Ensuring the suitability of JPEG 2000 for preservation
In my presentation during the Wellcome Trust’s JPEG 2000 seminar I discussed the suitability of JPEG 2000 (and more specifically its JP2 format) for long-term preservation. I highlighted the erroneous restriction in the JP2 (and JPX) format specification that only allows ICC profiles of the ‘input’ class to be used. This effectively prohibits the use of all working colour spaces such as Adobe RGB, which are defined using ‘display device’ profiles. I also showed how different software vendors interpret the format specification in subtly different ways, and how such issues can create problems in the long term, such as the loss of colour space and resolution information after some future migration.
This leads us to the question to which extent we can predict a specific file format’s suitability for long-term preservation. The answer is not that straightforward. The Library of Congress assesses file formats against 7 ‘sustainability factors’, whereas the National Archives have formulated a list of 12 criteria. It is beyond the scope of this blog post to present a detailed analysis of the extent to which JP2 lives up to either set of criteria. However, it is interesting to have a look at whether these criteria could have been helpful in identifying the issues covered by my presentation.
First, both the LoC’s ‘sustainability factors’ and the TNA criteria acknowledge the importance of having published specifications of a file format. The LoC uses a ‘Disclosure’ factor, which refers to “the existence of complete documentation, preferably subject to external expert evaluation”. TNA take this one step further by also defining a ‘Documentation Quality’ criterion, which expresses the degree to which documentation is comprehensive, accurate and comprehensible. This last criterion largely covers the JPEG 2000 ICC issue, although it’s questionable how useful this would have been to identify it a priori. A problem with errors and ambiguities in format specifications is that they can be incredibly easy to overlook, and you may only become aware of them after discovering that different software products interpret the specifications in slightly different ways.
Formats that are widely used are typically well supported by an array of software tools, and such formats are unlikely to disappear into obsolescence. TNA expresses this through a ‘Ubiquity’ criterion, which essentially reflects a file format’s overall popularity. The definition of the LoC’s ‘Adoption’ factor includes a list of criteria that can be used as “evidence of adoption”. The first set of criteria here includes “bundling of tools with personal computers, native support in Web browsers or market-leading content creation tools, and the existence of many competing products for creation, manipulation, or rendering of digital objects in the format”. Note that JP2 isn’t doing particularly well when measured against any of these criteria. However, the LoC list adds that “a format that has been reviewed by other archival institutions and accepted as a preferred or supported archival format also provides evidence of adoption”. This certainly seems to be the case for JP2. But how relevant is this, really? Going back to the ICC profiles issue: the JP2 file format has been around for about 10 years now, and its acceptance by the archival community has been growing steadily over the last 5 years or so. Yet, this whole issue seems to have gone unnoticed in the archival community for all those years, and I think this is slightly worrying.
Now let’s imagine for a moment that JP2 would have been picked up by the digital photography and graphic design communities. For such uses the ability to do proper colour management is a basic prerequisite, and limiting the support of ICC profiles to the ‘input’ class would have made the format virtually useless to these user communities. My guess is that in this -entirely fictional- scenario, the format specification would have either improved quickly (based on feedback from the user community), or the respective user communities would have simply stopped using the format altogether. The problem here seems to be that very few people in the archiving community are even aware of such things as colour spaces and colour management, let alone their importance within the context of preservation. With more established formats such as TIFF this may not be as much of a problem, if only because TIFF has been ‘road tested’ for decades by the photography and graphic design communities. As an archiving community we cannot fall back to any similar ‘road testing’ in the case of JP2. And this brings me to my next point.
Importance of hands-on experience
Preservation criteria such as those of the LoC or TNA are invaluable for assessing the suitability of a format for preservation, but I believe it is equally important to have actual hands-on experience with the tools that are used for creating, modifying, and reading the format. For instance, the TNA criteria use the number of software tools that support a given format as an indicator for the extent of current software support of that format. But knowing the number of tools says nothing about how good or useful these tools actually are! In the case of JP2, quite a large number of (mostly free or open-source) tools exist that, under the hood, are using the open JasPer library. JasPer is known to have performance and stability issues that make it unsuitable for most professional applications (for which, I should emphasise, it was never developed in the first place!). These issues affect all software tools that are using JasPer. So, only counting the number of available tools may be simply missing the point without incorporating any additional quality criteria. But how would you define these?
Part of the answer, I think, is that assessing a format’s suitability for long-term preservation is not a purely top-down process. Most of the software-related issues that I showed in my presentation were found by simply experimenting with actual files, encoders and characterisation tools: convert a TIFF to JP2; convert it back to TIFF; use existing metadata-extraction and characterisation tools such as ExifTool and JHOVE to analyse the in- and output files; try to understand the output of these tools; compare the output before and after the conversion, and so on. Such experiments are extremely useful for getting a feel for the strengths and weaknesses of specific software tools, and they can reveal problems that are not readily captured by pre-defined criteria. In some cases, their results may be used to refine existing criteria, or even add new ones.
Final notes on preservation criteria
Although I wouldn’t downplay the importance of preservation criteria such as those used by the LoC or TNA, I think it’s important to realise that such criteria are largely based on theoretical considerations. In most cases they are not based on any empirical data, and as a result their predictive value is largely unknown. For example, an interesting blog post by David Rosenthal argues that preserving the specifications of a file format doesn’t contribute anything to practical digital preservation. According to Rosenthal, the availability of working open-source rendering software is much more important, and he explains how “formats with open source renderers are, for all practical purposes, immune from format obsolescence”.
This takes us directly to the lack of JPEG 2000-related activity in the open source community, which I also referred to in my presentation. Perhaps the best way to ensure sustainability of JPEG 2000 and the JP2 format would be to invest in a truly open JP2 software library, and release this under a free software license. This could either take the form of the development of a completely new library, or investing in the improvement and further development of an existing one, such as OpenJPEG. This would require an investment from the archival community, but the payoff may be well worth it.
This blog entry was largely inspired by an e-mail discussion that was started by Richard Clark, and in particular by a contribution to this discussion by William Kilbride.
Originally published at the Wellcome Library Blog