Convert png to monochrome bmp - android

Question

I am developing an application in which I draw black n white drawing on canvas and save it in png format

now I bought a pos printer for which I need drawing in monochrome bmp format , how can I save canvas as monochrome bmp on sd card or while reading read png file as monochrome bmp?

here is my code to save canvas on sd card

Bitmap returnedBitmap = Bitmap.createBitmap(mContent.getWidth(),
            mContent.getHeight(), Bitmap.Config.ARGB_8888);
    Canvas canvas = new Canvas(returnedBitmap);

//  Drawable bgDrawable = mContent.getBackground();

    canvas.drawColor(Color.TRANSPARENT);
    mContent.draw(canvas);




    String root = Environment.getExternalStorageDirectory().toString();
    File myDir = new File(root);    
    myDir.mkdirs();
    Random generator = new Random();
    int n = 10000;
    n = generator.nextInt(n);
    String fname = "abc.bmp";

    File file = new File (myDir, fname);
    Log.d("SingatureView",file.getAbsolutePath().toString());


    if (file.exists ()) file.delete (); 
    try {

           FileOutputStream out = new FileOutputStream(file);
//     returnedBitmap = Bitmap.createScaledBitmap(returnedBitmap, 50, 30, false);
           returnedBitmap.compress(Bitmap.CompressFormat.PNG, 0, out);
           out.flush();
           out.close();

    } catch (Exception e) {
           e.printStackTrace();
    }

and to read file

String filePath = Environment.getExternalStorageDirectory()+File.separator+"abc.bmp";


   File file = new File(filePath);
   int size = (int) file.length();
   byte[] bytes = new byte[size];
   try {
       BufferedInputStream buf = new BufferedInputStream(new FileInputStream(file));
       buf.read(bytes, 0, bytes.length);
       buf.close();
   } catch (FileNotFoundException e) {
       // TODO Auto-generated catch block
       e.printStackTrace();
   } catch (IOException e) {
       // TODO Auto-generated catch block
       e.printStackTrace();
   }
  printer.PrintBmp(bytes, bytes.length, 0, 0);