TIFF 6.0 Specification
Final—June 3, 1992
Comments for TIFF Readers
TIFF readers that send a grayscale image to a halftone output device, whether it is
a binary laser printer or a PostScript imagesetter should make an effort to maintain
the highlight and shadow placement. This requires two steps. First, determine the
highlight and shadow gray level of a particular image. Second, communicate that
information to the halftone engine.
To determine the highlight and shadow gray levels, begin by looking for a
HalftoneHints field. If it exists, it takes precedence. The first word represents the
gray level of the highlight and the second word represents the gray level of the
shadow. If the image is a colorimetric image (i.e. it has a GrayResponseCurve
field or a TransferFunction field) but does not contain a HalftoneHints field, then
the gamut mapping techniques described earlier should be used to determine the
highlight and shadow values. If neither of these conditions are true, then the file
should be treated as if a HalftoneHints field had indicated a highlight at gray level
1 and a shadow at gray level 2**BitsPerPixel-2 (or vice-versa depending on the
PhotometricInterpretation field). Once the highlight and shadow gray levels have
been determined, the next step is to communicate this information to the halftone
module. The halftone module may exist within the same application as the TIFF
reader, it may exist within a separate printer driver, or it may exist within the
Raster Image Processor (RIP) of the printer itself. Whether the halftone process is
a simple dither pattern or a general purpose spot function, it has some gray level at
which the lightest printable tint will be rendered. The HalftoneHint concept is best
implemented in an environment where this lightest printable tint is easily and
consistently specified.
There are several ways in which an application can communicate the highlight
and shadow to the halftone function. Some environments may allow the applica-
tion to pass the highlight and shadow to the halftone module explicitly along with
the image. This is the best approach, but many environments do not yet provide
this capability. Other environments may provide fixed gray levels at which the
highlight and shadow will be rendered. For these cases, the application should
build a tone map that matches the highlight and shadow specified in the image to
the highlight and shadow gray level of the halftone module. This approach re-
quires more work by the application software, but will provide excellent results.
Some environments will not have any consistent concept of highlight and shadow
at all. In these environments, the best an application can do is characterize each of
the supported printers and save the observed highlight and shadow gray levels.
The application can then use these values to achieve the desired results, providing
the environment doesn’t change.
Once the highlight and shadow areas are selected, care should be taken to appro-
priately map intermediate gray levels to those expected by the halftone engine,
which may or may not be linear Reflectance. Note that although CCDs are linear
intensity detectors and many TIFF files are stored as linear intensity, most output
devices require significant tone compensation (sometimes called gamma correc-
tion) to correctly display or print linear data. Be aware that the
PhotometricInterpretation value of 0, 1 implies linear data because no gamma is
specified. The PhotometricInterpretation value of 2 (RGB data) specifies the
NTSC gamma of 2.2 as a default. If a GrayResponseCurve field or a
TransferFunction field is present, it may define something other than the default.
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