有读者问到 “”“从安卓模拟器路径用opencv读取图像失败”，您这个问题解决了吗？”

我刚开始将c++算法嵌入到安卓或java的时候也遇到过这个基本的问题，因为两者机制不一样，很容易搞混。

在安卓中是不能直接用opencv的imread函数的，必须以安卓自己的方式读图，然后将buffer传递给接口函数。我的算法接口用了jni封装，下面做个一简单的例子进行示例说明，希望对像我这样的安卓初学者有点帮助。

java读图并转换到字节数组中

方式一： 从res文件夹读取(传入c++接口有问题)

/*

* test_image为图像的名称， 测试图像放在app\src\main\res\drawable文件夹下

*/

Bitmap bitmap = BitmapFactory.decodeResource(getResources(), R.drawable.test_image);

int height = bitmap.getHeight();

int width = bitmap.getWidth();

byte[] data = bitmap.getNinePatchChunk();

进入c++函数后，测试一下data数组是否正确，不行的话就用下面的转换一下（将Bitmap对象读到字节数组中）：

ByteArrayOutputStream baos = new ByteArrayOutputStream();

bitmap.compress(Bitmap.CompressFormat.JPEG, 100, baos);

byte[] datas = baos.toByteArray();

存在一个问题： 默认读入的图像格式是ARGB_8888类型，且每个通道占1个byte

方式二：从assets文件夹读取（传入c++接口测试ok, 与VS中的结果一致）

/*

* images/lena_gray.jpg ， 测试图像放在app\src\main\assets\images文件夹下

* 用到的接口函数为ReadImageFromLocal

*/

package com.example.builtinalgorithm;import androidx.appcompat.app.AppCompatActivity;import android.content.res.AssetManager;
import android.graphics.Bitmap;
import android.graphics.BitmapFactory;
import android.graphics.Color;
import android.graphics.Rect;
import android.os.Bundle;
import android.util.Log;
import android.widget.TextView;import java.io.ByteArrayOutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.nio.ByteBuffer;public class MainActivity extends AppCompatActivity {// Used to load the 'textdetection-lib' library on application startup.//static {//    System.loadLibrary("textdetection-lib");//}@Overrideprotected void onCreate(Bundle savedInstanceState) {super.onCreate(savedInstanceState);setContentView(R.layout.activity_main);String str = stringFromJNI();AssetManager assetManager = getAssets();String[] files = null;try {//files = assetManager.list("image"); //image 表示assets文件夹下的子文件夹名称files = assetManager.list("");} catch (IOException e) {Log.e("tag", e.getMessage());str += e.getMessage();str += "\n";}//InputStream in = getAssets().open("file:///android_asset/frozen_east_text_detection.pb");// Example of a call to a native methodTextView tv = findViewById(R.id.sample_text);String assetFile = "file number is: " + Integer.toString (files.length) +" First file name is: "+ files[0] + "\n";str += assetFile;assetFile = "file number is: " + Integer.toString (files.length) +" Second file name is: "+ files[1] + "\n";str += assetFile;assetFile = "file number is: " + Integer.toString (files.length) +" Third file name is: "+ files[2] + "\n";str += assetFile;//str += ReadNetFromLocal(files[0]); //F:/text_detect/models/east/frozen_east_text_detection.pb "file:///android_asset/frozen_east_text_detection.pb"//Bitmap bitmap = BitmapFactory.decodeResource(getResources(), R.drawable.lena_gray);InputStream inputStream = null;try {inputStream = getResources().getAssets().open("images/lena_gray.jpg");} catch (IOException e) {e.printStackTrace();str += e.getMessage();str += "\n";}// 直接读取 ARGB_8888 占4个byte//Bitmap bitmap = BitmapFactory.decodeStream(inputStream);// 灰度图像只需要一个通道表示：转换为一个通道，占1byte,这时只有alpha通道BitmapFactory.Options options = new BitmapFactory.Options();options.inPreferredConfig = Bitmap.Config.ALPHA_8;Rect rect = new Rect();Bitmap bitmap = BitmapFactory.decodeStream(inputStream, rect, options);// lena_gray.jpg大小为512*512，第一个像素值为163int height = bitmap.getHeight();int width = bitmap.getWidth();int color = bitmap.getPixel(0,0);int a = Color.alpha(color);int bytes = bitmap.getByteCount();str += Integer.toString(width) + " " + Integer.toString(height) +" " + Integer.toString(bytes) +" " + Integer.toString(a) + "\n";// Bitmap转换为byte[]ByteBuffer buf = ByteBuffer.allocate(bytes);bitmap.copyPixelsToBuffer(buf);byte[] datas = buf.array();str += Integer.toString(datas.length) + "\n";str += ReadImageFromLocal(datas, width, height,1);tv.setText(str);}/*** A native method that is implemented by the 'native-lib' native library,* which is packaged with this application.*/public native String stringFromJNI();public native String ReadNetFromLocal(String path);public native String ReadImageFromLocal(byte[] imgdata, int width, int height, int channels);

下面展示一下native-cpp中的接口函数。这部分是用来测试传入的buffer是否正确。主要查看通道、pixel，然后对传入的buffer进行简单的均值方差计算，验证与c++版本下的结果是否一样即可。

#include <jni.h>
#include <string>#include <opencv2/opencv.hpp>
using namespace cv;
using namespace std;
using namespace cv::dnn;extern "C" JNIEXPORT jstring JNICALL
Java_com_example_builtinalgorithm_MainActivity_ReadImageFromLocal(JNIEnv *env,jobject /* this */, jbyteArray imgdata, jint width, jint height, jint channels) {
//    jboolean iscopy;
//    const char* nativeString = env->GetStringUTFChars(js, &iscopy);
//    string file(nativeString);std::string tips = "In cpp function: \n";int ch= channels;int cols = width;int rows = height;jsize len = env->GetArrayLength(imgdata);tips += "GetArrayLength: ";tips += to_string((int)len);tips += "\n";
//    jbyte *jbarray = (jbyte *)malloc(len * sizeof(jbyte));
//    env->GetByteArrayRegion(imgdata, 0, len, jbarray);jbyte* jbarray = env->GetByteArrayElements(imgdata,0);unsigned char *dDate = (unsigned char*)jbarray;try {//Mat img = imread(file);Mat img;int type = CV_8UC1;if(ch==3){type = CV_8UC3;Mat tmp(rows,cols,type,dDate);cvtColor(tmp,img,COLOR_RGB2GRAY);}else if(ch==1){type = CV_8UC1;Mat tmp(rows,cols,type,dDate);img = tmp;}else if(ch==4){type = CV_8UC4;Mat tmp(rows,cols,type,dDate);cvtColor(tmp,img,COLOR_RGBA2GRAY);}else{tips += "param:channels is error";return env->NewStringUTF(tips.c_str());}tips += to_string(cols);tips += "  ";tips += to_string(rows);tips += "  ";int pixl = *(img.ptr<uchar>(0)+0);tips += to_string(pixl);tips += "\n";Scalar a,b;meanStdDev(img, a,b);tips += "Load image is ok: ";tips += to_string(a[0]);tips += "  ";tips += to_string(b[0]);tips += "\n";}catch(cv::Exception e){String error = e.msg;tips += error;tips += "Load image error. \n";}return env->NewStringUTF(tips.c_str());
}

关于CMakeList.txt中的opencv配置这里再重复一遍，我当前用的native-lib名称改了一下：textdetection-lib，其他出现opencv字眼的地方都是opencv的配置

# For more information about using CMake with Android Studio, read the
# documentation: https://d.android.com/studio/projects/add-native-code.html# Sets the minimum version of CMake required to build the native library.cmake_minimum_required(VERSION 3.4.1)# 2019.10.31
set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -std=c++11")
set(opencv_411_dir E:/opencv-4.1.1-android-sdk/OpenCV-android-sdk/sdk/native/jni)
set(app_dir F:/android/AndroidStudioProjects/BuiltinAlgorithm/app)include_directories(${opencv_411_dir}/include)
add_library(opencv-lib SHARED IMPORTED)
set_target_properties(opencv-lib PROPERTIES IMPORTED_LOCATION ${app_dir}/src/main/jniLibs/${ANDROID_ABI}/libopencv_java4.so)# Creates and names a library, sets it as either STATIC
# or SHARED, and provides the relative paths to its source code.
# You can define multiple libraries, and CMake builds them for you.
# Gradle automatically packages shared libraries with your APK.add_library( # Sets the name of the library.textdetection-lib# Sets the library as a shared library.SHARED# Provides a relative path to your source file(s).textdetection-lib.cpp)# Searches for a specified prebuilt library and stores the path as a
# variable. Because CMake includes system libraries in the search path by
# default, you only need to specify the name of the public NDK library
# you want to add. CMake verifies that the library exists before
# completing its build.find_library( # Sets the name of the path variable.log-lib# Specifies the name of the NDK library that# you want CMake to locate.log)# Specifies libraries CMake should link to your target library. You
# can link multiple libraries, such as libraries you define in this
# build script, prebuilt third-party libraries, or system libraries.target_link_libraries( # Specifies the target library.textdetection-libopencv-lib #2019.10.31# Links the target library to the log library# included in the NDK.${log-lib})