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Document:	ULTRIX Worksystem Software Color Extensions for the Display PostScript® System
Order Number:	AA-PANAA-TE
Revision:	000
Pages:	31
Original Filename:

OCR Text

COLOR
EXTENSIONS
Order No. AA-PANAA-TE

ADOBE SYSTEMS
INCORPORATED

POSTSCRIPI' Language Color Extensions
October 25, 1989
Copyright © 1988, 1989 by Adobe Systems Incorporated.
All rights reserved.
POSTSCRIPT is a registered trademark of Adobe Systems
Incorporated.
The infonnation in this document is furnished for infonnational use
only, is subject to change without notice, and should not be construed
as a commitment by Adobe Systems Incorporated. Adobe Systems
Incorporated assumes no responsibility or liability for any errors or
inaccuracies that may appear in this document The software described
in this document is furnished under license and may only be used or
copied in accordance with the tenns of such license.
No part of this publication may be reproduced, stored in a retrieval
system, or transmitted in any fonn or by any means, electronic,
mechanical, recording, or otherwise, without the prior written
permission of Adobe Systems Incorporated.
This document replaces the previous version dated October 3, 1988.

1
2
3

About This Manual 1
About the POSTSCRIPT Language Color Extensions
New Features 3
3.1
CMYK Color Specification 3
3.2
Black Generation and Undercolor Removal 4
3.3
Direct CMYK Color Specification 6
3.4
Color Screens, Transfer Functions, and Images 7
The colorimage Operator 7
3.5
3.6
Color Implementations 12
Operators 13

Index 23

iii

1 ABOUT THIS MANUAL
This document contains:
• A description of the extensions to the POSTSCRIPT® language that support new color functionality.
• Detailed information on the associated color operators.
Section 2 provides a general introduction to color functionality
in the POSTSCRIPT language.
Section 3 discusses the extended color capabilities of the
POSTSCRIPT language.
Section 4 contains an alphabetical listing of descriptions for all
color operators that have been added to the POSTSCRIPT language.

2 ABOUT THE POSTSCRIPT LANGUAGE COLOR EXTENSIONS
The POSTSCRIPT language has been extended to provide more
complete color functionality. This includes cyan-magentayellow-black (CMYK) color specification, black generation and
undercolor removal functions, screen and transfer functions for
four separate color components, and extension of the image concept to a colorimage operator that accepts mUltiple color components.
Earlier versions of the POSTSCRIPf language support color using
the setrgbcolor and sethsbcolor operators, which enable the
POSTSCRIPT interpreter to paint filled regions, strokes, image
masks, and characters in color. On black-and-white machines,
these operators generate an equivalent gray shade, which is
printed or displayed.
To support color more fully, the POSTSCRIPT language has been
extended to provide the following functions:
• Most significantly, multiple color images: the color image
operator renders a multiple color image; it functions
analogously to the the image operator, but uses red-greenblue (RGB) or cyan-magenta-yellow-black (CMYK) color
input and generates full-color output.

2 ABOUT THE POSTSCRIPT LANGUAGE COLOR EXTENSIONS

• Halftone screen defmitions: the setcolorscreen operator
specifies halftone screen defmitions for red, green, blue,
and gray, or cyan, magenta, yellow, and black concurrently;
it is the logical expansion of setscreen as it takes the same
three arguments to defme each screen for each printing ink.
• Color correction: the setcolortransfer operator sets the
transfer function parameters for red, green, blue, and gray;
it is an expansion of settransfer to four color components.
The setblackgeneration operator provides a black generation function that establishes a black component from a
cyan, magenta, and yellow specification.
The
setundercolorremoval operator provides undercolor
removal from the cyan, magenta, and yellow components to
compensate for the addition of black by the black generation function.
• CMYK color specification: the setcmykcolor operator allows the user to set the current color in the graphics state to
a cyan-magenta-yellow-black color directly, bypassing the
color correction operators.
The POSTSCRIPT language supports one-color, three-color, and
four-color output devices. The color devices can be binary (onebit-per-pixel per color component) or gray-scale (multiple-bitsper-pixel per color component, representing a range of intensities
of each color component). A binary device uses halftoning to
produce intermediate shades of its color components. If a device
has eight-bits-per-pixel per component, it is called a full
gray-scale device, and uses no halftoning. Devices with more
than one and fewer than eight bits per pixel use a combination of
built-in intensities and halftoning to produce the full range of
desired shades of their color components. Three-color devices
may be either red-green-blue (RGB), typically for displays and
film recorders, or cyan-magenta-yellow (CMY) for printers.
Four-color devices are cyan-magenta-yellow-black (CMYK) for
color printers and color separation making devices.
The color operators described in this document are available in
two forms. New versions of the POSTSCRIPT interpreter have
these operators built in. For older versions of the POSTSCRIPT
interpreter, a package of POSTSCRIPT language programs that
emulate these operators will be available.

POSTSCRIPT Language Color Extensions

3 NEW FEATURES
The color extensions to the POSTSCRIPT language include the
CMYK color specification, black generation, and undercolor
removal for tunable conversion from RGB to CMYK color
specification, and operators that expand the scope of screens,
transfer functions, and images to full color input and output.
These new color features are described conceptually in the sections below.

3.1

CMYK COLOR SPECIFICATION
Colors are formed either by adding light to black or by subtracting light from white. Computer displays and film recorders typically add colors, while printing inks typically subtract colors.
These two methods for forming colors give rise to the two major
complementary color specifications, the additive RGB specification and the subtractive CMYK specification.
Accordingly, a color component in these specifications either indicates the amount of light it reflects or the amount of light it
absorbs. Each one of three standard printing process colors,
cyan, magenta, and yellow, absorb one of the standard light components, red, green, and blue, respectively. Black, a fourth standard printing process color, absorbs all components of light. In
the red-green-blue (RGB) color specification, each of its red,
green, and blue components is associated with a real number between 0.0 and 1.0, inclusive, where 0.0 represents dark (no light)
and 1.0 represents full light. In the cyan-magenta-yellow-black
(CMYK) color specification, each of the four components are
associated with real numbers between 0.0 and 1.0 inclusive,
where 0.0 represents full light (no ink), and 1.0 represents dark
(full ink).
The following equations demonstrate the relationship between
the RGB and CMYK color specifications. Since cyan is the absence of red light,

3 NEW FEATURES

cyan

= 1.0 - red.

Similarly,
magenta = 1.0 - green
and
yellow

= 1.0 - blue,

so that a color that is 0.2 red, 0.7 green, and 0.4 blue can also be
expressed as 1.0 - 0.2 =0.8 cyan, 1.0 - 0.7 = 0.3 magenta, and
1.0 - 0.4 = 0.6 yellow. To improve the fidelity of blacks and
grays, a fourth process color, black, is often available on color
printers. Just as red in the RGB specification is the opposite of
cyan in the CMYK specification, a black value is the opposite to
a POSTSCRIPT language gray value; that is,
black
3.2

= 1.0 - gray.

BLACK GENERATION AND UNDERCOLOR REMOVAL

Logically, cyan, magenta, and yellow are all that are needed to
generate a printing color completely. Thus an equal percentage
of cyan, magenta, and yellow should create the equivalent percentage of black. In reality, colored printing inks do not mix perfectly, and such-combinations often form dark brown shades instead. Thus, it is often desirable to substitute real black ink for
the mixed-black portion of a color to obtain a truer color rendition on a printer.
Black generation is the process of calculating the amount of
black to be used when trying to print a particular color. Undercolor removal is the process of reducing the amount of cyan,
magenta, and yellow components to compensate for the amount
of black that was added by the black generation. Flexibility in
performing these functions is important for achieving good
results under different printing conditions.
The setblackgeneration operator provides the functionality to
generate extra black, no black, or a black value equal to all or a
fraction of the minimum values of cyan, magenta and yellow
(see Section 4). Its argument is a procedure that takes one

POSTSCRIPT Language Color Extensions

numeric argument, the minimum value of user cyan, magenta,
and yellow color components, and returns a single numeric
result, the user black value (where a user color component value
is that specified in the POSTSCRIPT language program before application of the corresponding transfer function). This procedure
is automatically applied whenever setrgbcolor, the three-color
case of colorimage (see Section 3), or sethsbcolor specify a
color. This user black value is then mapped to a device black
value by applying the gray transfer function to its difference
from 1.0 and subtracting the result from 1.0 (see
setcolortransfer in Section 4). The black generation function is
not applied when setgray, setcmykcolor (see Section 4), or the
one- or four-color cases of colorimage specify colors. This computed black value is used only when outputting on four-color
output devices.
The setundercolorremoval operator (see Section 4) provides
functionality to remove some amount of color from each of the
cyan, magenta, and yellow components. This amount could be
exactly the same amount as was generated to make the black
component, zero (so no color is removed from the cyan, magenta
and yellow components), some fraction of the black amount, or
even a negative amount. Like setblackgeneration, this operator
permits considerable flexibility in color correction.
The argument to setundercolorremoval is a procedure that takes
one numeric argument, the minimum value of user cyan,
magenta, and yellow color components, and returns a single
numeric result that is subtracted from each of these original user
color components. This procedure is applied whenever
setrgbcolor, the three-color case of colorimage (see Section 4),
or sethsbcolor specify a color. After subtracting the value
generated in the above mapping from the color components and
resetting negative values to 0.0 and values greater than 1.0 to
1.0, each component is subtracted from 1.0 to yield red, green,
and blue components. Each of these components is mapped into
a device color component using its respective transfer function
(see setcolortransfer in Section 4). Undercolor removal is not
applied when setgray, setcmykcolor, or the one- or four-color
cases of colorimage specify a color. Undercolor removal is used
only when outputting on four-color devices.

3 NEW FEATURES

The following equations define the complete color transformation process from RGB to CMYK. The values redu' grnu'
bluu' gryu' cynu' magu' yelu' and blku are the (input) values of
user red, green, blue, gray, cyan, magenta, yellow, and black,
respectively. UCR() is the undercolor removal function. BG() is
the black generation function. RedT, GrnT, B luT, and GryT are
the red, green, blue, and gray transfer functions (see
setcolortransfer in Section 4), respectively. The values redd'
grnd' blud' gryd' cynd' magd, yeld, and blkd are the (output)
values of device red, green, blue, gray, cyan, magenta, yellow,
and black, respectively.
cynu = 1.0 - red u
mag u = 1.0 - grn u
yel u = 1.0 - blu u
gryu = 1.0
blku = 0.0
k = Min(cyn u' mag u' yel u)
u = UCR(k)
red d = RedT(1.0 - Min(1.0, Max(O.O, cyn u - u)))
grnd = GrnT(1.0 - Min(1.0, Max(O.O, mag u - u)))
blu d = BluT(1.0 - Min(1.0, Max(O.O, yel u - u)))
gryd = GryT(1.0 - BG(k))
cynd = 1.0 - red d
magd = 1.0 - grnd
yel d = 1.0 - blud
blkd = 1.0 - gry d

3.3

DIRECT CMYK COLOR SPECIFICATION
For the most demanding cases, color matching can require more
complicated methods than those described above. The
setcmykcolor operator and the four-color case of the
colorimage operator bypass the black generation and undercolor
removal operations, allowing the knowledgeable user to specify
the cyan, magenta, yellow, and black color components for a
particular device. This operator provides no correction other than
the transfer functions setcolortransfer specifies (see Section 4);
the results are device dependent.
The following equations define the complete color transformation process for the setcmykcolor operator and the four-color

POSTSCRIPT Language Color Extensions

case of the colorimage operator. The values are as defmed in
Black Generation and Undercolor Removal above. These equations are equivalent to those given above except for the omission
of the black generation and undercolor removal steps.
red u = 1.0 - cynu
grnu = 1.0 - mag u
blu u = 1.0 - yel u
gryu = 1.0 - blku
redd = RedT(red u)
grnd = GrnT(grn u)
blu d = BluT(blu u)
gryd = GryT(gry u)
cynd = 1.0 - redd
mag d = 1.0 - grnd
yeld = 1.0 - blu d
blkd = 1.0 - gry d

3.4

COLOR SCREENS, TRANSFER FUNCTIONS, AND IMAGES

The
operators
setcolorscreen,
currentcolorscreen,
setcolortransfer, and currentcolortransfer provide an expansion of the operators setscreen, currentscreen, settransfer, and
currenttransfer, respectively, by setting up a screen and a transfer function for each color component. The colorimage operator
provides an expansion of the image operator to allow samples of
one, three, or four color components. (See Section 4 for more
details.)
3.5

THE COLORIMAGE OPERATOR

colorimage is the logical expansion of image to handle sampled
images whose samples are composed of more than one color
component. The initial arguments to colorimage are the same as
those for image. The final arguments differ according to the
number of color components per sample and according to the
encoding method.
The arguments to colorimage are as follows:
width height bits/component matrix proc o [... proc nCO!ors_1] multiproc ncolors

3 NEW FEATURES

(See Section 5 for precise definitions of these arguments.)
ncolors describes the number of color components in each
sample. Legal values for ncolors are 1 (gray-level samples only),
3 (RGB samples), or 4 (CMYK samples). multiproc distinguishes between encoding methods. The possibilities for
multiproc are false (single procedure, color components are
bunched together) or true (mUltiple procedures, one per color,
components are separated into strings of like colors).
The legal variations of ncolors and multiproc allow the following possibilities (where proc subscripts have been changed to
words to indicate the purpose of each proc):

w h b/c matrix proc gray false 1
w h b/c matrix proc gray true 1
w h b/c matrix proc rgb false 3
w h b/c matrix proc red prOCgreen procb1ue true 3
w h b/c matrix proc cmyk false 4
w h b/c matrix proc cyan prOCmagenta ProCyellow procblack true 4
The fIrst two variations here are both equivalent to

w h bls matrix proc gray image
Data formats for coIorimage operator. As indicated above, the
coIorimage operator has two forms, distinguished by its
multiproc argument.
The single-procedure form is most useful if sample input is taken
from a source that has already merged the color components.
This form provides samples for which each RGB triple or
CMYK quadruple is packed together in the string result of the
procedure, using one of the following bit formats (where the
high-order bit is shown on the left):
Bits/

compo
1 RGBRGBRG
2 RRGGBBRR
4 RRRRGGGG
8 RRRRRRRR
or

POSTSCRIPT Language Color Extensions

RGB Format

BRGBRGBR GBRGBRGB
GGBBRRGG BBRRGGBB
BBBBRRRR GGGGBBBB
GGGGGGGG BBBBBBBB

RGBRGBRG
RRGGBBRR
RRRRGGGG
RRRRRRRR

Bltsl
compo
1
2
4
8

CMYK Format
CMYKCMYK
CCMMVYKK
CCCCMMMM
CCCCCCCC

CMYKCMYK
CCMMYYKK
YYYYKKKK
MMMMMMMM

CMYKCMYK
CCMMVYKK
CCCCMMMM
YYYYYYYY

CMYKCMYK
CCMMVYKK
YYYYKKKK
KKKKKKKK

The multiple-procedure form expects each procedure to return a
string of values for only one color component per sample, using
the same format as strings returned by the proc argument of the
image operator. For a three-color image, proco returns red
values, proc 1 returns green values, and proc2 returns blue values.
For a four-color image, proco returns cyan values, proc 1 returns
magenta values, proc2 returns yellow values, and proc3 returns
black values. The colorimage operator calls each of these
procedures in tum, starting with proco and continuing with
procl' proc2' and, if available, proc3. When the colorimage
operator needs more samples, it calls these procedures again in
the same order. The color procedures must use separate strings
for the three or four results of the three or four procedures, i.e.,
reusing the red string for the green values can result in some of
the red values being lost. Also, the three or four procedures must
return strings of identical lengths within each cycle of three or
four calls.
The multiple-procedure form is most useful when color sample
data are taken from separate color scanner passes. The
colorimage operator requires the color data to be interleaved, as
it must have all three or four components of any sample at the
same time to do its work. The single-procedure form interleaves
the data at the sample level; this may be convenient only if the
data are already in that form when preparing the POSTSCRIPT
language page description. The multiple-procedure form allows
interleaving at a much coarser level. Typically, each procedure
of the multiple-procedure form returns components for some
number of scan lines of samples, where the number of components returned at each call is limited by the string storage
available in the POSTSCRIPT interpreter.
Examples for colorimage operator. The following examples illustrate the use of the colorimage operator:

3 NEW FEATURES

EXAMPLE 1:

Irgbstr 192 string def

% string to hold 256 two-bit samples
% each of red, green, and blue data
45 140 translate
% locate lower left corner of image
132 132 scale
% map image to 132 point square
256 256 2
% dimensions of source image
[256 0 0 -256 0 256] % map unit square to source
{currentfile
% read image data from program file
rgbstr readhexstring pop}
% single proc, 3 colors, bit format:
false 3
% rrggbbrr ggbbrrgg bbrrggbb ...
colorimage
94a1 bec8cOb371 a3a5c4d281 ...
(98304 hex digits of image data)

The above is a one-procedure 2-bit RGB image. The base-4 representation of the hex data is

21102201 2332 ...
which is composed of the following color samples:

r=2 g= 1 b= 1 r=O g=2 b=2 r=O g= 1 b=2 r=3 g=3 b=2 ...
EXAMPLE 2:
Irstr 256 string def
Igstr 256 string def

% string to hold 256 8-bit red samples
% string to hold 256 8-bit green samples
% (distinct from rstr)
Ibstr 256 string def
% string to hold 256 8-bit blue samples
% (distinct from rstr and bstr)
% locate lower left corner of image
45 140 translate
132 132 scale
% map image to 132 point square
256 256 8
% dimensions of source image
[25600 -2560256] % map unit square to source
{currentfile rstr readhexstring pop}
% read red data from program file
{currentfile gstr readhexstring pop}
% read green data from program file
{currentfile bstr readhexstring pop}
% read blue data from program file
true 3
% multiple proc, 3 colors
colorimage
7b5e606969615365556a6a66 ...
(512"hex digits of red data)
88868d848a92878578787a82 .. ,
(512 hex digits of green data)
62717c7b736e707d7b6a7c79...
(512 hex digits of blue data)
7d8b8d8c837d8b8e9284878e ...
(512 hex digits of red data)
2788b838b8e8e86868988908 ...
(512 hex digits of green data)
81817d857f85858290949487...
(512 hex digits of blue data)
... (390144 more hex digits of RGB data, cycling as above)

POSTSCRIPT Language Color Extensions

The above is a three-procedure 8-bit RGB image. The initial samples
for each color, in hex, are

red:
green:
blue:

7b
88
62

5e
86
71

60
8d
7c

69
84
7b

EXAMPLE 3:
Icstr 128 string def
Imstr 128 string def

% string to hold 256 4-bit cyan samples
% string to hold 256 4-bit magenta samples
% (distinct from cstr)
Iystr 128 string def
% string to hold 256 4-bit yellow samples
% (distinct from cstr and mstr)
Ikstr 128 string def
% string to hold 256 4-bit black samples
% (distinct from cstr, mstr, and ystr)
% locate lower left corner of image
45 140 translate
% map image to 132 point square
132 132 scale
256 256 4
% dimensions of source image
[256 0 0 -256 0 256] % map unit square to source
{currentfile cstr readhexstring pop}
% read cyan data from program file
{currentfile mstr readhexstring pop}
% read magenta data from program file
{currentfile ystr readhexstring pop}
% read yellow data from program file
{currentfile kstr readhexstring pop}
% read black data from program file
true 4
% multiple proc, 4 colors
colorimage
e1d8caa57b655b6779606b72...
(256 hex digits of cyan data)
6bdbb867b9fb6a4859569989 ...
(256 hex digits of magenta data)
996796e639ccOb29f94736c7...
(256 hex digits of yellow data)
c9cOcadOd3cad2b7cge2d7d8 ...
(256 hex digits of black data)
5d2d6d7d4d3d1 d4d6c9d4d9d ...
(256 hex digits of cyan data)
4cdcfd4d6d1 d8d7d5d4d2d2d ...
(256 hex digits of magenta data)
d2b7cge2d7d8d8cbbac2d9d8...
(256 hex digits of yellow data)
88ae96632a70f6f4d8d9d9d8 ...
(256 hex digits of black data)
... (260096 more hex digits of CMYK data, cycling as above)

The above is a four-procedure four-bit CMYK image.
samples for each color, in hex, are

cyan:
magenta:
yellow:
black:

The initial

e 1 d 8 c a a 5
6 b d b b 8 6 7

9 9 679 6 e 6
c 9 c 0 cad 0

3 NEW FEATURES

EXAMPLE 4:
Icstr 1024 string def
Imstr 1024 string def

% string to hold 1024 8-bit cyan samples
% string to hold 1024 8-bit magenta samples
% (distinct from cstr)
Iystr 1024 string def
% string to hold 1024 8-bit yellow samples
% (distinct from cstr and mstr)
Ikstr 1024 string def
% string to hold 1024 8-bit black samples
% (distinct from cstr, mstr, and ystr)
letile (img/smp.c) (r) file def % binary file containing 1048576 8-bit
% cyan samples
Imfile (img/smp.m) (r) file def % binary file containing 1048576 8-bit
% magenta samples
Iyfile (img/smp.y) (r) file def % binary file containing 1048576 8-bit
% yellow samples
Ikfile (img/smp.k) (r) file def % binary file containing 1048576 8-bit
% black samples
36 126 translate
% locate lower left corner of image
540 540 scale
% map image to 540 point square
1024 1024 8
% dimensions of source image
[1024 0 0 -1024 0 1024]
% map unit square to source
{cfile cstr readstring pop}
% read cyan data from imglsmp.c
% read magenta data from img/smp.m
{mfile mstr readstring pop}
{yfile ystr readstring pop}
% read yellow data from imglsmp.y
{kfile kstr readstring pop}
% read black data from img/smp.k
true 4
% multiple proc, 4 colors
colorimage

The above is a four-procedure 8-bit CMYK image, with cyan,
magenta, yellow, and black samples taken from the files
img/smp.c, img/smp.m, img/smp.y, and img/smp.k, respectively.
This example only applies to a POSTSCRIPT interpreter that has a
file system.

3.6

COLOR IMPLEMENTATIONS
Each POSTSCRIPT .interpreter contains default color output
methods that correspond to the printer to which it is attached. If
the attached printer is a direct-color binary device, the standard
output method produces three- or four-color output. If the attached printer is a gray-scale color device, then the POSTSCRIPT
interpreter uses a gray-scale three- or four-color output method.
If the attached printer is a black-and-white device, then the
default output method produces a single black-and-white rendition of each page described.

POSTSCRIPT Language Color Extensions

Black-and-white printers also have the capability of producing
color separations. These are three or four black-and-white pages
for each page described. Each output page corresponds to the
output for one color component. These separations are only useful if they are used in another printing process in which they are
combined with color to form the intended full-color output.
The method by which a POSTSCRIPT language program informs
the POSTSCRIPT interpreter that color separations are desired is
still to be determined. This color separation capability will not be
provided in the backward compatibility package, as it requires
considerable internal changes in the POSTSCRIPT interpreter.

4 OPERATORS
The following pages contain an alphabetical listing of the new
POSTSCRIPT color operators.

4 OPERATORS

colorlmage

width height bits/component matrix proc 0 [... procncolorS-1] multiproc
ncolors colorimage -

renders a sampled image, whose samples contain one, three, or
four color components on the current page. The fIrst four arguments are the same as those for the image operator. The
bits/component argument applies equally to all color components. colorimage permits its proci arguments to return RGB
or CMYK sample values rather than single-color (gray) values as
are returned by the proc argument of the image operator.
The ncolors argument, a 1, 3, or 4, is the number of color components represented in the samples. If ncolors is 1, the samples
have only one component, a gray component, and the operation
of colorimage is equivalent to that of image with the same fIve
initial arguments. If the ncolors argument is 3, the colorimage
operator takes RGB (light-high) samples. If the ncolors argument is 4, the colorimage operator takes CMYK (dark-high)
samples. On a four-color (CMYK) machine, the POSTSCRIPT interpreter converts a three-color (RGB) image to CMYK using
the black generation and undercolor removal functions, whereas
a four-color (CMYK) image bypasses these operations.
The multiproc argument is a boolean that distinguishes between
two forms of the colorimage operator: false indicates the singleprocedure form, argument (proco) , and true indicates the
multiple-procedure form, which requires one procedure argument per sample color (procO ... procncolors-l)' i.e., three procedure arguments for RGB samples or four-procedure arguments
for CMYK samples. If the ncolors argument is 1, then there is
only one procedure argument, proco' regardless of the value of
the multiproc argument. For a detailed description of the data
formats and how the proci procedures are called, see Data formats for colorimage Operator in Section 4, New Features.
Use of setcolorscreen, setcolortransfer, setscreen, or
settransfer by any of the proci procedures causes unpredictable
results. Use of the colorimage operator after a setcachedevice
within the context of a BuildChar procedure is not permitted (an
undefined error results).
ERRORS:

Iimitcheck, rangecheck, stack underflow , typecheck,
undefined, undefinedresult

POSTSCRIPT Language Color Extensions

currentblackgeneration

- currentblackgeneration proc

returns the current black generation function in the graphics state
(see setblackgeneration).
ERRORS:

stackoverflow
currentcmykcolor

- currentcmykcolor cyan magenta yellow black

returns the four components of the current color in the graphics
state according to the cyan-magenta-yellow-black color
specification (see setcmykcolor).
Note that the currentgray operator returns a weighted average
of all four color components. Applying it is the equivalent of the
following use of currentcmykcolor:
1.0 currentcmykcolor 4 1 roll 0.11 mul 3 1 roll 0.59 mul
exch 0.30 mul add add add sub dup 0.0 It {pop O.O} if
ERRORS:

stackoverflow
currentcolorscreen

- currentcolorscreen ric-frequency ric-angie r/c-proc g/m-frequency
g/m-angle g/m-proc b/y-frequency b/y-angle b/y-proc glb-frequency
g/b-angle glb-proc

returns all 12 current halftone screen parameters in the graphics
state (see setcolorscreen).
The currentcolorscreen operator is the logical expansion of
currentscreen to four color components. Applying the
currentscreen operator returns the three parameters describing
the gray/black screen. It is the equivalent of the following use of
currentcolorscreen:'
currentcolorscreen 12 3 roll 9 {pop} repeat
ERRORS:

stackoverflow

4 OPERATORS

currentcolortransfer

- currentcolortransfer redproc greenproc blueproc grayproc

returns the current transfer functions in the graphics state for
each of the four color components (see setcolortransfer).
The currentcolortransfer operator is the logical expansion of
currenttransfer to four color components. Applying the
currenttransfer operator returns the gray transfer function. It is
the equivalent of the following use of currentcolortransfer:
currentcolortransfer 41 roll pop pop pop
ERRORS:

stackoverflow
currentundercolorremoval

- currentundercolorremoval proc

returns the current undercolor removal function in the graphics
state (see setundercolorremoval).
ERRORS:

stackoverflow

POSTSCRIPT Language Color Extensions

setblackgeneration

proc setblackgeneration -

sets the current black generation function parameter in the
graphics state. The proc operand must be a POSTSCRIPT language procedure that can be called with a number in the range
0.0 to 1.0 (inclusive) on the operand stack and which returns a
number in the same range. This procedure maps the minimum of
the user cyan, magenta, and yellow color components to user
black values.
For additional information, see section 3.
EXAMPLE:

{dup .75 Ie {pop O.O} {.75 sub 4.0 mul} ifelse} setblackgeneration

This sets the black component to zero when the minimum of cyan,
magenta, and yellow is less than or equal to .75, while minima greater
than .75 produce a black component that increases linearly from 0.0 (at
a minimum of .75) to 1.0 (when user cyan, magenta, and yellow all
have values of 1.0).

The use of setblackgeneration after a setcachedevice operation
within the scope of a BuildChar procedure is not permitted (an
undefined error results).
ERRORS:

stackunderflow, typecheck

4 OPERATORS

setcmykcolor

cyan magenta yellow black setcmykcolor -

sets the current color parameter in the graphics state to a color
described by the parameters cyan, magenta, yellow, and black,
each of which must be a number in the range 0.0 to 1.0. This
establishes the color used subsequently to paint shapes such as
lines, areas, and characters on the current page. This operator
bypasses the black generation and undercolor removal operations.
For additional information, see section 3.
Note that applying the setgray operator sets the gray color component to its single argument value and the red, green, and blue
color components to 1.0. It is the equivalent of the following use
of setcmykcolor:
0.0 0.0 0.0 1.0 5 -1 roll sub setcmykcolor

The use of setcmykcolor after a setcachedevice operation
within the scope of a BuildChar procedure is not permitted (an
undefined error results).
ERRORS:

stackunderflow, typecheck

POSTSCRIPT Language Color Extensions

setcolorscreen

rIc-frequency rIc-angIe r/c-proc g/m-frequency g/m-angle g/m-proc
b/y-frequency b/y-angle b/y-proc g/b-frequency g/b-angle g/b-proc

setcolorscreen -

sets the current halftone screen definitions for red/cyan (r/c) ,
green/magenta (g/m), blue/yellow(b/y), and gray/black (g/b) output color components in the graphics state. Each of the
rIc-frequency, glm-Jrequency, bly{requency, and glb-Jrequency
operands is a number that specifies the screen frequency for one
output color component, measured in halftone cells per inch in
device space. The rIc-angle, glm-angle, bly-angle, and glb-angle
operands specify the number of degrees by which their respective halftone screens are rotated with respect to the device coordinate system. The rlc-proc, glm-proc, b/y-proc, and g/b-proc
operands are each a POSTSCRIPT language procedure (as for
setscreen). They define one color component's spot function to
determine the order in which pixels within a halftone cell of that
color component are assigned a light value to produce any
desired shade of that color component. As with setscreen, the
pixels with the highest values returned by this function become
dark soonest at the lightest shades of that color, and the pixels
with the lowest values becomes dark latest at the darkest shades
of that color. The red/cyan, green/magenta, and blue/yellow
screens have no effect on a black-and-white device, the
gray/black screen has no effect on an ROB or CMY device, and
no screens have any effect on a full (8-bits-per-pixel) gray-scale
device.
Many binary color printers (those that print each dot at full intensity or not at all) require different angles for each colorcomponent halftone to have attractive output. Each color printer
containing a POSTSCRIPT interpreter has a default color screen
chosen to look good on that printer.
The setcolorscreen operator is the logical expansion of
setscreen to four color components. It takes the same three types
of arguments as setscreen, but repeated four times. Applying the
setscreen operator sets all four screens equally. It is the equivalent of the following use of setcolorscreen:
3 copy 6 copy setcolorscreen

EXAMPLE:

4 OPERATORS

% 50 line dot screen with 75 degree cyan,
% 15 degree magenta,
% 0 degree yellow, and 45 degree black angled screens,
% which are standard for color printing
% 50 halftone cells per inch
/sfreq 50 def
/sproc {dup mul exch dup mul add 1 exch sub} def
% dot-screen spot function
% 75 degree red (cyan) screen
sfreq 75 /sproc load
sfreq 15 /sproc load
% 15 degree green (magenta) screen
sfreq 0 /sproc load
% 0 degree blue (yellow) screen
sfreq 45 /sproc load
% 45 degree gray (black) screen
setcolorscreen
ERRORS:

Iimitcheck, rangecheck, stackunderflow, typecheck

POSTSCRIPT Language Color Extensions

setcolortransfer

redproc greenproc blueproc grayproc setcolortransfer -

sets the current transfer function parameters for red, green, blue,
and gray in the graphics state. Each operand must be a
POSTSCRIPT language procedure that may be called with a number in the range 0.0 to 1.0 (inclusive) on the operand stack and
which will return a number in the same range. These procedures
map user values of the color components (e.g., those specified by
setrgbcolor and adjusted by setblackgeneration and
setundercolorremoval, or 1.0 minus those specified by
setcmykcolor) to device color components (for halftones, a
weighted average of the lightness of pixels in a halftone cell).
Only those transfer functions corresponding to color components
supported by a device will have an effect on that device's output.
For example, redproc, greenproc, and blueproc will have no effect on a black-and-white device, while grayproc will have no
effect on an ROB device.
The single-color settransfer operator takes a single procedure
argument whose purpose is to provide gamma correction for a
printer's halftoning response. That operator is useful for a variety
of effects beyond its original intention as a gray response correction function, but it is useful only in the context of a single output color, as on black-and-white printers. The setcolortransfer
operator is the logical expansion of settransfer to four color
components; it takes four function arguments, each similar in
purpose to the function argument of settransfer, but each function separately controls the response for each of the red (1.0
minus cyan), green (1.0 minus magenta), blue (1.0 minus
yellow) and gray (1.0 minus black) components, respectively.
Applying the settransfer operator sets all four transfer functions
equally. It is the equivalent of the following use of
setcolortransfer:
dup dup dup setcolortransfer
EXAMPLE:

{} {} {dup mul} {} setcolortransfer

This sets device blue as the square of user blue and leaves the other
color components unchanged.

Calling settransfer with the argument

4 OPERATORS

{1 exch sub}

to invert an output image is not guaranteed to work if any of the
following operators are used in generating the image:
colorimage, setcmykcolor, setcolortransfer, sethsbcolor, and
setrgbcolor. In the case of a device with four color components,
inversion can be more complicated than merely inverting all of
the components.
The use of setcolortransfer after a setcachedevice operation
within the scope of a BuiIdChar procedure is not permitted (an
undefined error results).
ERRORS:

stack underflow, typecheck
setundercolorremoval

proc setundercolorremoval -

sets the current undercolor removal function parameter in the
graphics state. The proc operand must be a POSTSCRIPT language procedure that may be called with a number in the range
0.0 to 1.0 (inclusive) on the operand stack and which will return
a number in the range -1.0 (to increase the color components) to
+1.0 (to decrease the color components). This procedure maps
the minimum of the user cyan, magenta, and yellow color components to a value to be subtracted from each of these same components.
For additional information, see section 3.
EXAMPLE:

{currentblackgeneration exec .5 mul} setundercolorremoval

This sets the undercolor removal to be half of the black component
from black generation.

The use of setundercolorremovaI after a setcachedevice operation within the scope of a BuiIdChar procedure is not permitted
(an undefined error results).
ERRORS:

stackunderflow, typecheck

POSTSCRIPT Language Color Extensions

black generation 4
CMYK color 3, 6
color images 7
color screens 7
color transfer functions 7
colorimage 7, 8, 9, 21
colorimage 15
currentblackgeneration 15
currentcmykcolor 15
currentcolorscreen 15
currentcolortransfer 16
currentundercolorremoval 16
setblackgeneration 17
setcmykcolor 21
setcmykcolor 18
setcolorscreen 20
setcolortransfer 21
setcolortransfer 22
sethsbcolor 21
setrgbcolor 21
settransfer 20
setundercolorremoval 22
undercolor removal 4

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