byte[] readCodewords() throws FormatException {

    FormatInformation formatInfo = readFormatInformation();
    Version version = readVersion();

    // Get the data mask for the format used in this QR Code. This will exclude
    // some bits from reading as we wind through the bit matrix.
    DataMask dataMask = DataMask.forReference((int) formatInfo.getDataMask());
    int dimension = bitMatrix.getHeight();
    dataMask.unmaskBitMatrix(bitMatrix, dimension);

    BitMatrix functionPattern = version.buildFunctionPattern();

    boolean readingUp = true;
    byte[] result = new byte[version.getTotalCodewords()];
    int resultOffset = 0;
    int currentByte = 0;
    int bitsRead = 0;
    // Read columns in pairs, from right to left
    for (int j = dimension - 1; j > 0; j -= 2) {
      if (j == 6) {
        // Skip whole column with vertical alignment pattern;
        // saves time and makes the other code proceed more cleanly
        j--;
      }
      // Read alternatingly from bottom to top then top to bottom
      for (int count = 0; count < dimension; count++) {
        int i = readingUp ? dimension - 1 - count : count;
        for (int col = 0; col < 2; col++) {
          // Ignore bits covered by the function pattern
          if (!functionPattern.get(j - col, i)) {
            // Read a bit
            bitsRead++;
            currentByte <<= 1;
            if (bitMatrix.get(j - col, i)) {
              currentByte |= 1;
            }
            // If we've made a whole byte, save it off
            if (bitsRead == 8) {
              result[resultOffset++] = (byte) currentByte;
              bitsRead = 0;
              currentByte = 0;
            }
          }
        }
      }
      readingUp ^= true; // readingUp = !readingUp; // switch directions
    }
    if (resultOffset != version.getTotalCodewords()) {
      throw FormatException.getFormatInstance();
    }
    return result;
  }