基于Android studio3.6的JNI教程之opencv实例详解

基本环境：

Android studio3.6

NDK:r14b（尽量使用该版本）

Opencv3.4.1 android sdk

(1)新建工程OpenCVDemo，选择，一定要选择Native c++类型，最后要选c++14支持。

(2)File->Project Structure->SDK Location,设置这3个路径，NDK选择r14b。

(3)任意找一张图片，复制到res/drawable。

(4)修改布局文件res/layout/ activity_main.xml

<?xml version='1.0' encoding='utf-8'?><RelativeLayout xmlns:android='http://schemas.android.com/apk/res/android' android:layout_width='match_parent' android:layout_height='match_parent'> <ImageView android: android:layout_width='match_parent' android:layout_height='match_parent' /> <LinearLayout android:layout_width='match_parent' android:layout_height='wrap_content' android:layout_alignParentBottom='true' android:orientation='horizontal'> <Button android: android:layout_width='match_parent' android:layout_height='wrap_content' android:layout_weight='1' android:text='show' /> <Button android: android:layout_width='match_parent' android:layout_height='wrap_content' android:layout_weight='1' android:text='process' /> </LinearLayout></RelativeLayout>

(5)修改java文件，app/src/main/java/ com.example.opencvdemo/ MainActivity

主要修改包括修改

继承OnClickListener类，

修改onCreate方法

增加c++的接口函数，getEdge

实现点击按钮的方法，

整体代码如下，

(6)Termi

package com.example.opencvdemo;import androidx.appcompat.app.AppCompatActivity;import android.graphics.Bitmap;import android.graphics.BitmapFactory;import android.os.Bundle;import android.view.View;import android.widget.ImageView;import android.widget.TextView;public class MainActivity extends AppCompatActivity implements View.OnClickListener{ // Used to load the ’native-lib’ library on application startup. static { System.loadLibrary('native-lib'); } @Override protected void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.activity_main); imageView = findViewById(R.id.imageView); findViewById(R.id.show).setOnClickListener(this); findViewById(R.id.process).setOnClickListener(this); } /** * A native method that is implemented by the ’native-lib’ native library, * which is packaged with this application. */ //获得Canny边缘 public native void getEdge(Object bitmap); private ImageView imageView; @Override public void onClick(View v) { if (v.getId() == R.id.show) { Bitmap bitmap = BitmapFactory.decodeResource(getResources(), R.drawable.test); imageView.setImageBitmap(bitmap); } else { Bitmap bitmap = BitmapFactory.decodeResource(getResources(), R.drawable.test); getEdge(bitmap); imageView.setImageBitmap(bitmap); } }}

nal下进入appsrcmainjava这一层目录，执行，

javah com.example.opencvdemo.MainActivity

将生成的com_example_opencvdemo_MainActivity.h，剪切到app/src/main/cpp目录下。

(7)修改app/src/main/cpp下面的native-lib.cpp，主要通过c++实现getEdge方法，主要代码如下，

#include <jni.h>#include <string>#include 'com_example_opencvdemo_MainActivity.h'#include <android/bitmap.h>#include <opencv2/opencv.hpp>using namespace cv;extern 'C'JNIEXPORT void JNICALLJava_com_example_opencvdemo_MainActivity_getEdge(JNIEnv *env, jobject obj, jobject bitmap){ // TODO: implement getEdge() AndroidBitmapInfo info; void *pixels; CV_Assert(AndroidBitmap_getInfo(env, bitmap, &info) >= 0); CV_Assert(info.format == ANDROID_BITMAP_FORMAT_RGBA_8888 || info.format == ANDROID_BITMAP_FORMAT_RGB_565); CV_Assert(AndroidBitmap_lockPixels(env, bitmap, &pixels) >= 0); CV_Assert(pixels); if (info.format == ANDROID_BITMAP_FORMAT_RGBA_8888) { Mat temp(info.height, info.width, CV_8UC4, pixels); Mat gray; cvtColor(temp, gray, COLOR_RGBA2GRAY); Canny(gray, gray, 125, 225); cvtColor(gray, temp, COLOR_GRAY2RGBA); } else { Mat temp(info.height, info.width, CV_8UC2, pixels); Mat gray; cvtColor(temp, gray, COLOR_RGB2GRAY); Canny(gray, gray, 125, 225); cvtColor(gray, temp, COLOR_GRAY2RGB); } AndroidBitmap_unlockPixels(env, bitmap);}

(8)修改CMakeLists.txt

包括增加opencv包含路径，增加opencv链接，增加目标库的链接（OpenCV_LIBS和jnigraphics）

全部代码如下，

# 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)# 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.#设置OpenCV-android-sdk路径set( OpenCV_DIR E:/Android/OpenCV-android-sdk/sdk/native/jni )find_package(OpenCV REQUIRED )if(OpenCV_FOUND) include_directories(${OpenCV_INCLUDE_DIRS}) message(STATUS 'OpenCV library status:') message(STATUS ' version: ${OpenCV_VERSION}') message(STATUS ' libraries: ${OpenCV_LIBS}') message(STATUS ' include path: ${OpenCV_INCLUDE_DIRS}')else(OpenCV_FOUND) message(FATAL_ERROR 'OpenCV library not found')endif(OpenCV_FOUND) set(CMAKE_SHARED_LINKER_FLAGS '${CMAKE_SHARED_LINKER_FLAGS} -Wl,--exclude-libs,libippicv.a -Wl,--exclude-libs,libippiw.a') add_library( # Sets the name of the library. native-lib # Sets the library as a shared library. SHARED # Provides a relative path to your source file(s). native-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. native-lib ${OpenCV_LIBS} jnigraphics # Links the target library to the log library # included in the NDK. ${log-lib} )

(9)修改app/build.gradle

主要增加cmake的cppFlags，arguments

全部代码如下，

apply plugin: ’com.android.application’android { compileSdkVersion 29 buildToolsVersion '29.0.3' defaultConfig { applicationId 'com.example.opencvdemo' minSdkVersion 16 targetSdkVersion 29 versionCode 1 versionName '1.0' testInstrumentationRunner 'androidx.test.runner.AndroidJUnitRunner' externalNativeBuild { cmake { cppFlags '-std=c++14 -frtti -fexceptions' arguments ’-DANDROID_STL=gnustl_shared’ //支持C++异常处理标准模板快,ndk16+需要注释 //abiFilters 'armeabi-v7a', 'arm64-v8a', 'x86_64' } } } buildTypes { release { minifyEnabled false proguardFiles getDefaultProguardFile(’proguard-android-optimize.txt’), ’proguard-rules.pro’ } } externalNativeBuild { cmake { path 'src/main/cpp/CMakeLists.txt' version '3.10.2' } }}dependencies { implementation fileTree(dir: ’libs’, include: [’*.jar’]) implementation ’androidx.appcompat:appcompat:1.1.0’ implementation ’androidx.constraintlayout:constraintlayout:1.1.3’ testImplementation ’junit:junit:4.12’ androidTestImplementation ’androidx.test.ext:junit:1.1.1’ androidTestImplementation ’androidx.test.espresso:espresso-core:3.2.0’}

(10)整体目录结构如下，

运行程序，

代码链接：

References:

https://www.jianshu.com/p/6e16c0429044

https://www.bilibili.com/video/av55834524/

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