imread works in a normal c++ program, but not in android with ndk native

Question

#include <jni.h>
#include <string>

#include <opencv2/opencv.hpp>
using namespace cv;

#include <iostream>

extern "C" JNIEXPORT jstring JNICALL
Java_com_example_cppinandroid_MainActivity_stringFromJNI( JNIEnv* env,
                            jobject)
{
    Mat image;
    image = imread( "/home/<name>/a.png", 1 );

    if ( !image.data )
    {
        return env->NewStringUTF( "No data in image!" );
    } else
    {
        return env->NewStringUTF( "Data is in image" );
    }
}

That's what I have in Native Cpp part of the android project.
When I run the application with emulator, it always shows: "No data in image!" text.

I have compiled this program separately as a normal c++ opencv program. It is working perfectly fine.
I have given the absolute path too. Is that not enough?

Do I have to write something in build.gradle also?

I am using Android studio, if that matters.

Neargye · Accepted Answer · 2020-05-11T11:48:22.517

Android directories do not match Linux at all.

A good solution is to find your file through Java and pass the path to the file as a parameter to function Java_com_example_cppinandroid_MainActivity_stringFromJNI.

If your file is in public files, you can try "/sdcard/a.png" - but the path may vary between different android shells.

The possible solution from reading from public files:

Java:

File f = new File(Environment.getExternalStorageDirectory().getAbsolutePath()+ File.separator + fileName);
if (f.exists()) {
foo(f.getAbsolutePath());

NDK:

extern "C" JNIEXPORT jstring JNICALL
Java_com_example_cppinandroid_MainActivity_stringFromJNI(JNIEnv* env,
                            jobject, jstring javaPath)
{
    const char* path = env->GetStringUTFChars(javaPath, 0);
    Mat image;
    image = imread( path, 1 );
    env->ReleaseStringUTFChars(javaPath, path );
    if ( !image.data )
    {
        return env->NewStringUTF( "No data in image!" );
    } else
    {
        return env->NewStringUTF( "Data is in image" );
    }
}

A better way to store a picture in assets and read AAssetManager like Android read text file from asset folder using C (ndk) or How to properly pass an asset FileDescriptor to FFmpeg using JNI in Android.

Thank you. So, you mean to say that we just cannot fetch the file directly by giving absolute address in C++ in Android. There isn't a way to do it? — Aquarius_Girl, May 09 '20 at 15:58
You can get the absolute path to public files through Environment.getExternalStorageDirectory().getAbsolutePath(). This method is easier to debug. — Neargye, May 11 '20 at 11:49

imread works in a normal c++ program, but not in android with ndk native

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