OpenCV是图像操作的工具，针对于Android平台，他们也出了相关的sdk，就用最新的4.1.1接入并学习。
下载：https://opencv.org/releases/

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1、创建一个支持ndk开发的工程。

2、编辑cmake文件

# 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)
#支持-std=gnu++11
set(CMAKE_VERBOSE_MAKEFILE on)
set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -std=gnu++11")


#配置加载native依赖
include_directories(${CMAKE_CURRENT_SOURCE_DIR}/opencv/include)

# 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.
        native-lib

        # Sets the library as a shared library.
        SHARED

        # Provides a relative path to your source file(s).
        native-lib.cpp)

#动态方式加载
add_library( opencv_java4 SHARED IMPORTED )

# 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.

#引入libopencv_java4.so文件
set_target_properties(opencv_java4
        PROPERTIES IMPORTED_LOCATION
        ${CMAKE_CURRENT_SOURCE_DIR}/../jniLibs/armeabi-v7a/libopencv_java4.so
        )

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_java4
        # Links the target library to the log library
        # included in the NDK.
        ${log-lib})

注意点：
a、opencv得支持-std=gnu++11

set(CMAKE_VERBOSE_MAKEFILE on)
set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -std=gnu++11")

b、添加所有的头文件，设置他的引入路径

include_directories(${CMAKE_CURRENT_SOURCE_DIR}/opencv/include)

c、动态方式加载opencv

add_library( opencv_java4 SHARED IMPORTED )

这里的opencv_java4这个命名必须是这样，他其实是我们依赖的libopencv_java4.so去掉lib头去掉.so后缀留下的，用其他的名字比如随便命名opencv就会报libopencv.so找不到。这个要特别注意。
d、依赖opencv的so文件

set_target_properties(opencv_java4
        PROPERTIES IMPORTED_LOCATION
        ${CMAKE_CURRENT_SOURCE_DIR}/../jniLibs/armeabi-v7a/libopencv_java4.so
        )

这里的${CMAKE_CURRENT_SOURCE_DIR}指的是你的cmake文件存放路径，然后基于这个路径自己设置找到so文件路径。
e、最后连接依赖关系

target_link_libraries( # Specifies the target library.
        native-lib
        opencv_java4
        # Links the target library to the log library
        # included in the NDK.
        ${log-lib})

3、放入opencv sdk文件

1、根据cmake先来放入我们的头文件

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将整个include文件夹存入到cpp文件中，这个我为了分类，所以多创了一个opencv的文件夹并放入，毕竟我们一个项目不止一个三方依赖库。

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这里，我只想支持arm-v7a的cpu，所以我只创了这个文件夹，libc++_shared.so文件不属于opencv中，那么他在哪呢？进入我们的ndk目录，他就在这，ndk目录\sources\cxx-stl\llvm-libc++\libs\armeabi-v7a,注意一定要是armeabi-v7a，因为我们应用的是这个，不然会报错，是64位和32位不同报错。

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externalNativeBuild {
            cmake {
                cppFlags "-std=c++11 -frtti -fexceptions"
                abiFilters 'armeabi-v7a'
            }
        }
        ndk {
            abiFilters "armeabi-v7a"
        }

4、gradle.properties文件中允许对ndk限制

# Project-wide Gradle settings.
# IDE (e.g. Android Studio) users:
# Gradle settings configured through the IDE *will override*
# any settings specified in this file.
# For more details on how to configure your build environment visit
# http://www.gradle.org/docs/current/userguide/build_environment.html
# Specifies the JVM arguments used for the daemon process.
# The setting is particularly useful for tweaking memory settings.
org.gradle.jvmargs=-Xmx1536m
# When configured, Gradle will run in incubating parallel mode.
# This option should only be used with decoupled projects. More details, visit
# http://www.gradle.org/docs/current/userguide/multi_project_builds.html#sec:decoupled_projects
# org.gradle.parallel=true
android.useDeprecatedNdk=true

4、测试图片转为灰度图

1、创建自己的工具类
OpencvImageUtile引入两个so

public class OpencvImageUtile {
    static {
        System.loadLibrary("native-lib");
        System.loadLibrary("opencv_java4");
    }
    //转灰度
    public static Bitmap goGray(Bitmap bitmap){
        Mat sSrc =new Mat();
        Mat sDst=new Mat();
        org.opencv.android.Utils.bitmapToMat(bitmap, sSrc);
        Imgproc.cvtColor(sSrc, sDst, Imgproc.COLOR_BGRA2GRAY);
        org.opencv.android.Utils.matToBitmap(sDst, bitmap);
        sSrc.release();
        sDst.release();
        return bitmap;
    }
}

然后他会报org.opencv.android.Utils等文件找不到，我们可以去opencv sdk中找

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2、activity调用

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微信图片_20190912115741.jpg

5、工程git

https://github.com/yanjinloving/OpencvDemo