一、创建支持C++的android基础模版项目
新建项目，勾选【Include C++ support】，后续一直按【next】按钮取默认设置即可。
image.png
最终生成的目录结构如下图：
image.png
此时直接build并运行能正确看到效果。
打开MainActivity，可以看到生成的示例代码：
public class MainActivity extends AppCompatActivity {
// 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);
// Example of a call to a native method
TextView tv = (TextView) findViewById(R.id.sample_text);
tv.setText(stringFromJNI());
}
/**
* A native method that is implemented by the 'native-lib' native library,
* which is packaged with this application.
*/
public native String stringFromJNI();
}
再打开 native-lib.cpp 文件，也可以看到自动生成的JNI风格代码：
#include
#include
extern "C" JNIEXPORT jstring
JNICALL
Java_com_woodstream_opencvdemo_MainActivity_stringFromJNI(
JNIEnv *env,
jobject /* this */) {
std::string hello = "Hello from C++";
return env->NewStringUTF(hello.c_str());
}
其中
Java_com_woodstream_opencvdemo_MainActivity_stringFromJNI就包含了java对应方法的不少信息。
为了看看IDE是怎么操作的，回到MainActivity，写入一个新的方法：
public native int[] gray(int[] buf, int w, int h);
image.png
此时方法名会标红，在IDE的智能提示下选择Create function xxx，会自动跳转到native-lib.cpp文件，并且自动添加了代码：
extern "C"
JNIEXPORT jintArray JNICALL
Java_com_woodstream_opencvdemo_MainActivity_gray(JNIEnv *env, jobject instance, jintArray buf_,
jint w, jint h) {
jint *buf = env->GetIntArrayElements(buf_, NULL);
// TODO
env->ReleaseIntArrayElements(buf_, buf, 0);
}
就这么简单的步骤就可以进行JNI的开发，可见Android Studio（下称AS）大为减少我们编写JNI的工作量，而它又是怎么工作的？常规的makefile什么的不用了？
二、默认CMake的构建方式
- 可以看到目录有一个CMakeLists.txt文件。
-
打开build.gradle，看到有外部native编译的配置：
image.png -
打开app/build/intermediates/cmake目录，看到其子目录下生成了so文件：
image.png
从这几个信息可以知道AS默认是通过CMake来完成上述操作的，当然也支持android.mk的方式，但比后者要方便不少。
三、用CMake的方式集成OpenCV
首先，可以先看下此文：Android 接入 OpenCV库的三种方式
除非你是用java写图像处理算法，不然不建议用第一种方式，因为图像识别实现考虑由C++来完成，所以选用第二种方式，此时如下步骤搭建OpenCV的环境：
- https://opencv.org/releases.html下载源码包并解压。
-
新建jniLibs文件夹，把OpenCV中sdk/native/libs内容复制到其下：
image.png - 修改CMakeLists.txt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.
set(pathToOpenCv /Users/woodstream/Documents/dev/projects/android/OpenCV-android-sdk)#设置OpenCv的路径变量
cmake_minimum_required(VERSION 3.4.1)
#支持-std=gnu++11
set(CMAKE_VERBOSE_MAKEFILE on)
set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -std=gnu++11")
#配置加载native依赖
include_directories(${pathToOpenCv}/sdk/native/jni/include)
#动态方式加载
add_library(lib_opencv STATIC IMPORTED ) #表示创建一个导入库，静态方式
#引入libopencv_java3.so文件
set_target_properties(lib_opencv
PROPERTIES
IMPORTED_LOCATION ${PROJECT_SOURCE_DIR}/src/main/jniLibs/${}/libopencv_java3.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.
#自己的源文件
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).
src/main/cpp/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
# Links the target library to the log library
# included in the NDK.
${log-lib} android -ljnigraphics lib_opencv)
至此环境已经搭好了（主要修改pathToOpenCv、set_target_properties的IMPORTED_LOCATION、add_library的source file）。
四、编写代码测试效果
完善C++文件native-lib.app先前生成的gray方法:
#include
#include
#include
#include
#include
#include
using namespace cv;
using namespace std;
extern "C"
JNIEXPORT jintArray JNICALL
Java_com_eastcom_cpptest_MainActivity_gray(JNIEnv *env, jobject instance, jintArray buf, jint w,
jint h) {
jint *cbuf = env->GetIntArrayElements(buf, JNI_FALSE );
if (cbuf == NULL) {
return 0;
}
Mat imgData(h, w, CV_8UC4, (unsigned char *) cbuf);
uchar* ptr = imgData.ptr(0);
for(int i = 0; i < w*h; i ++){
//计算公式：Y(亮度) = 0.299*R + 0.587*G + 0.114*B
//对于一个int四字节，其彩色值存储方式为：BGRA
int grayScale = (int)(ptr[4*i+2]*0.299 + ptr[4*i+1]*0.587 + ptr[4*i+0]*0.114);
ptr[4*i+1] = grayScale;
ptr[4*i+2] = grayScale;
ptr[4*i+0] = grayScale;
}
int size = w * h;
jintArray result = env->NewIntArray(size);
env->SetIntArrayRegion(result, 0, size, cbuf);
env->ReleaseIntArrayElements(buf, cbuf, 0);
return result;
}
MainActivity改成：
public class MainActivity extends AppCompatActivity {
// 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);
Bitmap bitmap = ((BitmapDrawable) getResources().getDrawable(
R.mipmap.pic_test)).getBitmap();
int w = bitmap.getWidth(), h = bitmap.getHeight();
int[] pix = new int[w * h];
bitmap.getPixels(pix, 0, w, 0, 0, w, h);
// 调用JNI实现的gray方法
int [] resultPixes = gray(pix,w,h);
Bitmap result = Bitmap.createBitmap(w,h, Bitmap.Config.RGB_565);
result.setPixels(resultPixes, 0, w, 0, 0,w, h);
ImageView img = (ImageView)findViewById(R.id.img2);
img.setImageBitmap(result);
}
/**
* A native method that is implemented by the 'native-lib' native library,
* which is packaged with this application.
*/
public native int[] gray(int[] buf, int w, int h);
public native String stringFromJNI();
}
activity_main.xml布局文件改成：
此时编译运行可以看到效果如图：
运行图