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，

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);
        }
    }


}

(6)Terminal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 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：

https://download.csdn.net/download/qq_14845119/12249464

网上还找到了另外一种调用方法。

上面的方法的优点是，相对比较简单，缺点是没有把opencv的so和.h加进工程里面，所以我更推荐下面的这种。

下面的方法还有一个优势，支持ndk16+的版本，比如ndk21

这里只写需要修改的有区别的地方。

（1）在cpp中增加include文件夹，里面包含opencv，opencv2的库。

（2）修改CmakeLists，增加include_directories，add_library，set_target_properties。

target_link_libraries增加libopencv_java3。

全部的代码如下，

# 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
include_directories(include)
add_library(libopencv_java3 SHARED IMPORTED)
set_target_properties(libopencv_java3 PROPERTIES IMPORTED_LOCATION
        ${CMAKE_SOURCE_DIR}/../jniLibs/${ANDROID_ABI}/libopencv_java3.so)


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
        libopencv_java3
        jnigraphics
        # Links the target library to the log library
        # included in the NDK.
        ${log-lib} )

（3）build.grade增加ndk的设置，

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++11 -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"
            }
        }

        ndk {
            abiFilters 'armeabi-v7a', 'arm64-v8a' //,'x86', 'x86_64', 'armeabi'
            stl "gnustl_static"
        }
    }

    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'
}

（4）main下面新建jniLibs，加入不同平台下的libopencv_java3.so

代码链接：

https://download.csdn.net/download/qq_14845119/12251699

References:

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

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