JNI code for the blogpost - http://jayrambhia.com/blog/android-camerapreview-opencv/

ImageProcessing.cpp

#include <stdio.h>
#include <jni.h>

#include <opencv2/core/core.hpp>
#include <opencv2/highgui/highgui.hpp>
#include <opencv2/imgproc/imgproc.hpp>

#include <android/log.h>

#define APPNAME "Exposure jni"
#define LOGD(TAG) __android_log_print(ANDROID_LOG_DEBUG , APPNAME,TAG);

using namespace std;
using namespace cv;

extern "C"
jboolean
Java_my_project_MyRealTimeImageProcessing_CameraPreview_ImageProcessing(
    JNIEnv* env, jobject thiz,
    jint width, jint height,
    jbyteArray NV21FrameData, jintArray outPixels)
{
  jbyte * pNV21FrameData = env->GetByteArrayElements(NV21FrameData, 0);
  jint * poutPixels = env->GetIntArrayElements(outPixels, 0);

 
  Mat mGray(height + height/2, width, CV_8UC1, (unsigned char *)pNV21FrameData);
  Mat blur;
  Mat mResult(height, width, CV_8UC4, (unsigned char *)poutPixels);
  
  cvtColor(mGray, blur, CV_YUV2BGR_NV21);
  GaussianBlur(blur, blur, Size(5, 5), 5, 0);
  cvtColor(blur, mResult, CV_RGB2RGBA); // somehow this seems to work instead of CV_BGR2RGBA
 
  env->ReleaseByteArrayElements(NV21FrameData, pNV21FrameData, 0);
  env->ReleaseIntArrayElements(outPixels, poutPixels, 0);
  return true;
}