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498
CHAPTER 6 Rendering
KEY TYPE VALUE
TransferFunction function or name (Optional) A transfer function, which overrides the current transfer
function in the graphics state for the same component. This entry is re-
quired if the dictionary is a component of a type 5 halftone (see “Type
5 Halftones” on page 505) and represents either a nonprimary or non-
standard primary color component (see Section 6.3, “Transfer Func-
tions”). The name Identity may be used to specify the identity
function.
If the AccurateScreens entry has a value of true, a highly precise halftoning algo-
rithm is substituted in place of the standard one. If AccurateScreens is false or not
present, ordinary halftoning is used. Accurate halftoning achieves the requested
screen frequency and angle with very high accuracy, whereas ordinary halftoning
adjusts them so that a single screen cell is quantized to device pixels. High accu-
racy is important mainly for making color separations on high-resolution devic-
es. However, it may be computationally expensive and therefore is ordinarily
disabled.
In principle, PDF permits the use of halftone screens with arbitrarily large cells—
in other words, arbitrarily low frequencies. However, cells that are very large
relative to the device resolution or that are oriented at unfavorable angles may ex-
ceed the capacity of available memory. If this happens, an error occurs. The
AccurateScreens feature often requires very large amounts of memory to achieve
the highest accuracy.
Example 6.1 shows a halftone dictionary for a type 1 halftone.
Example 6.1
28 0 obj
<< /Type /Halftone
/HalftoneType 1
/Frequency 120
/Angle 30
/SpotFunction /CosineDot
/TransferFunction /Identity
>>
endobj
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