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:id="@+id/imageView"
 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:id="@+id/show"
 android:layout_width="match_parent"
 android:layout_height="wrap_content"
 android:layout_weight="1"
 android:text="show" />
 <Button
 android:id="@+id/process"
 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app\src\main\java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 JNICALL
Java_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/

总结

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