Hot to convert a Mat object to byte[] data object in "onPreviewFrame" and show it in camera preview

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In my android app, I am trying to take frames in "onPreviewFrame" and convert them to Mat object so i can do on them some image processing, but i don't know how to convert them back to byte[] type and make them be shown on camera preview.

Does any one know how to do that?

Here is my onPreviewFrame function, i am trying to change frame color to grey:

public void onPreviewFrame(byte[] data, Camera arg1)
{
    Mat mRgba = new Mat(PreviewSizeWidth, PreviewSizeHeight, CvType.CV_8UC3);
    mRgba.put(0, 0, data);

    Imgproc.cvtColor(mRgba, mRgba, Imgproc.COLOR_BGR2GRAY);
    mRgba.put(0, 0, data);

    mCamera.addCallbackBuffer(data);

}

Thanks in advance!