Over the last few weeks I’ve been working on the design of a workflow that the KB is planning to use for the migration of a collection of (mostly old) TIFF images to JP2. One major risk of such a migration is that hardware failures during the migration process may result in corrupted images. For instance, one could imagine a brief network or power interruption that occurs while an image is being written to disk. In that case data may be missing from the written file. Ideally we would be able to detect such errors using format validation tools such as JHOVE. Some time ago Paul Wheatley reported that the BL at some point were dealing with corrupted, incomplete JP2 files that were nevertheless deemed “well-formed and valid” by JHOVE. So I started doing some experiments in which I deliberately butchered up some images, and subsequently checked to what extent existing tools would detect this.
I started out with removing some trailing bytes from a lossily compressed JP2 image. As it turned out, I could remove most of the image code stream (reducing the original 2 MB image to a mere 4 kilobytes!), but JHOVE would still say the file was “well-formed and valid”. I was also able to open and render these files with viewer applications such as Adobe Photoshop, Kakadu’s viewer and Irfanview. The behaviour of the viewer apps isn’t really a surprise, since the ability to render an image without having to load the entire code stream is actually one of the features that make JPEG 2000 so interesting for many access applications. JHOVE’s behaviour was a bit more surprising, and perhaps slightly worrying.
This made me wonder about a way to detect incomplete code streams in JP2 files. A quick glance at the standard revealed that image code streams should always be terminated by a two-byte ‘end of codestream marker’. As this is something that is straightforward to check, I fired up Python and ended up writing a very simple JP2 file structure checker. Since the image code stream in JP2 does not have to be located at the end of the file (even though it usually is), it is necessary to do a superficial parsing of JP2’s ‘box’ structure (which is documented here). So I thought I might as well include an additional check that verifies if the JP2 contains all required boxes.
In brief, when jp2StructCheck analyses a file, it first parses the top-level box structure, and collects the unique identifiers (or marker codes) of all boxes. If it encounters the box that contains the code stream, it checks if the code stream is terminated by a valid end-of-codestream marker. Finally, it checks if the file contains all the compulsory/required top-level boxes. These are:
- JPEG 2000 signature box
- File Type box
- JP2 Header box
- Contiguous Codestream box
In order to test the box checking mechanism I did some additional image butchering, where I deliberately changed the tags of existing boxes so that they wouldn’t be recognised. When I subsequently ran these images through JHOVE, this revealed some additional surprises. For instance, after changing the markers of the Contiguous Codestream box or even the JP2 Header box (which effectively makes them unrecognisable), JHOVE would still report these images as “well-formed and valid” (although in the case of the missing JP2 Header box JHOVE did report an error).
Limitations of jp2StructCheck
It is important to note here that jp2StructCheck only checks the top-level boxes. In case of a superbox (which is a box that contains child boxes), it does not recurse into its child boxes. For example, it does not check if a JP2 Header box (which is a superbox) contains a Colour Specification box (which is required by the standard). So the scope of the tool is limited to a rather superficial check of the general file structure. It is not a JP2 validator, and it is certainly not a replacement for JHOVE (which performs a more in-depth analysis)! The main scope is to be able to detect certain types of file corruption that may occur as a result of hardware failure (e.g. network interruptions) during the creation of an image.
In addition, the fact that a code stream is terminated by and end-of-codestream marker is no guarantee that the code stream is complete. For instance, if due to some hardware failure some part of the middle of the codestream is not written, jp2StructCheck will not detect this! It may be possible to improve the level of error detection by including additional codestream markers. This is something I might have a look at at some later point.
I created a Github repository that contains the source code of jp2StructCheck, some documentation, and a small data set with some test images.
As some people may not want to install Python on their system, I also created a binary distribution that should work on most Windows systems.
The documentation (in PDF format) is here.
Finally, use this link to download the test images.
I’m curious to hear if anyone finds jp2StructCheck useful at all, so please feel free to use the comment fields below for your feedback (including reports on any bugs that may exist).
Update (March 2012)
By now jp2StructCheck which is a full-fledged JP2 validator. See this more recent blog post for details.
Johan van der Knijff
KB / National Library of the Netherlands