hello World JNI!");

c++ is different from c
so to make it work
use
env->NewStringUTF("Hy");

How to turn on/off native log cat in ndk at runtime

I use this snippet code to turn log on or off
#define DEBUG 1
#if DEBUG
#include <android/log.h>
#define LOG_TAG "native_log"
#define LOGD(...) __android_log_print(ANDROID_LOG_DEBUG,LOG_TAG,__VA_ARGS__)
#else
# define LOGD(...) do {} while (0) // do nothing
#endif
// use it
LOGD("%s : %d","value", val);
It worked fine by turn DEBUG flag on/off. The problem is I want to do it at run time in java side. What I want like this:
// java
private native void nativeSetDebug(boolean flag);
// jni
JNIEXPORT void JNICALL Java_com_my_package_Native_nativeSetDebug(JNIEnv *env, jobject thiz, jboolean flag){
// what should I do in this method?
}
Since the macros in c++ are replaced by the preprocessor by their value before source file even compiles, so I'm looking for another approach. Is there any ideas?

macro file
extern bool useDebug;
#include <android/log.h>
#define LOG_TAG "native_log"
#define LOGD(...) if(useDebug){__android_log_print(ANDROID_LOG_DEBUG,LOG_TAG,__VA_ARGS__)}
C file
bool useDebug = true;
JNIEXPORT void JNICALL Java_com_my_package_Native_nativeSetDebug(JNIEnv *env, jobject thiz, jboolean flag){
useDebug = flag;
}
The extern is important, otherwise each file including the header will define its own variable and they won't be set correctly.

"Not declared in this scope" error in Android JNI C++

I have this JNI code that uses C++.
#include <eigenfaces_jni.h>
#include <opencv2/core/core.hpp>
#include <opencv2/contrib/contrib.hpp>
#include <opencv2/highgui/highgui.hpp>
#include <string>
#include <vector>
#include <iostream>
#include <fstream>
#include <sstream>
using namespace std;
using namespace cv;
JNIEXPORT jstring JNICALL Java_com_example_facedetector_FaceTracker_nativeCreateObject
(JNIEnv *env, jclass, jstring JFileDir){
// These vectors hold the images and corresponding labels.
vector<Mat> images;
const char *str = env->GetStringUTFChars(jFileDir, NULL);
const char *buf;
/*Releasing the Java String once you have got the C string
is very important!!!*/
env->ReleaseStringUTFChars(jFileDir, str);
Mat im = imread(jFileDir);
if (im.empty())
{
cout << "Cannot load image!" << endl;
buf = "Can load image";
}
else{
buf = "Cannot load image";
}
return env->NewStringUTF("Hello");
}
However, there is always an error at
const char *str = env->GetStringUTFChars(jFileDir, NULL);
which states that
- Symbol 'jFileDir' could not be resolved
- 'jFileDir' was not declared in this scope
- Invalid arguments ' Candidates are: const char * GetStringUTFChars(_jstring *, unsigned
char *) '
Tried the solution in this link Type 'std::string' could not be resolved.
Still not working. Any help to solve this problem would be appreciated. Thank you.

You've declared a function parameter called JFileDir, but nothing called jFileDir. C++ is case-sensitive.

Well, since I don't have enough reputation points to commend, I have to write this as an answer. Check your function parameter list (JNIEnv *env, jclass, jstring JFileDir), what is jclass about, I feel something is missing here.

Problems with JNI and C++

I did an Opencv's application en windows and now I am using JNI to convert this code to Android but I am having some problems.
In concrete my native code not do nothing.
This is my Java class where I define my native methods:
package com.example.telo3;
import org.opencv.core.Mat;
public class Process {
static {
System.loadLibrary("nativo");
}
public Process(){
dir=inicializar_nativo();
}
public void Procesar(Mat framedetect, Mat framedraw){
procesar_nativo(dir,framedetect.getNativeObjAddr(),framedraw.getNativeObjAddr());
}
private long dir;
private static native long inicializar_nativo();
private static native void procesar_nativo(long thiz, long framedetect, long framedraw);
}
This is my JNI code:
#include "nativo.h"
#include <opencv2/objdetect/objdetect.hpp>
#include <opencv2/highgui/highgui.hpp>
#include <opencv2/imgproc/imgproc.hpp>
#include "opencv2/video/tracking.hpp"
#include <iostream>
#include <stdio.h>
#include "FaceDetector.h"
#include "Draw.h"
#include "Almacena.h"
#include "Runnable.h"
using namespace std;
using namespace cv;
#include <android/log.h>
#define LOG_TAG "NATIVO"
#define LOGD(...) ((void)__android_log_print(ANDROID_LOG_DEBUG, LOG_TAG, __VA_ARGS__))
struct variables {
Almacena almacena;
Draw draw;
FaceDetector face_detector;
};
JNIEXPORT jlong JNICALL Java_com_example_telo3_Process_inicializar_1nativo(
JNIEnv *, jobject) {
long dir = (long) new variables();
return (dir);
}
JNIEXPORT void JNICALL Java_com_example_telo3_Process_procesar_1nativo(JNIEnv *,
jobject, jlong dir, jlong framedetect, jlong framedraw) {
Mat* telo =(Mat*)framedetect;
Mat* telo2= (Mat*)framedraw;
((variables*)dir)->almacena = ((variables*)dir)->face_detector.Detect(*telo);
//almacena = face_detector.Detect(frame_gray);
((variables*)dir)->draw.Dibujar(*telo2,((variables*)dir)->almacena);
//frame_capturado = draw.Dibujar(frame_capturado, almacena);
if( (((variables*)dir)->almacena.get_faces()).size() ==0){
LOGD("no detecto caras");
}
}
I think that I use the Jni correctly but the function Detect not works correctly because when I uses this if return 0.

Looks like a simple typo for me.
Your package is "com.example.telo3"
Your class is "Process"
Your function is "inicializar_nativo()"
So the c++ function has to be declared as
JNIEXPORT jlong JNICALL Java_com_example_telo3_Process_inicializar_nativo(...)
You have declared it as
JNIEXPORT jlong JNICALL Java_com_example_telo3_Process_inicializar_1nativo(...)
Did you get an unsatisfied link exception or such?