ESC/POS thermal printer, how to center a bitmap image in Android?

Question

I'm writing a simple app in Android to print through a ESC/POS thermal printer. I've just a problem. The app generate a QR Code (with zxing library), convert it in a bitmap and send it to the printer. The printer print it, but I'm not able to center it. Instead with text I have no positioning issue.

This is the code to print the text and the QR.

byte[] INIT = {27, 64};
byte[] FEED_LINE = {10};

byte[] SELECT_FONT_A = {27, 33, 0};

byte[] FONT_B = {0x1B, 0x4D, 0x01};
byte[] ALLINEA_SX = {0x1B, 0x61, 0x00};
byte[] ALLINEA_CT = {0x1B, 0x61, 0x01};
byte[] ALLINEA_DX = {0x1B, 0x61, 0x02};
byte[] GRASSETTO_ON = {0x1B, 0x47, 0x11};
byte[] GRASSETTO_OFF = {0x1B, 0x47, 0x00};
byte[] SET_6 = {0x1B, 0x52, 0x06};
byte[] CODICI = {0x1B, 0x74, 0x13};
byte[] EURO = {(byte) 0xD5};
byte[] PALLINO = {(byte) 0x5B};
byte[] FONT_3X = {0x1D, 0x21, 0x21};
byte[] FONT_2X = {0x1D, 0x21, 0x11};
byte[] FONT_1X = {0x1D, 0x21, 0x00};

byte[] SET_BAR_CODE_HEIGHT = {29, 104, 100};
byte[] PRINT_BAR_CODE_1 = {29, 107, 2};
byte[] SEND_NULL_BYTE = {0x00};

byte[] SELECT_PRINT_SHEET = {0x1B, 0x63, 0x30, 0x02};
byte[] FEED_PAPER_AND_CUT = {0x1D, 0x56, 66, 0x00};

byte[] SELECT_CYRILLIC_CHARACTER_CODE_TABLE = {0x1B, 0x74, 0x11};

byte[] SELECT_BIT_IMAGE_MODE = {0x1B, 0x2A, 33, 127, 3};
byte[] SET_LINE_SPACING_24 = {0x1B, 0x33, 24};
byte[] SET_LINE_SPACING_30 = {0x1B, 0x33, 30};

byte[] TRANSMIT_DLE_PRINTER_STATUS = {0x10, 0x04, 0x01};
byte[] TRANSMIT_DLE_OFFLINE_PRINTER_STATUS = {0x10, 0x04, 0x02};
byte[] TRANSMIT_DLE_ERROR_STATUS = {0x10, 0x04, 0x03};
byte[] TRANSMIT_DLE_ROLL_PAPER_SENSOR_STATUS = {0x10, 0x04, 0x04};

mService.write(INIT);
mService.write(ALLINEA_CT); //text to center
mService.write(SELECT_FONT_A);
mService.write(FONT_2X);
mService.write("Hello stackoverflow!!\n".getBytes());
mService.write(FONT_1X);
mService.write(ALLINEA_SX); //text to left
mService.write("Thanks for help!\n".getBytes());


int QRCODE_IMAGE_HEIGHT = 255;
int QRCODE_IMAGE_WIDTH = 255;

QRCodeWriter qrWriter = new QRCodeWriter();
try {
    Hashtable<EncodeHintType, ErrorCorrectionLevel> hints = new Hashtable<EncodeHintType, ErrorCorrectionLevel>();
    hints.put(EncodeHintType.ERROR_CORRECTION, ErrorCorrectionLevel.H);
BitMatrix bitMatrix = qrWriter.encode("https://stackoverflow.com",
        BarcodeFormat.QR_CODE,
        QRCODE_IMAGE_WIDTH,
        QRCODE_IMAGE_HEIGHT, hints);

    int bitMatrixWidth = bitMatrix.getWidth();

    int bitMatrixHeight = bitMatrix.getHeight();

    int[] pixels = new int[bitMatrixWidth * bitMatrixHeight];

    for (int y = 0; y < bitMatrixHeight; y++) {
        int offset = y * bitMatrixWidth;

        for (int x = 0; x < bitMatrixWidth; x++) {

            pixels[offset + x] = bitMatrix.get(x, y) ?
                    getResources().getColor(R.color.QRCodeBlackColor):getResources().getColor(R.color.QRCodeWhiteColor);
        }
    }
    Bitmap bitmap = Bitmap.createBitmap(bitMatrixWidth, bitMatrixHeight, Bitmap.Config.ARGB_4444);

    bitmap.setPixels(pixels, 0, 255, 0, 0, bitMatrixWidth, bitMatrixHeight);

    Bitmap bmp = BitmapFactory.decodeResource(getResources(), R.drawable.k_city5);

    Bitmap bmOverlay = Bitmap.createBitmap(bitmap.getWidth(), bitmap.getHeight(), bitmap.getConfig());
    Canvas canvas = new Canvas(bmOverlay);
    canvas.drawBitmap(bitmap, new Matrix(), null);
    canvas.drawBitmap(bmp, 94, 94, null); //34x34
    //canvas.drawBitmap(bmp, 86, 86, null); //42x42

    if(bmOverlay!=null) {
        byte[] command = Utils.decodeBitmap(bmOverlay);
        mService.write(ALLINEA_CT); //This command should center my QR, but it's not working
        mService.write(command);
        mService.write(FEED_LINE);
        mService.write(FEED_LINE);
    }

This is the result of the printing:

Answer 1

Pad the image on the left until it looks right. See Android: padding left a bitmap with white color.

You can do this via trial and error, or take the width of the printer in dots (available in its data sheet) and do the maths:

padding = (page width - image width) / 2

There are commands in ESC/POS to centre text, and on some printers, this will centre an image. There are also alternative ways to print an image at a given location (page mode), or in a way that aligns more like text (column format), but padding the image is a much simpler modification to your example.