[android with opencv ] RGBA to signal channel

edit

I want to extract green channel from input.rgba(), my code are below:

enter code here
      mRgba = inputFrame.rgba();
      Rect r = new Rect(150,50,200,200); 
      Core.rectangle(mRgba, r.tl(), r.br(), yellow, 3);
      Mat sub = mRgba.submat(r);
      Mat gchannel = new Mat();
      Core.extractChannel(sub, sub, 1);
      sub.copyTo(mRgba.submat(r));
      return mRgba;

but is doesn't work . Could anybody help me