Inserting large amount of XMP data into jpg using multiple packets?

Question

My problem is I am trying to insert a large amount of RDF data into a jpeg image, specifically into the XMP headers. The RDF is specific to my application, with custom namespaces etc. However this should not affect the process of inserting RDF. I can do this with a small amount of RDF data, however I reach the XMP packet size limit when I try to insert anything larger.

I am using Java and the Apache Sanselan lib, however I'm open to use other libs.

Below is the code I am using in a test app to do this, however I do not know how to split it over multiple XMP packets in order for me to insert all the data I need to.

Any help or pointers in the right direction would be greatly appreciated :)

Thanks

private static File writeXmpToFile(File file, String xmpXmlAsString)
                throws FileNotFoundException, ImageReadException, IOException,
                ImageWriteException {

        String XmpXmlAsString = xmpXmlAsString;

        File fileWithXmpXml = new File(file.getParent(), file.getName()+ ".added-xmp" + ".jpg");
        OutputStream os = null;
        try {
                os = new BufferedOutputStream(new FileOutputStream(fileWithXmpXml));
                new JpegXmpRewriter().updateXmpXml(new ByteSourceFile(file), os, XmpXmlAsString);
        } finally {
                if (os != null) {
                        os.close();
                }
                os = null;
        }
        return fileWithXmpXml;
}

Answer 1

Quoting Adobe XMP Specification part 3:

Following the normal rules for JPEG sections, the header plus the following data can be at most 65535 bytes long. If the XMP packet is not split across multiple APP1 sections, the size of the XMP packet can be at most 65502 bytes. It is unusual for XMP to exceed this size; typically, it is around 2 KB.

If the serialized XMP packet becomes larger than the 64 KB limit, you can divide it into a main portion (StandardXMP) and an extended portion (ExtendedXMP), and store it in multiple JPEG marker segment. A reader must check for the existence of ExtendedXMP, and if it is present, integrate the data with the main XMP. Each portion (standard and extended) is a fully formed XMP metadata tree, although only the standard portion contains a complete packet wrapper. If the data is more than twice the 64 KB limit, the extended portion can also be split and stored in multiple marker segments; in this case, the split portions are not fully formed metadata trees.

When ExtendedXMP is required, the metadata must be split according to some algorithm that assigns more important data to the main portion, and less important data to the extended portions or portions.

Most of the metadata managing code currently only deals with the standard XMP part which fits into a single APP1 segment - either reading or writing. Exiftool seems to be fine with ExtendedXMP.

For the ExtendedXMP structure and how it is included in JPEG multiple segments, I answered a question here.

Basically, you need to split the entire XMP data into two parts: a standard XMP which will be fit into one APP1 segment with packet wrapper and an ExtendedXMP which is also a well-formed XMP but with no packet wrapper. The ExtendedXMP part could be any size and if it exceed the limit of a single APP1, it will be split and insert into multiple APP1 segment.

The above linked code could also insert large XMP data as long as you split it into two parts and keep the standard XMP part smaller than one APP1 segment limit.