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I am trying to use openCV in my android project and trying to run this native code but I don't know how to use this parameter

JNIEXPORT jint JNICALL Java_com_example_helloopencvactivity_nativecalls_filepath
    (JNIEnv * env, jobject jo, jstring str1, jstring str2) {
    cv::Mat img1 = cv::imread("");
}

I tried using this

const char *nativeString = (*env)->GetStringUTFChars(env, str1, 0);
cv::Mat img1 = cv::imread(nativeString);

but i am getting this error error: no matching function for call to '_JNIEnv::GetStringUTFChars

I need to pass the file path from android file system to openCV's native code for processing, the passing element is string and should be read by imread

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first in the java side, my codes looks like：

 private String path="/mnt/sdcard/";

 InitFeature(width,height,path);

 public native void InitFeature(int width, int height,String path);

then in the jni side, it's:

//passed from the java part and release after InitFreature
const char* nPath;
//to save the globle path
char* g_path;

JNIEXPORT void JNICALL Java_org_opencv_samples_tutorial6_Tutorial2Activity_InitFeature(JNIEnv* env, jobject,jint width,jint height,jstring path);

JNIEXPORT void JNICALL Java_org_opencv_samples_tutorial6_Tutorial2Activity_InitFeature(JNIEnv* env, jobject,jint width,jint height,jstring path)
{

//jsize path_size;
nPath=env->GetStringUTFChars(path,NULL);
//path_size=env->GetArrayLength(path);

LOGD("path: %s \n",nPath);

int length=strlen(nPath);

LOGD("length: %d \n",length);

g_path=new char[length+1];

memcpy(g_path,nPath,length+1);

LOGD("path_2: %s \n",g_path);
LOGD("length: %d \n",strlen(g_path));

char l_path[128];

strcpy(l_path,g_path);
strcat(l_path,"color.jpg");

LOGD("path_3: %s \n",l_path);
LOGD("length: %d \n",sizeof(l_path));

m_width=width;
m_height=height;

center.x=float (m_width/2.0+0.5);//float (Img_tmp->width/2.0+0.5);
center.y=float (m_width/2.0+0.5);//float (Img_tmp->height/2.0+0.5);

    env->ReleaseStringUTFChars(path,nPath);

}

since I have different native calls, one to initiate features(shown here) and others to process every frame, and after you env->ReleaseStringUTFChars(path,nPath); the string would be invisible to jni part. I have the copy the string to a global char* g_path;

and a little sample is here as well, the file path is "/mnt/sdcard/color.jpg" and check those logs.

then you can use imread() to get this jpg. I use other libs like libjpg, so I am not showing the codes here.

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1 Comment

I am trying to write cv::imwrite(jenv->GetStringUTFChars(file_Path_Android, NULL),Mat_File) but always get false ? how to write a Mat to android's cache directory from jni code ??

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