Display-screen Viewing and Printed Output
The students and researchers who use Library of Congress collections online desire the ability to view the images on their computer display screens and to print copies, typically on a laser printer. Most students and researchers use current-generation color-capable display systems with resolutions of 1024 x 768 or 1280 x 1024 pixels; their printers are likely to be capable of printing at settings of 300 or 600 dpi.
For the foreseeable future, access to Library of Congress collections will be provided using software associated with the World Wide Web protocols for Internet.
Informal experiments by the Library of Congress suggest that the image type that works best for display may not be the type that works best for printing. Display systems often produce the greatest legibility (and thus the best results) with a grayscale image. But printers often do best with a bitonal image. When a grayscale image is printed it must be "halftoned" and this tends to break up small features like fine print.
Generally speaking, students and researchers using Library of Congress text-based (as opposed to pictorial) collections place greater importance on the printed output than on screen display. They do not always view a document page as an end in itself, but typically will use the information that they find in the documents when they write their own articles or reports. Although some researchers may "carry away the document" on a floppy disk, most will prefer to print it and carry away a sheet of paper for later reference.
In past paper-scanning projects, the preference for printed output over screen display has led the Library to favor bitonal images. More recent explorations, however, have shown that a laser printer's representation of a grayscale image can be very good. In one informal experiment, for example, some manuscript pages were scanned from the original paper at 150 and 300 dpi. Using graphic-arts software, the laser printer was set for 600 dpi output (which affected the way in which the halftoning occurred) and the resulting paper copy was very legible.
The Library wishes to create images that will display and print successfully for researchers working in contexts like those described above, with the greatest emphasis placed on successful printing.
Scaling at Output Time, Capture Resolution for High-detail Content, and Capture of Large Pages in Segments
The researchers who access Library of Congress collections via Internet employ a variety of software packages, ranging from modest freeware associated with World Wide Web browsers to sophisticated graphic arts software for image handling. With varying degrees of effectiveness, this software scales (changes the sizes of) the images at display and print time. As noted above, the Library findings thus far suggest that, for documents (as distinct from pictorial matter), printed output is of greater importance to users than screen display. Typically, a researcher's personal computer will have a laser printer as a peripheral device; the Library's digital images must be conveniently printed within such a system.
The Library recognizes the emergent state of software associated with the World Wide Web and it well aware of the shortage of available tools for certain image types, especially viewing and printing software appropriate for bitonal images, especially bitonal images with TIFF headers and ITU group 4 compression. In fact, the Library is planning to make a special arrangement to offer viewing and printing software for this purpose to researchers who wish to use Library collections via the World Wide Web.
Although some researchers wishing to print document images may be limited to software like that described in the preceding paragraph, many others will have additional graphic arts or other software (not intended for use "within" the World Wide Web environment) capable of handling raster-scanned images.
A problem arises, however, in the case of fine print, especially when large page dimensions and fine print co-occur. In these cases, the contractor shall create segmented images of the large pages so that each segment, when scaled and printed in the manner described above, shall remain legible. These images may be created by scanning parts of the document separately or by capturing one large image and dividing it into segments in post-processing. Each segment shall be captured with an overlap that repeates on the joining images. In the latter case, the deliverables to the Library shall be the large image and all of the segments.
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