Digital Representation and Compression of William Blake's Hand-Colored Engravings
Vladimir Misic, University of Rochester, USA
Kari Kraus, University of Rochester, USA
http://henry.ee.rochester.edu:8080/~misicv/CTLTR/CTLTR.PPT

Mini-Workshop: Image File Formats

This paper demonstrates and discusses the application of Mixed Raster Content (MRC) compression scheme to digital reproductions of William Blake's hand-colored etchings and engravings. Although Blake is our focus, much of what we have to say and show can be extrapolated to the graphic arts more generally. MRC, for example, has the potential to transform the way we study and represent different states of an engraved plate, and consequently could help retool the genre of the catalogue raisonne for the digital age. More broadly, it is our feeling that the image processing advances presented here will be of interest to humanists working in a variety of image-intensive areas and disciplines, ranging from art history to conservation science to editorial theory. Although we want to underscore the versatility of the technology, the paper takes a case study approach, considering the achievement of MRC within the history of Blake reproduction and editing.

Originally developed by Xerox Corporation as a compression scheme to efficiently combine different compression requirements for text and image content in scanned digital documents, MRC is for the first time being deployed to the compression of non-compound documents. Unlike the first installment of the technology, which was a response to composite text and image data, the version of MRC being developed by the co-author is designed with the inherent compositeness of colored prints and other graphic media in mind. The documents we are dealing with, then, are still "compound" in the broader, non-technical sense of the term, but the semiological analysis is performed entirely at the level of the image, exclusive of a consideration of text.

Used in conjunction with the new JPEG 2000 compression standard, MRC enables not only image compression but also the accurate discrimination of printed intaglio lines from finishing work in colored prints, paving the way for separate processing of each. Whereas other compression models treat the formal content of an image as invariant, MRC is sensitive to the heterogeneous character of the pictorial composition as a whole. By treating printed (engraved and/or etched) lines separately from colored image layers, MRC minimizes image degradation and the inevitable loss of information. The result is superior raster imagery: digital facsimiles that show exceptional fidelity to their prototypes, as a comparison between an original scan of one of Blake's commercial print illustrations and its MRC reconstruction makes evident. Because applied segmentation can essentially lift the color overlay of an impression, it additionally provides the student of Blake the unique opportunity to recreate the underlying copperplate image and model the artist's coloring process.

A visual demonstration of MRC can be found at (http://henry.ee.rochester.edu:8080/~misicv/CTLTR/CTLTR.PPT) .