How to use Android-OpenCV JNI API

This article shows the steps to build the OpenCV JNI application under the Android.
Before type the copy, we need setup the environment.

Step1: First, we need Android JNI environment, you can refer to articles Install the Cygwin and Install the NDK.

Step2:Then, of course, we need the OpenCV library, you can refer to the article How to use OpenCV under Android to install the OpenCV library. But there is a little difference, is the install directory. The OpenCV library  must installed in the Cygwin directory. In our example, I install the OpenCV library in the I:\Cygwin\JNI\OpenCV-2.4.0

Step3:Create a text file named "includeOpenCV.mk", the file content shows below.
OPENCV_MK_PATH:=../OpenCV-2.4.0/share/OpenCV/OpenCV.mk

Step4:Now, we can start to create the OpenCV-JNI project.
   Open the Eclipse, and load the workspace.

Step5:Eclipse menu bar->File->New->Android Project.The new project directory should be in the same directory as the OpenCV-2.4.0 library.

Step6:Create the JNI and libs directories in the project directory. And setup the auto-compile configuration. Please to refer the article Install the NDK

Step7:Create our JNI c++ code.

 
/*
*  first-opencvjni.cpp
*/
#include 

#include <opencv2/core/core.hpp>
#include <opencv2/imgproc/imgproc_c.h>


using namespace cv;

extern "C"
jboolean
Java_my_project_MyFirstOpenCVJNI_MyFirstOpenCVJNI_CannyJNI( 
  JNIEnv* env, jobject thiz, 
  jint height, jint width, jintArray in, jintArray out)
{
 //get the data pointer.
 jint* _in = env->GetIntArrayElements(in, 0);
    jint* _out = env->GetIntArrayElements(out, 0);


 //Build the Mat structure for input data
 Mat mSrc(height, width, CV_8UC4, (unsigned char *)_in);
 //Build the Mat structure for output data
 Mat mOut(height, width, CV_8UC4, (unsigned char *)_out);

 //Convert Mat to IplImage
 IplImage mSrcImg = mSrc;
 IplImage mOutImg = mOut;

 //Create the gray image for input data.
 IplImage * mSrcGrayImg = cvCreateImage(cvGetSize(&mSrcImg), mSrcImg.depth, 1);
 IplImage * mOutGrayImg = cvCreateImage(cvGetSize(&mSrcImg), mSrcImg.depth, 1);

 //Convert to Gray image
 cvCvtColor(&mSrcImg, mSrcGrayImg, CV_BGR2GRAY);

 //Do canny
 cvCanny(mSrcGrayImg, mOutGrayImg, 80, 100, 3);


 //Convert Gray image to bitmap BGR
 cvCvtColor(mOutGrayImg, &mOutImg, CV_GRAY2BGR);

 //release the pointer. 
    env->ReleaseIntArrayElements(in, _in, 0);
    env->ReleaseIntArrayElements(out, _out, 0);
 return true;
}

Step8:Create the make file for Android JNI and OpenCV.

 
#
#  Android.mk
#

LOCAL_PATH := $(call my-dir)

include $(CLEAR_VARS)

OPENCV_LIB_TYPE:=STATIC
OPENCV_INSTALL_MODULES:=on

include ../includeOpenCV.mk
include $(OPENCV_MK_PATH)

LOCAL_MODULE    := first-opencvjni
LOCAL_SRC_FILES := first-opencvjni.cpp
LOCAL_LDLIBS +=  -llog -ldl

include $(BUILD_SHARED_LIBRARY)

 

#
#  Application.mk
#

APP_STL := gnustl_static
APP_CPPFLAGS := -frtti -fexceptions
APP_ABI := armeabi-v7a
APP_MODULES := first-opencvjni

Step9:Put the input image to do the canny() processes.In our example is foot.png.

Step10:The entire project files in the Package Explorer.

Step11:When we do the thing right , the libs directory should have the library file "libfirst-opencvjni.so".
Step12:And out APK will be shown in the bin directory.
Step13:Run the APK in the real Phone and result will be this.