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Back to Issue 431
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President's Report
Robert Buckley ISCC President
In this column I usually write about ISCC news and upcoming meetings. However, you can read about the fall Baltimore meetings and the “Black and White” Meeting in Portland elsewhere in this issue or on our re-designed website. And the new Board candidates will be featured in the next issue.
So instead I will write about archival color, one of my current interests in color and part of a broader interest in the long-term preservation of digital images and documents.
One focus of this interest is the CIE Technical Committee on Archival Color Imaging. This committee is part of CIE Division 8 on Imaging Technology, which was established in 1998. The first round of committees in Division 8 looked at topics such as gamut mapping, color appearance, color encoding and fluorescence, all in the context of images and imaging media. I chaired the committee on the Communication of Color Information, which issued a report on color encodings. With Archival Color, my interest has shifted to the communication of color information into the future.
The Archival Color committee, formally known as CIE TC8-09, is one of the second set of committees formed in the Division. According to its terms of reference, the committee is "to recommend a set of techniques for the accurate capture, encoding and long-term preservation of color descriptions of digital images that are either born digital or the result of digitizing 2D static physical objects, including documents, maps, photographic materials and paintings." Pretty broad.
"Accurate" implies that the born digital image or the digitized object is the reference and that the objective is being able to recreate it or an acceptable facsimile of it in the future. This leaves aside the question of restoration or other interpretation based on the digital material. An interesting question though is what does accurate capture of a black-and-white or color negative mean? Presumably it means capturing the tone range of original analog material and with it all the rendering options offered by it.
While capture puts the emphasis on input rather than output, the method of capture and subsequent encoding enables future rendering options and should therefore not be limited to current rendering options. It is here that questions of multiple views, goniometric methods and spectral versus trichromatic measurements come into play. For example, conference papers in recent years have discussed goniometric imaging systems to capture surface texture in objects and spectral systems to capture surface reflectance in objects with multiple pigments or inks. However for reasons that have to do with feasibility and cost, trichromatic capture and encodings are still the norm.
The question then becomes selecting an encoding that will capture the range of colors in the original without loss. “Without loss” means that colors that are distinguishable in the original are distinguishable in the description and can be recreated, perhaps allowing for some round-off errors. Therefore gamut compression is not part of the capture process.
Different materials can have different ranges of colors, and a number of RGB-based color encoding have been proposed over the years. Familiar ones are sRGB and its follow-ons. Adobe RGB 98 and Pro Photo RGB, all of which are output encodings with defined viewing conditions. (Pro Photo RGB has an analogous input encoding.) The progression has been to grow the size of the color space and the range of colors that can be encoded. The usual tradeoff is covering as many colors as possible or as required versus using as many bits as can be afforded. In the limit is XYZ, which can cover all physically realizable colors. XYZ with a 12-bit non-linear encoding is what Digital Cinema uses for distributing digital movies to theaters.
All these choices are made in the context of long-term preservation. According to the Open Archival Information System (OAIS) Reference Model, a widely-cited recommendation in the field of digital preservation, long term is “long enough to be concerned with the impacts of changing technologies, including support for new media and data formats, or with a changing user community.” The goal is to make choices that have a high degree of sustainability and that can survive changing technologies or minimize their dependence on them.
With all this as background, what the Archival Color committee is doing now is focusing on a few specific applications, both to narrow the scope and test the approach. Interested in learning more or contributing to the discussion? Feel free to contact me.
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