TIFF 6.0 Specification
Final—June 3, 1992
Section 17: HalftoneHints
This section describes a scheme for properly placing highlights and shadows in
halftoned images.
Introduction
The single most easily recognized failing of continuous tone images is the incor-
rect placement of highlight and shadow. It is critical that a halftone process be
capable of printing the lightest areas of the image as the smallest halftone spot
capable of the output device, at the specified printer resolution and screen ruling.
Specular highlights (small ultra-white areas) as well as the shadow areas should
be printable as paper only.
Consistency in highlight and shadow placement allows the user to obtain predict-
able results on a wide variety of halftone output devices. Proper implementation
of theHalftoneHints field will provide a significant step toward device indepen-
dent imaging, such that low cost printers may to be used as effective proofing
devices for images which will later be halftoned on a high-resolution imagesetter.
The HalftoneHints Field
HalftoneHints
Tag
= 321 (141.H)
Type = SHORT
N
=2
The purpose of the HalftoneHints field is to convey to the halftone function the
range of gray levels within a colorimetrically-specified image that should retain
tonal detail. The field contains two values of sixteen bits each and, therefore, is
contained wholly within the field itself; no offset is required. The first word speci-
fies the highlight gray level which should be halftoned at the lightest printable tint
of the final output device. The second word specifies the shadow gray level which
should be halftoned at the darkest printable tint of the final output device. Portions
of the image which are whiter than the highlight gray level will quickly, if not
immediately, fade to specular highlights. There is no default value specified, since
the highlight and shadow gray levels are a function of the subject matter of a par-
ticular image.
Appropriate values may be derived algorithmically or may be specified by the
user, either directly or indirectly.
The HalftoneHints field, as defined here, defines an achromatic function. It can be
used just as effectively with color images as with monochrome images. When
used with opponent color spaces such as CIE L*a*b* or YCbCr, it refers to the
achromatic component only; L* in the case of CIELab, and Y in the case of
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