SECTION 6.3
485
Transfer Functions
some other color space. The output is the transformed component value to be
transmitted to the device (after halftoning, if necessary).
Both the input and the output of a transfer function are always interpreted as if
the corresponding color component were additive (red, green, blue, or gray): the
greater the numeric value, the lighter the color. If the component is subtractive
(cyan, magenta, yellow, black, or a spot color), it is converted to additive form by
subtracting it from 1.0 before it is passed to the transfer function. The output of
the function is always in additive form and is passed on to the halftone function
in that form.
In PDF 1.2, transfer functions are defined as PDF function objects (see Section
•
The
current transfer function
parameter in the graphics state consists of either a
single transfer function or an array of four separate transfer functions, one each
for red, green, blue, and gray or their complements cyan, magenta, yellow, and
black. (If only a single function is specified, it applies to all components.) An
RGB
device uses the first three, a monochrome device uses the gray transfer
function only, and a
CMYK
device uses all four. The current transfer function
can be specified as the value of the
TR
or
TR2
entry in a graphics state parameter
dictionary; see Table 4.8 on page 220.
•
The
current halftone
parameter in the graphics state can specify transfer func-
tions as optional entries in
halftone dictionaries
(see Section 6.4.4, “Halftone
color components or for any component in devices whose native color space
uses components other than the ones listed above. A transfer function specified
in a halftone dictionary overrides the corresponding one specified by the cur-
rent transfer function parameter in the graphics state.
In addition to their intended use for gamma correction, transfer functions can be
used to produce a variety of special, device-dependent effects. For example, on a
monochrome device, the PostScript calculator function
{ 1 exch sub }
inverts the output colors, producing a negative rendition of the page. In general,
this method does not work for color devices; inversion can be more complicated
than merely inverting each of the components. Because transfer functions pro-