TIFF 6.0 Specification
Final—June 3, 1992
Some Background on the Halftone Process
To obtain the best results when printing a continuous-tone raster image, it is sel-
dom desirable to simply reproduce the tones of the original on the printed page.
Most often there is some gamut mapping required. Often this is because the tonal
range of the original extends beyond the tonal range of the output medium. In
some cases, the tone range of the original is within the gamut of the output me-
dium, but it may be more pleasing to expand the tone of the image to fill the range
of the output. Given that the tone of the original is to be adjusted, there is a whole
range of possibilities for the level of sophistication that may be undertaken by a
software application.
Printing monochrome output is far less sophisticated than printing color output.
For monochrome output the first priority is to control the placement of the high-
light and the shadow. Ideally, a quality halftone will have sufficient levels of gray
so that a standard observer cannot distinguish the interface between any two adja-
cent levels of gray. In practice, however, there is often a significant step between
the tone of the paper and the tone of the lightest printable tint. Although usually
less severe, the problem is similar between solid ink and the darkest printable tint.
Since the dynamic range between the lightest printable tint and the darkest print-
able tint is usually less than one would like, it is common to maximize the tone of
the image within these bounds. Not all images will have a highlight (an area of the
image which is desirable to print as light as possible while still retaining tonal
detail). If one exists, it should be carefully controlled to print at the lightest print-
able tint of the output medium. Similarly, the darkest areas of the image to retain
tonal detail should be printed as the darkest printable tint of the output medium.
Tones lighter or darker than these may be clipped at the limits of the paper and
ink. Satisfactory results may be obtained in monochrome work by doing nothing
more than a perceptually-linear mapping of the image between these rigorously
controlled endpoints. This level of sophistication is sufficient for many mid-range
applications, although the results often appear flatter (i.e. lower in contrast) than
desired.
The next step is to increase contrast slightly in the tonal range of the image that
contains the most important subject matter. To perform this step well requires
considerably more information about the image and about the press. To know
where to add contrast, the algorithm must have access to first the keyness of the
image; the tone range which the user considers most important. To know how
much contrast to add, the algorithm must have access to the absolute tone of the
original and the dynamic range of the output device so that it may calculate the
amount of tone compression to which the image is actually subjected.
Most images are called normal key. The important subject areas of a normal key
image are in the midtones. These images do well when a so-called “sympathetic
curve” is applied, which increases the contrast in midtones slightly at the expense
of contrast in the lighter and darker tones. White china on a white tablecloth is an
example of a high key image. High key images benefit from higher contrast in
lighter tones, with less contrast needed in the midtones and darker tones. Low key
images have important subject matter in the darker tones and benefit from increas-
ing the contrast in the darker tones. Specifying the keyness of an image might be
attempted by automatic techniques, but it will likely fail without user input. For
example, a photo of a bride in a white wedding dress it may be a high key image if
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