It’s been more than two years now since I wrote my D-Lib paper JPEG 2000 for Long-term Preservation: JP2 as a Preservation Format. From time to time people ask me about the status of the issues that are mentioned in that paper, so here’s a long overdue update.
Issues addressed in the 2011 paper
- The specification was overly restrictive on the embedding of ICC profiles. By only allowing input profiles, this ruled out the use of display profiles. In practice this meant that widely-used working colour spaces such as Adobe RGB and eciRGB 2 could not be used in JP2 without violating the standard.
- JP2 makes a distinction between capture resolution and default display resolution, which are stored in two designated sets of header fields. However, the specification was not clear in which case either set of fields should be used.
This lead to the situation that not all software products were interpreting the specification in the same way. For instance, some encoders would (silently) produce files in JPX format whenever they encountered an input image with an embedded display ICC profile. Other encoders would embed the profile, changing the profile class in the process, whereas others yet would ignore the limitation altogether and embed the profile without complaining. Similarly, some software products would only write (and read) the capture resolution fields (while ignoring any default display ones), whereas the opposite was true for other products. This in turn raised various interoperability issues, many of which are potential risks in a long-term preservation context.
The 2011 paper concluded that "[t]hese issues could be remedied by some small adjustments of JP2’s format specification, which would create minimal backward compatibility problems, if any at all".
Amendment to the standard
So, enter the amendment "Updated ICC profile support and resolution clarification", which was published by ISO earlier this year. This amendment remedies the above issues by applying the following changes to the existing JP2 format specification:
- The Restricted ICC profile method now permits the use of display profiles (previously only input profiles were allowed). The other restrictions (e.g. that ICC profiles should be of either the Monochrome or the Three-Component Matrix-Based type) remain unchanged.
- It is more specific about the intended uses of the capture and the default display resolution boxes. Of particular interest here is that capture resolution now reflects the resolution at which the image samples were "captured or created". Previously the word "digitized" was used, which ruled out the case of born-digital materials. The use of the default display resolution box is also further clarified. In practice this means that the capture resolution is pretty much equivalent to the XResolution /YResolution fields in TIFF.
Note that the full amendment text is only available after purchase at ISO (previously an earlier draft was available for free, but apparently it was taken down recently).
Implementation of changes
Since a standard isn’t worth much unless it is used, let’s have a quick look at the three most popular JPEG 2000 implementations (a more elaborate overview is available here and here at the OPF File Format Risk Registry).
Recent versions of Kakadu‘s kdu_compress are now able to correctly handle ICC profiles. Compressing a TIFF that contains an ICC profile that meets the (updated) restricted ICC profile definition now produces a JP2 with the profile correctly embedded. Moreover, kdu_compress now uses the capture resolution fields to store the image’s resolution (as derived from the TIFF). Previously, the Kakadu demo applications were using the default display fields instead, which resulted in various interoperability issues, because most other decoders/encoders were using the capture fields. This is all solved in the latest version.
By default Aware and Luratech already used the capture resolution fields back in 2011, and this behaviour is now consistent with the updated standard. As for ICC profiles, Aware accepted display profiles without complaining in my 2011 tests, and with the amendment in effect, these images are now also valid JP2. Luratech used to handle display profiles by changing the profile class field tom input. That was in 2011, and I don’t know if anything has changed since then, but then again this behaviour never caused by problems in the first place.
Round-up and conclusions
The amendment to JP2 fixes the previous shortcomings that were mentioned in my 2011 D-Lib paper. Moreover, the behaviour of the three most popular (commercial) JPEG 2000 implementations now closely follows the updated specification, which should minimise any interoperability problems related to ICC profiles and resolution.