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Android camera gives you frames in YUV format. You need to convert it to RGB/RGBA image before drawing them on screen. You need something like this:

mRgba = new Mat();
mYuv = new Mat(getFrameHeight() + getFrameHeight() / 2, getFrameWidth(), CvType.CV_8UC1);
mBitmap = Bitmap.createBitmap(previewWidth, previewHeight, Bitmap.Config.ARGB_8888);

protected Bitmap processFrame(byte[] data) {
    mYuv.put(0, 0, data);    
    Imgproc.cvtColor(mYuv, mRgba, Imgproc.COLOR_YUV420sp2RGB, 4);

    //process mat with native code

    Utils.matToBitmap(mRgba, mBitmap);
    return mBitmap;
}

mGraySubmat = mYuv.submat(0, getFrameHeight(), 0, getFrameWidth()) gives you only Y plain of YUV image that is equivalent of grace scale.