code: private void CheckBilevelImage(IfdTags metadata, int n) {
      // Compression
      long comp = metadata.get(TiffTags.getTagId("Compression")).getFirstNumericValue();
      // if (comp != 1 && comp != 2 && comp != 32773)
      if (comp < 1)
        validation.addError("Invalid Compression", "IFD" + n, comp);
    }
docs: Check Bilevel Image.
  
  @param metadata the metadata
  @param n the IFD number