TIFF 6.0 Specification
Final—June 3, 1992
YCbCr. When used with tri-stimulus spaces such as RGB, it suggests to retain
tonal detail for all colors with an NTSC gray component within the bounds of the
R=G=B=Highlight to R=G=B=Shadow range.
Comments for TIFF Writers
TIFF writers are encouraged to include the HalftoneHints field in all color or
grayscale images where BitsPerSample >1. Although no default value is speci-
fied, prior to the introduction of this field it has been common practice to implic-
itly specify the highlight and shadow gray levels as 1 and 2**BitsperSample-2
and manipulate the image data to this definition. There are some disadvantages to
this technique, and it is not feasible for a fixed gamut colorimetric image type.
Appropriate values may be derived algorithmically or may be specified by the
user directly or indirectly. Automatic algorithms exist for analyzing the histogram
of the achromatic intensity of an image and defining the minimum and maximum
values as the highlight and shadow settings such that tonal detail is retained
throughout the image. This kind of algorithm may try to impose a highlight or
shadow where none really exists in the image, which may require user controls to
override the automatic setting.
It should be noted that the choice of the highlight and shadow values is somewhat
output dependent. For instance, in situations where the dynamic range of the
output medium is very limited (as in newsprint and, to a lesser degree, laser out-
put), it may be desirable for the user to clip some of the lightest or darkest tones to
avoid the reduced contrast resulting from compressing the tone of the entire im-
age. Different settings might be chosen for 150-line halftone printed on coated
stock. Keep in mind that these values may be adjusted later (which might not be
possible unless the image is stored as a colorimetric, fixed, full-gamut image), and
that more sophisticated page-layout applications may be capable of presenting a
user interface to consider these decisions at a point where the halftone process is
well understood.
It should be noted that although CCDs are linear intensity detectors, TIFF writers
may choose to manipulate the image to store gamma-compensated data. Gamma-
compensated data is more efficient at encoding an image than is linear intensity
data because it requires fewer BitsPerPixel to eliminate banding in the darker
tones. It also has the advantage of being closer to the tone response of the display
or printer and is, therefore, less likely to produce poor results from applications
that are not rigorous about their treatment of images. Be aware that the
PhotometricInterpretation value of 0 or 1 (grayscale) 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 data is written as something
other than the default, then a GrayResponseCurve field or a TransferFunction
field must be present to define the deviation. For grayscale data, be sure that the
densities in the GrayResponseCurve are consistent with the
PhotometricInterpretation field and the HalftoneHints field.
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