How to get grayscale value of a pixel in OpenCV for Android

Question

I want to get the gray scale value of a pixel in OpenCV for Android. The problem is that OpenCV for Android is very much undocumented and a lot of the methods are different. I have found answers to this problem, but none of them work for android.

import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.util.Log;
import android.view.Menu;
import android.view.MenuItem;
import android.view.SurfaceView;
import android.hardware.camera2.CameraCaptureSession;

import org.opencv.android.BaseLoaderCallback;
import org.opencv.android.CameraBridgeViewBase;
import org.opencv.android.CameraBridgeViewBase.CvCameraViewListener2;
import org.opencv.android.JavaCameraView;
import org.opencv.android.LoaderCallbackInterface;
import org.opencv.android.OpenCVLoader;
import org.opencv.core.Mat;

public class testActivity extends AppCompatActivity implements CvCameraViewListener2 {

private BaseLoaderCallback mLoaderCallback = new BaseLoaderCallback(this) {
    @Override
public void onManagerConnected(int status){
        switch (status){
            case LoaderCallbackInterface.SUCCESS:
            {
                mOpenCvCameraView.enableView();
                break;
            }
            default:
            {
                super.onManagerConnected(status);
            }
        }
    }

};
private JavaCameraView mOpenCvCameraView;


@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_test);

    mOpenCvCameraView = (JavaCameraView) findViewById(R.id.testVideoView);
    mOpenCvCameraView.setMaxFrameSize(240,320);
    mOpenCvCameraView.setVisibility(SurfaceView.VISIBLE);
    mOpenCvCameraView.setCvCameraViewListener(this);
}


@Override
public boolean onCreateOptionsMenu(Menu menu) {
    // Inflate the menu; this adds items to the action bar if it is present.
    //getMenuInflater().inflate(R.menu.menu_main, menu);
    return true;
}

@Override
public boolean onOptionsItemSelected(MenuItem item) {
    // Handle action bar item clicks here. The action bar will
    // automatically handle clicks on the Home/Up button, so long
    // as you specify a parent activity in AndroidManifest.xml.
    int id = item.getItemId();
    getSupportActionBar().hide();

    //noinspection SimplifiableIfStatement
    if (id == R.id.action_settings) {
        return true;
    }

    return super.onOptionsItemSelected(item);
}



public void onDestroy() {
    super.onDestroy();
    if (mOpenCvCameraView != null)
        mOpenCvCameraView.disableView();
}

public void onResume(){
    super.onResume();
    if (!OpenCVLoader.initDebug()) {
        OpenCVLoader.initAsync(OpenCVLoader.OPENCV_VERSION_2_4_9, this, mLoaderCallback);
    } else {
        mLoaderCallback.onManagerConnected(LoaderCallbackInterface.SUCCESS);
    }
}



@Override
public void onPause() {
    super.onPause();

    if (mOpenCvCameraView != null)
        mOpenCvCameraView.disableView();
}



@Override
public void onCameraViewStarted(int width, int height) {

}

@Override
public void onCameraViewStopped() {

}

@Override
public Mat onCameraFrame(CameraBridgeViewBase.CvCameraViewFrame inputFrame) {
    return inputFrame.gray();
}
}

Answer 1

I'll assume you mean "how to extract gray pixel from RGB Mat", because getting the gray pixel from Mat that represents a grayscale image is trivial.

What you can do is define submat of size 1x1 for instance

Mat bgrPixel= rgbMat.submat(x,y,x,y).clone();
Mat grayPixel = new Mat(1,1, CvType.CV_8UC1);
Imgproc.cvtColor(bgrPixel, grayPixel, Imgproc.COLOR_BGR2GRAY);

(maybe the indexes are x,y,x+1,y+1, I'm not sure) after that you can use your gray pixel.

It's not an elegant solution but it should work.