android renderscript ScriptIntrinsicColorMatrix setRGBtoYUV example usage

Question

as the title says, how to convert bitmap RGB back to YUV byte[] using ScriptIntrinsicColorMatrix? below is the sample code (cannot be decoded by zxing):

public byte[] getYUVBytes(Bitmap src, boolean initOutAllocOnce){    

    if(!initOutAllocOnce){
        outYUV = null;
    }
    if(outYUV == null){
        outYUV = Allocation.createSized(rs, Element.U8(rs), src.getByteCount());    
    }

    byte[] yuvData = new byte[src.getByteCount()];

    Allocation in;

    in = Allocation.createFromBitmap(rs, src,
            Allocation.MipmapControl.MIPMAP_NONE,
            Allocation.USAGE_SCRIPT);   

    scriptColor.setRGBtoYUV();

    scriptColor.forEach(in, outYUV);
    outYUV.copyTo(yuvData);

    return yuvData;
}

one thing i notice is from original camera YUV byte is 3110400 bytes but after using the ScriptIntrinsicColorMatrix convertion in becomes 8294400, which i think is wrong.

Reason for YUV -> BW -> YUV is I want to convert the image into black and white (not grayscale) and back to YUV for zxing to decode at the same time show the black and white in surfaceview (like a custom camera filter).

tried below code but its a bit slow (can be decoded by zxing).

  int[] intArray  = null;
          intArray = new int[bmp.getWidth() * bmp.getHeight()];  
          bmp.getPixels(intArray, 0, bmp.getWidth(), 0, 0, bmp.getWidth(), bmp.getHeight());  

          LuminanceSource source = new RGBLuminanceSource(cameraResolution.x, cameraResolution.y, intArray);
          data = source.getMatrix();

any other alternative for RGB to YUV that is fast? if ScriptIntrinsicColorMatrix class cannot be done?

please and thank you