PDF Reference sixth edition, Adobe Portable Document Format Version 1.7 (book 1) — page 7

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The transformation defined by the Gamma and Matrix entries in the CalRGB color

space dictionary is

G_R

G_G

G_B

X = L = XA × A

X_B × B

X_C × C

G_R

G_G

G_B

Y = M = YA × A

Y_B × B

Y_C × C

G_R

G_G

G_B

Z = N = ZA × A

Z_B × B

Z_C × C

In other words, the A, B, and C components are first decoded individually by the

gamma functions. The results are treated as a three-element vector and multi-

plied by Matrix (a 3-by-3 matrix) to obtain the L, M, and N components of the in-

termediate representation. Since there is no second stage, these are also the X, Y,

and Z components of the final representation.

Example 4.7 shows an example of a CalRGB color space for the CCIR XA/11-

recommended D65 white point with 1.8 gammas and Sony Trinitron phosphor

chromaticities.

Example 4.7

[

/CalRGB

<< /WhitePoint [ 0.9505 1.0000 1.0890 ]

/Gamma [ 1.8000 1.8000 1.8 000]

/Matrix [

0.4497 0.2446 0.0252

0.3163 0.6720 0.1412

0.1845 0.0833 0.9227

]

In some cases, the parameters of a CalRGB color space may be specified in terms

of the CIE 1931 chromaticity coordinates (x_R , y_R ), (x_G , y_G ), (x_B , y_B ) of the red,

green, and blue phosphors, respectively, and the chromaticity (x_W, y_W ) of the dif-

fuse white point corresponding to some linear RGB value (R, G, B), where usually

R = G = B = 1.0. Note that standard CIE notation uses lowercase letters to specify

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chromaticity coordinates and uppercase letters to specify tristimulus values. Giv-

en this information, Matrix and WhitePoint can be found as follows:

y_W ×((xG – xB)× yR – (xR – xB)× yG + (xR _– xG)×yB

)

y_R

(

x_G – x_B

)× y_W

–

(

x_W – x_B

)× y_G

(

x_W – x_G

)× y_B

Y_A

-----

× ---------------------------------------------------------------------------------------------------------------------------------

x_R

⎛1

x_R

⎞

X_A = YA

× -----

Z_A = YA

⎜--------------

- 1

⎟

y_R

⎝

⎠

y_G

(

x_R – x_B

)× y_W

–

(

x_W – x_B

)× y_R

(

x_W – x_R

)× y_B

Y_B

- -----

× -------------------------------------------------------------------------------------------------------------------------------

x_G

⎛1

x_G

⎞

X_B = YB

× ------

Z_B = YB

⎜---------------

- 1

⎟

y_G

⎝

⎠

y_B

(

x_R – x_G

)× y_W

–

(

x_W – x_G

)× y_R

(

x_W – x_R

)× y_G

Y_C

-----

× ---------------------------------------------------------------------------------------------------------------------------------

x_B

⎛1

x_B

⎞

X_C = YC

× -----

Z_C = YC

⎜--------------

- 1

⎟

y_B

⎝

⎠

X_W = X_A × R + X_B × G + X_C × B

Y_W = Y_A × R + Y_B × G + Y_C × B

Z_W = Z_A × R + Z_B × G + Z_C × B

Lab Color Spaces

A Lab color space is a CIE-based ABC color space with two transformation stages

(see Figure 4.14 on page 245). In this type of space, A, B, and C represent the L*,

a*, and b* components of a CIE 1976 L*a*b* space. The range of the first (L*)

component is always 0 to 100; the ranges of the second and third (a* and b*)

components are defined by the Range entry in the color space dictionary (see

Table 4.15).

Plate 3 illustrates the coordinates of a typical Lab color space; Plate 4 compares

the gamuts (ranges of representable colors) for L*a*b*, RGB, and CMYK spaces.

251

SECTION 4.5

Color Spaces

TABLE 4.15 Entries in a Lab color space dictionary

KEY

TYPE

VALUE

WhitePoint

array

(Required) An array of three numbers [ X_W Y_W Z_W ] specifying the tristimulus value,

in the CIE 1931 XYZ space, of the diffuse white point; see “CalRGB Color Spaces” on

page 247 for further discussion. The numbers X_W and Z_W must be positive, and Y_W

must be equal to 1.0.

BlackPoint

array

(Optional) An array of three numbers [ X_B Y_B Z_B ] specifying the tristimulus value, in

the CIE 1931 XYZ space, of the diffuse black point; see “CalRGB Color Spaces” on

page 247 for further discussion. All three of these numbers must be non-negative.

Default value: [ 0.0 0.0 0.0 ].

Range

array

(Optional) An array of four numbers [ a_min a_max b_min b_max ] specifying the range of

valid values for the a* and b* (B and C) components of the color space—that is,

a_min ≤ a* ≤ a_max

and

b_min ≤ b* ≤ b_max

Component values falling outside the specified range are adjusted to the nearest valid

value without error indication. Default value: [ −100 100 −100 100 ].

A Lab color space does not specify explicit decoding functions or matrix coef-

ficients for either stage of the transformation from L*a*b* space to XYZ space

(denoted by “Decode ABC,” “Matrix ABC,” “Decode LMN,” and “Matrix LMN” in

Figure 4.14 on page 245). Instead, these parameters have constant implicit values.

The first transformation stage is defined by the equations

L = L*⁺16

+ --------

116

500

M = L*⁺16

116

N = L*⁺16

- --------

116

200

The second transformation stage is given by

X = X_W × g(L)

Y = Y_W × g(M)

Z = Z_W × g(N)

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where the function g (x) is defined as

g(x)

x³

if x

≥ ---

108

⎛

⎞

g(x)

- ---

otherwise

841

⎝

29⎠

Example 4.8 defines the CIE

1976 L*a*b* space with the CCIR XA/11-

recommended D65 white point. The a* and b* components, although theoretical-

ly unbounded, are defined to lie in the useful range −128 to +127.

Example 4.8

[

/Lab

<< /WhitePoint [ 0.9505 1.0000 1.0890 ]

/Range [ −128 127 −128 127 ]

]

ICCBased Color Spaces

ICCBased color spaces (PDF 1.3) are based on a cross-platform color profile as

defined by the International Color Consortium (ICC). Unlike the CalGray,

CalRGB, and Lab color spaces, which are characterized by entries in the color

space dictionary, an ICCBased color space is characterized by a sequence of bytes

in a standard format. Details of the profile format can be found in the ICC speci-

fication (see the Bibliography).

An ICCBased color space is specified as an array:

[ /ICCBased stream ]

The stream contains the ICC profile. Besides the usual entries common to all

streams (see Table 3.4 on page 62), the profile stream has the additional entries

listed in Table 4.16.

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SECTION 4.5

Color Spaces

TABLE 4.16 Additional entries specific to an ICC profile stream dictionary

KEY

TYPE

VALUE

integer

(Required) The number of color components in the color space described by the ICC

profile data. This number must match the number of components actually in the ICC

profile. As of PDF 1.4, N must be 1, 3, or 4.

Alternate

array or

(Optional) An alternate color space to be used in case the one specified in the stream

name

data is not supported (for example, by applications designed for earlier versions of

PDF). The alternate space may be any valid color space (except a Pattern color space)

that has the number of components specified by N. If this entry is omitted and the ap-

plication does not understand the ICC profile data, the color space used is

DeviceGray, DeviceRGB, or DeviceCMYK, depending on whether the value of N is 1, 3,

or 4, respectively.

Note: There is no conversion of source color values, such as a tint transformation, when

using the alternate color space. Color values within the range of the ICCBased color space

might not be within the range of the alternate color space. In this case, the nearest values

within the range of the alternate space are substituted.

Range

array

(Optional) An array of 2 × N numbers [ min₀ max₀ min₁ max₁ … ] specifying the min-

imum and maximum valid values of the corresponding color components. These val-

ues must match the information in the ICC profile. Default value:

[ 0.0 1.0

0.0

1.0

… ].

Metadata

stream

(Optional; PDF 1.4) A metadata stream containing metadata for the color space (see

Section 10.2.2, “Metadata Streams”).

The ICC specification is an evolving standard. Table 4.17 shows the versions of

the ICC specification on which the ICCBased color spaces supported by PDF ver-

sions 1.3 and later are based. (Earlier versions of the ICC specification are also

supported.)

TABLE 4.17 ICC specification versions supported by ICCBased color spaces

PDF VERSION ICC SPECIFICATION VERSION

1.3

3.3

1.4

ICC.1:1998-09 and its addendum ICC.1A:1999-04

1.5

ICC.1:2001-12

1.6

ICC.1:2003-09

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PDF VERSION ICC SPECIFICATION VERSION

1.7

ICC.1:2004-10

PDF producers and consumers should follow these guidelines:

• A consumer that supports a given PDF version is required to support ICC pro-

files conforming to the corresponding version (and earlier versions) of the ICC

specification, as described above. It may optionally support later ICC versions.

• For the most predictable and consistent results, a producer of a given PDF ver-

sion should embed only profiles conforming to the corresponding version of

the ICC specification.

• A PDF producer may embed profiles conforming to a later ICC version, with

the understanding that the results will vary depending on the capabilities of the

consumer. The consumer might process the profile while ignoring newer

features, or it might fail altogether to process the profile. Therefore, it is recom-

mended that the producer provide an alternate color space (Alternate entry in

the ICCBased color space dictionary) containing a profile that is appropriate for

the PDF version.

PDF supports only the profile types shown in Table 4.18; other types may be sup-

ported in the future. (In particular, note that XYZ and 16-bit L*a*b* profiles are

not supported.) Each of the indicated fields must have one of the values listed for

that field in the second column of the table. (Profiles must satisfy both the criteria

shown in the table.) The terminology is taken from the ICC specifications.

TABLE 4.18 ICC profile types

HEADER FIELD

REQUIRED VALUE

deviceClass

icSigInputClass ('scnr')

icSigDisplayClass ('mntr')

icSigOutputClass ('prtr')

icSigColorSpaceClass ('spac')

colorSpace

icSigGrayData ('GRAY')

icSigRgbData ('RGB ')

icSigCmykData ('CMYK')

icSigLabData ('Lab ')

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SECTION 4.5

Color Spaces

The terminology used in PDF color spaces and ICC color profiles is similar, but

sometimes the same terms are used with different meanings. For example, the

default value for each component in an ICCBased color space is 0. The range of

each color component is a function of the color space specified by the profile and

is indicated in the ICC specification. The ranges for several ICC color spaces are

shown in Table 4.19.

TABLE 4.19 Ranges for typical ICC color spaces

ICC COLOR SPACE

COMPONENT RANGES

Gray

[ 0.0

1.0 ]

RGB

[ 0.0

1.0 ]

CMYK

[ 0.0

1.0 ]

L*a*b*

L*: [ 0 100 ]; a* and b*: [ −128 127 ]

Since the ICCBased color space is being used as a source color space, only the “to

CIE” profile information (AToB in ICC terminology) is used; the “from CIE”

(BToA) information is ignored when present. An ICC profile may also specify a

rendering intent, but PDF consumer applications ignore this information; the ren-

dering intent is specified in PDF by a separate parameter (see “Rendering Intents”

on page 260).

Note: The requirements stated above apply to an ICCBased color space that is used

to specify the source colors of graphics objects. When such a space is used as the

blending color space for a transparency group in the transparent imaging model

(see Sections 7.2.3, “Blending Color Space”; 7.3, “Transparency Groups”; and 7.5.5,

“Transparency Group XObjects”), it must have both “to CIE” (AToB) and “from

CIE” (BToA) information. This is because the group color space is used as both the

destination for objects being painted within the group and the source for the

group’s results. ICC profiles are also used in specifying output intents for matching

the color characteristics of a PDF document with those of a target output device or

production environment. When used in this context, they are subject to still other

constraints on the “to CIE” and “from CIE” information; see Section 10.10.4,

“Output Intents,” for details.

The representations of ICCBased color spaces are less compact than CalGray,

CalRGB, and Lab, but can represent a wider range of color spaces. In those cases

where a given color space can be expressed by more than one of the CIE-based

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color space families, the resulting colors are expected to be rendered similarly,

regardless of the method selected for representation.

One particular color space is the so-called “standard RGB” or sRGB, defined in

the International Electrotechnical Commission (IEC) document Colour Measure-

ment and Management in Multimedia Systems and Equipment (see the Bibliogra-

phy). In PDF, the sRGB color space can be expressed precisely only as an

ICCBased space, although it can be approximated by a CalRGB space.

Example 4.9 shows an ICCBased color space for a typical three-component RGB

space. The profile’s data has been encoded in hexadecimal representation for

readability; in actual practice, a lossless decompression filter such as FlateDecode

should be used.

Example 4.9

10 0 obj

% Color space

[ /ICCBased 15 0 R ]

endobj

15 0 obj

% ICC profile stream

<< /N 3

/Alternate /DeviceRGB

/Length 1605

/Filter

/ASCIIHexDecode

stream

00 00 02 0C 61 70 70 6C 02 00 00 00 6D 6E 74 72

52 47 42 20 58 59 5A 20 07 CB 00 02 00 16 00 0E

00 22 00 2C 61 63 73 70 41 50 50 4C 00 00 00 00

61 70 70 6C 00 00 04 01 00 00 00 00 00 00 00 02

00 00 00 00 00 00 F6 D4 00 01 00 00 00 00 D3 2B

00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00

00 00 00 09 64 65 73 63 00 00 00 F0 00 00 00 71

72 58 59 5A 00 00 01 64 00 00 00 14 67 58 59 5A

00 00 01 78 00 00 00 14 62 58 59 5A 00 00 01 8C

00 00 00 14 72 54 52 43 00 00 01 A0 00 00 00 0E

67 54 52 43 00 00 01 B0 00 00 00 0E 62 54 52 43

00 00 01 C0 00 00 00 0E 77 74 70 74 00 00 01 D0

00 00 00 14 63 70 72 74 00 00 01 E4 00 00 00 27

64 65 73 63 00 00 00 00 00 00 00 17 41 70 70 6C

257

SECTION 4.5

Color Spaces

65 20 31 33 22 20 52 47 42 20 53 74 61 6E 64 61

72 64 00 00 00 00 00 00 00 00 00 00 00 17 41 70

70 6C 65 20 31 33 22 20 52 47 42 20 53 74 61 6E

64 61 72 64 00 00 00 00 00 00 00 00 00 00 00 00

00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00

00 58 59 5A 58 59 5A 20 00 00 00 00 00 00 63 0A

00 00 35 0F 00 00 03 30 58 59 5A 20 00 00 00 00

00 00 53 3D 00 00 AE 37 00 00 15 76 58 59 5A 20

00 00 00 00 00 00 40 89 00 00 1C AF 00 00 BA 82

63 75 72 76 00 00 00 00 00 00 00 01 01 CC 63 75

63 75 72 76 00 00 00 00 00 00 00 01 01 CC 58 59

58 59 5A 20 00 00 00 00 00 00 F3 1B 00 01 00 00

00 01 67 E7 74 65 78 74 00 00 00 00 20 43 6F 70

79 72 69 67 68 74 20 41 70 70 6C 65 20 43 6F 6D

70 75 74 65 72 73 20 31 39 39 34 00 >

endstream

endobj

Default Color Spaces

Colors that are specified in a device color space (DeviceGray, DeviceRGB, or

DeviceCMYK) are device-dependent. By setting default color spaces (PDF 1.1), a

PDF document can request that such colors be systematically transformed

(remapped) into device-independent CIE-based color spaces. This capability can

be useful in a variety of circumstances:

• A document originally intended for one output device is redirected to a differ-

ent device.

• A document is intended to be compatible with applications designed for earlier

versions of PDF and thus cannot specify CIE-based colors directly.

• Color corrections or rendering intents need to be applied to device colors (see

“Rendering Intents” on page 260).

A color space is selected for painting each graphics object. This is either the cur-

rent color space parameter in the graphics state or a color space given as an entry

in an image XObject, inline image, or shading dictionary. Regardless of how the

color space is specified, it may be subject to remapping as described below.

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When a device color space is selected, the ColorSpace subdictionary of the cur-

rent resource dictionary (see Section 3.7.2, “Resource Dictionaries”) is checked

for the presence of an entry designating a corresponding default color space

(DefaultGray, DefaultRGB, or DefaultCMYK, corresponding to DeviceGray,

DeviceRGB, or DeviceCMYK, respectively). If such an entry is present, its value is

used as the color space for the operation currently being performed. (If the appli-

cation does not recognize this color space, no remapping occurs; the original de-

vice color space is used.)

Color values in the original device color space are passed unchanged to the

default color space, which must have the same number of components as the

original space. The default color space should be chosen to be compatible with

the original, taking into account the components’ ranges and whether the compo-

nents are additive or subtractive. If a color value lies outside the range of the de-

fault color space, it is adjusted to the nearest valid value.

Note: Any color space other than a Lab, Indexed, or Pattern color space may be used

as a default color space provided that it is compatible with the original device color

space as described above.

If the selected space is a special color space based on an underlying device color

space, the default color space is used in place of the underlying space. This applies

to the following color spaces:

• The underlying color space of a Pattern color space

• The base color space of an Indexed color space

• The alternate color space of a Separation or DeviceN color space (but only if the

alternate color space is actually selected)

See Section 4.5.5, “Special Color Spaces,” for details on these color spaces.

Note: There is no conversion of color values, such as a tint transformation, when us-

ing the default color space. Color values that are within the range of the device color

space might not be within the range of the default color space (particularly if the de-

fault is an ICCBased color space). In this case, the nearest values within the range of

the default space are used. For this reason, a Lab color space is not permitted as the

DefaultRGB color space.

259

SECTION 4.5

Color Spaces

Implicit Conversion of CIE-Based Color Spaces

In workflows in which PDF documents are intended for rendering on a specific

target output device (such as a printing press with particular inks and media), it is

often useful to specify the source colors for some or all of a document’s objects in

a CIE-based color space that matches the calibration of the intended device. The

resulting document, although tailored to the specific characteristics of the target

device, remains device-independent and will produce reasonable results if re-

targeted to a different output device. However, the expectation is that if the docu-

ment is printed on the intended target device, source colors that have been

specified in a color space matching the calibration of the device will pass through

unchanged, without conversion to and from the intermediate CIE 1931 XYZ

space as depicted in Figure 4.14 on page 245.

In particular, when colors intended for a CMYK output device are specified in an

ICCBased color space using a matching CMYK printing profile, converting such

colors from four components to three and back is unnecessary and results in a

loss of fidelity in the black component. In such cases, PDF consumer applications

may provide the ability for the user to specify a particular calibration to use for

printing, proofing, or previewing. This calibration is then considered to be that of

the native color space of the intended output device (typically DeviceCMYK), and

colors expressed in a CIE-based source color space matching it can be treated as if

they were specified directly in the device’s native color space. Note that the condi-

tions under which such implicit conversion is done cannot be specified in PDF,

since nothing in PDF describes the calibration of the output device (although an

output intent dictionary, if present, may suggest such a calibration; see Section

10.10.4, “Output Intents”). The conversion is completely hidden by the applica-

tion and plays no part in the interpretation of PDF color spaces.

When this type of implicit conversion is done, all of the semantics of the device

color space should also apply, even though they do not apply to CIE-based spaces

in general. In particular:

• The nonzero overprint mode (see Section 4.5.6, “Overprint Control”) deter-

mines the interpretation of color component values in the space.

• If the space is used as the blending color space for a transparency group in the

transparent imaging model (see Sections 7.2.3, “Blending Color Space”; 7.3,

“Transparency Groups”; and 7.5.5, “Transparency Group XObjects”), compo-

nents of the space, such as Cyan, can be selected in a Separation or DeviceN col-

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or space used within the group (see “Separation Color Spaces” on page 264 and

“DeviceN Color Spaces” on page 268).

• Likewise, any uses of device color spaces for objects within such a transparency

group have well-defined conversions to the group color space.

Note: A source color space can be specified directly (for example, with an ICCBased

color space) or indirectly using the default color space mechanism (for example,

DefaultCMYK; see “Default Color Spaces” on page 257). The implicit conversion of a

CIE-based color space to a device space should not depend on whether the CIE-

based space is specified directly or indirectly.

Rendering Intents

Although CIE-based color specifications are theoretically device-independent,

they are subject to practical limitations in the color reproduction capabilities of

the output device. Such limitations may sometimes require compromises to be

made among various properties of a color specification when rendering colors for

a given device. Specifying a rendering intent (PDF 1.1) allows a PDF file to set pri-

orities regarding which of these properties to preserve and which to sacrifice. For

example, the PDF file might request that colors falling within the output device’s

gamut (the range of colors it can reproduce) be rendered exactly while sacrificing

the accuracy of out-of-gamut colors, or that a scanned image such as a photo-

graph be rendered in a perceptually pleasing manner at the cost of strict colori-

metric accuracy.

Rendering intents are specified with the ri operator (see Section 4.3.3, “Graphics

State Operators”), the RI entry in a graphics state parameter dictionary (see Sec-

tion 4.3.4), and with the Intent entry in image dictionaries (Section 4.8.4, “Image

Dictionaries”). The value is a name identifying the rendering intent. Table 4.20

lists the standard rendering intents recognized in the initial release of PDF viewer

applications from Adobe Systems; Plate 5 illustrates their effects. These intents

have been deliberately chosen to correspond closely to those defined by the Inter-

national Color Consortium (ICC), an industry organization that has developed

standards for device-independent color. Note, however, that the exact set of ren-

dering intents supported may vary from one output device to another; a particu-

lar device may not support all possible intents or may support additional ones

beyond those listed in the table. If the application does not recognize the speci-

fied name, it uses the RelativeColorimetric intent by default.

261

SECTION 4.5

Color Spaces

See Section 7.6.4, “Rendering Parameters and Transparency,” and in particular

“Rendering Intent and Color Conversions” on page 574, for further discussion of

the role of rendering intents in the transparent imaging model.

TABLE 4.20 Rendering intents

NAME

DESCRIPTION

AbsoluteColorimetric

Colors are represented solely with respect to the light source; no

correction is made for the output medium’s white point (such as

the color of unprinted paper). Thus, for example, a monitor’s

white point, which is bluish compared to that of a printer’s pa-

per, would be reproduced with a blue cast. In-gamut colors are

reproduced exactly; out-of-gamut colors are mapped to the

nearest value within the reproducible gamut. This style of repro-

duction has the advantage of providing exact color matches

from one output medium to another. It has the disadvantage of

causing colors with Y values between the medium’s white point

and 1.0 to be out of gamut. A typical use might be for logos and

solid colors that require exact reproduction across different me-

dia.

RelativeColorimetric

Colors are represented with respect to the combination of the

light source and the output medium’s white point (such as the

color of unprinted paper). Thus, for example, a monitor’s white

point would be reproduced on a printer by simply leaving the

paper unmarked, ignoring color differences between the two

media. In-gamut colors are reproduced exactly; out-of-gamut

colors are mapped to the nearest value within the reproducible

gamut. This style of reproduction has the advantage of adapting

for the varying white points of different output media. It has the

disadvantage of not providing exact color matches from one me-

dium to another. A typical use might be for vector graphics.

Saturation

Colors are represented in a manner that preserves or emphasizes

saturation. Reproduction of in-gamut colors may or may not be

colorimetrically accurate. A typical use might be for business

graphics, where saturation is the most important attribute of the

color.

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NAME

DESCRIPTION

Perceptual

Colors are represented in a manner that provides a pleasing per-

ceptual appearance. To preserve color relationships, both in-

gamut and out-of-gamut colors are generally modified from

their precise colorimetric values. A typical use might be for

scanned images.

4.5.5

Special Color Spaces

Special color spaces add features or properties to an underlying color space.

There are four special color space families: Pattern, Indexed, Separation, and

DeviceN.

Pattern Color Spaces

A Pattern color space (PDF 1.2) enables a PDF content stream to paint an area

with a pattern rather than a single color. The pattern may be either a tiling pattern

(type 1) or a shading pattern (type 2). Section 4.6, “Patterns,” discusses patterns in

detail.

Indexed Color Spaces

An Indexed color space allows a PDF content stream to use small integers as indi-

ces into a color map or color table of arbitrary colors in some other space. A PDF

consumer application treats each sample value as an index into the color table

and uses the color value it finds there. This technique can considerably reduce the

amount of data required to represent a sampled image—for example, by using

8-bit index values as samples instead of 24-bit RGB color values.

An Indexed color space is defined by a four-element array:

[ /Indexed base hival lookup ]

The first element is the color space family name Indexed. The remaining ele-

ments are parameters that an Indexed color space requires; their meanings are

discussed below. Setting the current stroking or nonstroking color space to an

Indexed color space initializes the corresponding current color to 0.

263

SECTION 4.5

Color Spaces

The base parameter is an array or name that identifies the base color space in

which the values in the color table are to be interpreted. It can be any device or

CIE-based color space or (in PDF 1.3) a Separation or DeviceN space, but not a

Pattern space or another Indexed space. For example, if the base color space is

DeviceRGB, the values in the color table are to be interpreted as red, green, and

blue components; if the base color space is a CIE-based ABC space such as a

CalRGB or Lab space, the values are to be interpreted as A, B, and C components.

Note: Attempting to use a Separation or DeviceN color space as the base for an

Indexed color space generates an error in PDF 1.2.

The hival parameter is an integer that specifies the maximum valid index value. In

other words, the color table is to be indexed by integers in the range 0 to hival.

hival can be no greater than 255, which is the integer required to index a table

with 8-bit index values.

The color table is defined by the lookup parameter, which can be either a stream

or (in PDF 1.2) a byte string. It provides the mapping between index values and

the corresponding colors in the base color space.

The color table data must be m × (hival + 1) bytes long, where m is the number of

color components in the base color space. Each byte is an unsigned integer in the

range 0 to 255 that is scaled to the range of the corresponding color component in

the base color space; that is, 0 corresponds to the minimum value in the range for

that component, and 255 corresponds to the maximum.

Note: PostScript uses a different interpretation of an Indexed color space’s color ta-

ble. In PostScript, the component value is always scaled to the range 0.0 to 1.0, re-

gardless of the range of color values in the base color space.

The color components for each entry in the table appear consecutively in the

string or stream. For example, if the base color space is DeviceRGB and the

indexed color space contains two colors, the order of bytes in the string or stream

is R₀ G₀ B₀ R₁ G₁ B₁ , where letters denote the color component and numeric

subscripts denote the table entry.

Example 4.10 illustrates the specification of an Indexed color space that maps

8-bit index values to three-component color values in the DeviceRGB color space.

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Example 4.10

[

/Indexed

/DeviceRGB

255

< 000000 FF0000 00FF00 0000FF B57342 … >

]

The example shows only the first five color values in the lookup string; in all, there

should be 256 color values and the string should be 768 bytes long. Having

established this color space, the program can now specify colors as single-compo-

nent values in the range 0 to 255. For example, a color value of 4 selects an RGB

color whose components are coded as the hexadecimal integers B5, 73, and 42.

Dividing these by 255 and scaling the results to the range 0.0 to 1.0 yields a color

with red, green, and blue components of 0.710, 0.451, and 0.259, respectively.

Although an Indexed color space is useful mainly for images, index values can

also be used with the color selection operators SC, SCN, sc, and scn. For example:

123 sc

selects the same color as does an image sample value of 123. The index value

should be an integer in the range 0 to hival. If the value is a real number, it is

rounded to the nearest integer; if it is outside the range 0 to hival, it is adjusted to

the nearest value within that range.

Separation Color Spaces

Color output devices produce full color by combining primary or process

colorants in varying amounts. On an additive color device such as a display, the

primary colorants consist of red, green, and blue phosphors; on a subtractive de-

vice such as a printer, they typically consist of cyan, magenta, yellow, and some-

times black inks. In addition, some devices can apply special colorants, often

called spot colorants, to produce effects that cannot be achieved with the standard

process colorants alone. Examples include metallic and fluorescent colors and

special textures.

When printing a page, most devices produce a single composite page on which all

process colorants (and spot colorants, if any) are combined. However, some de-

vices, such as imagesetters, produce a separate, monochromatic rendition of the

page, called a separation, for each colorant. When the separations are later com-

265

SECTION 4.5

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bined—on a printing press, for example—and the proper inks or other colorants

are applied to them, the result is a full-color page.

A Separation color space (PDF 1.2) provides a means for specifying the use of

additional colorants or for isolating the control of individual color components of

a device color space for a subtractive device. When such a space is the current

color space, the current color is a single-component value, called a tint, that con-

trols the application of the given colorant or color components only.

Note: The term separation is often misused as a synonym for an individual device

colorant. In the context of this discussion, a printing system that produces separa-

tions generates a separate piece of physical medium (generally film) for each color-

ant. It is these pieces of physical medium that are correctly referred to as separations.

A particular colorant properly constitutes a separation only if the device is generat-

ing physical separations, one of which corresponds to the given colorant. The

Separation color space is so named for historical reasons, but it has evolved to the

broader purpose of controlling the application of individual colorants in general, re-

gardless of whether they are actually realized as physical separations.

Note also that the operation of a Separation color space itself is independent of the

characteristics of any particular output device. Depending on the device, the space

may or may not correspond to a true, physical separation or to an actual colorant.

For example, a Separation color space could be used to control the application of a

single process colorant (such as cyan) on a composite device that does not produce

physical separations, or could represent a color (such as orange) for which no specif-

ic colorant exists on the device. A Separation color space provides consistent, pre-

dictable behavior, even on devices that cannot directly generate the requested color.

A Separation color space is defined as follows:

[ /Separation name alternateSpace tintTransform ]

In other words, it is a four-element array whose first element is the color space

family name Separation. The remaining elements are parameters that a

Separation color space requires; their meanings are discussed below.

A color value in a Separation color space consists of a single tint component in

the range 0.0 to 1.0. The value 0.0 represents the minimum amount of colorant

that can be applied; 1.0 represents the maximum. Tints are always treated as

subtractive colors, even if the device produces output for the designated compo-

nent by an additive method. Thus, a tint value of 0.0 denotes the lightest color

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that can be achieved with the given colorant, and 1.0 is the darkest. (This conven-

tion is the same as for DeviceCMYK color components but opposite to the one for

DeviceGray and DeviceRGB.) The initial value for both the stroking and non-

stroking color in the graphics state is 1.0. The SCN and scn operators respectively

set the current stroking and nonstroking color to a tint value. A sampled image

with single-component samples can also be used as a source of tint values.

The name parameter is a name object specifying the name of the colorant that

this Separation color space is intended to represent (or one of the special names

All or None; see below). Such colorant names are arbitrary, and there can be any

number of them, subject to implementation limits.

The special colorant name All refers collectively to all colorants available on an

output device, including those for the standard process colorants. When a

Separation space with this colorant name is the current color space, painting

operators apply tint values to all available colorants at once. This is useful for pur-

poses such as painting registration targets in the same place on every separation.

Such marks are typically painted as the last step in composing a page to ensure

that they are not overwritten by subsequent painting operations.

The special colorant name None never produces any visible output. Painting op-

erations in a Separation space with this colorant name have no effect on the cur-

rent page.

All devices support Separation color spaces with the colorant names All and

None, even if they do not support any others. Separation spaces with either of

these colorant names ignore the alternateSpace and tintTransform parameters (dis-

cussed below), although valid values must still be provided.

At the moment the color space is set to a Separation space, the consumer applica-

tion determines whether the device has an available colorant corresponding to

the name of the requested space. If so, the application ignores the alternateSpace

and tintTransform parameters; subsequent painting operations within the space

apply the designated colorant directly, according to the tint values supplied.

Note: The preceding paragraph applies only to subtractive output devices such as

printers and imagesetters. For an additive device such as a computer display, a

Separation color space never applies a process colorant directly; it always reverts to

the alternate color space as described below. This is because the model of applying

process colorants independently does not work as intended on an additive device; for

267

SECTION 4.5

Color Spaces

instance, painting tints of the Red component on a white background produces a

result that varies from white to cyan.

Note that this exception applies only to colorants for additive devices, not to the spe-

cific names Red, Green, and Blue. In contrast, a printer might have a (subtractive)

ink named, for example, Red, which should work as a Separation color space just

the same as any other supported colorant.

If the colorant name associated with a Separation color space does not cor-

respond to a colorant available on the device, the application arranges for subse-

quent painting operations to be performed in an alternate color space. The

intended colors can be approximated by colors in a device or CIE-based color

space, which are then rendered with the usual primary or process colorants:

• The alternateSpace parameter must be an array or name object that identifies

the alternate color space, which can be any device or CIE-based color space but

not another special color space (Pattern, Indexed, Separation, or DeviceN).

• The tintTransform parameter must be a function (see Section 3.9, “Functions”).

During subsequent painting operations, an application calls this function to

transform a tint value into color component values in the alternate color space.

The function is called with the tint value and must return the corresponding

color component values. That is, the number of components and the interpre-

tation of their values depend on the alternate color space.

Note: Painting in the alternate color space may produce a good approximation of

the intended color when only opaque objects are painted. However, it does not cor-

rectly represent the interactions between an object and its backdrop when the object

is painted with transparency or when overprinting (see Section 4.5.6, “Overprint

Control”) is enabled.

Example 4.11 illustrates the specification of a Separation color space (object 5)

that is intended to produce a color named LogoGreen. If the output device has no

colorant corresponding to this color, DeviceCMYK is used as the alternate color

space, and the tint transformation function (object 12) maps tint values linearly

into shades of a CMYK color value approximating the LogoGreen color.

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Example 4.11

5 0 obj

% Color space

[

/Separation

/LogoGreen

/DeviceCMYK

12 0 R

]

endobj

12 0 obj

% Tint transformation function

<< /FunctionType 4

/Domain [ 0.0 1.0 ]

/Range [ 0.0 1.0

0.0

1.0

0.0

1.0

0.0 1.0 ]

/Length 62

stream

{ dup 0.84 mul

exch 0.00 exch dup 0.44 mul

exch 0.21 mul

}

endstream

endobj

See Section 7.6.2, “Spot Colors and Transparency,” for further discussion of the

role of Separation color spaces in the transparent imaging model.

DeviceN Color Spaces

DeviceN color spaces (PDF 1.3) can contain an arbitrary number of color compo-

nents. They provide greater flexibility than is possible with standard device color

spaces such as DeviceCMYK or with individual Separation color spaces. For ex-

ample, it is possible to create a DeviceN color space consisting of only the cyan,

magenta, and yellow color components, with the black component excluded.

DeviceN color spaces are used in applications such as these:

• High-fidelity color is the use of more than the standard CMYK process colo-

rants to produce an extended gamut, or range of colors. A popular example is

269

SECTION 4.5

Color Spaces

the PANTONE Hexachrome system, which uses six colorants: the usual cyan,

magenta, yellow, and black, plus orange and green.

• Multitone color systems use a single-component image to specify multiple color

components. In a duotone, for example, a single-component image can be used

to specify both the black component and a spot color component. The tone

reproduction is generally different for the different components. For example,

the black component might be painted with the exact sample data from the sin-

gle-component image; the spot color component might be generated as a

nonlinear function of the image data in a manner that emphasizes the shadows.

Plate 6 shows an example that uses black and magenta color components. In

Plate 7, a single-component grayscale image is used to generate a quadtone re-

sult that uses four colorants: black and three PANTONE spot colors. See Exam-

ple 4.21 on page 282 for the code used to generate this image.

DeviceN was designed to represent color spaces containing multiple components

that correspond to colorants of some target device. As with Separation color

spaces, PDF consumer applications must be able to approximate the colorants if

they are not available on the current output device, such as a display. To accom-

plish this, the color space definition provides a tint transformation function that

can be used to convert all the components to an alternate color space.

PDF 1.6 extends the meaning of DeviceN to include color spaces that are referred

to as NChannel color spaces. Such color spaces may contain an arbitrary number

of spot and process components, which may or may not correspond to specific

device colorants (the process components must be from a single process color

space). They provide information about each component that allows applications

more flexibility in converting colors. For example, they may use their own blend-

ing algorithms for on-screen viewing and composite printing, rather than being

required to use a specified tint transformation function. These color spaces are

identified by a value of NChannel for the Subtype entry of the attributes dictio-

nary (see Table 4.21). A value of DeviceN for the Subtype entry, or no value,

means that only the previous features are supported. PDF consumer applications

that do not support PDF 1.6 treat these color spaces as normal DeviceN color

spaces and use the tint transformation function as appropriate. Producer applica-

tions using the NChannel features should follow certain guidelines, as noted

throughout this section, to achieve good backward compatibility.

DeviceN color spaces are defined in a similar way to Separation color spaces—in

fact, a Separation color space can be defined as a DeviceN color space with only

one component.

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A DeviceN color space is specified as follows:

[ /DeviceN names alternateSpace tintTransform ]

[ /DeviceN names alternateSpace tintTransform attributes ]

It is a four- or five-element array whose first element is the color space family

name DeviceN. The remaining elements are parameters that a DeviceN color

space requires.

The names parameter is an array of name objects specifying the individual color

components. The length of the array determines the number of components in

the DeviceN color space, which is subject to an implementation limit; see Appen-

dix C.The component names must all be different from one another, except for

the name None, which can be repeated as described later in this section. (The

special name All, used by Separation color spaces, is not allowed.)

Color values are tint components in the range 0.0 to 1.0:

• For DeviceN color spaces that do not have a subtype of NChannel, 0.0 always

represents the minimum amount of colorant; 1.0 represents the maximum.

Tints are always treated as subtractive colors, even if the device produces out-

put for the designated component by an additive method. Thus, a tint value of

0.0 denotes the lightest color that can be achieved with the given colorant, and

1.0 the darkest. (This convention is the same one as for DeviceCMYK color

components but opposite to the one for DeviceGray and DeviceRGB.)

• For NChannel color spaces, values for additive process colors (such as RGB) are

specified in their natural form, where 1.0 represents maximum intensity of col-

or.

When this space is set to the current color space (using the CS or cs operators),

each component is given an initial value of 1.0. The SCN and scn operators re-

spectively set the current stroking and nonstroking color. Operand values sup-

plied to SCN or scn are interpreted as color component values in the order in

which the colors are given in the names array, as are the values in a sampled im-

age that uses a DeviceN color space.

The alternateSpace parameter is an array or name object that can be any device or

CIE-based color space but not another special color space (Pattern, Indexed,

Separation, or DeviceN). When the color space is set to a DeviceN space, if any of

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SECTION 4.5

Color Spaces

the component names in the color space do not correspond to a colorant avail-

able on the device, the PDF consumer application can perform subsequent paint-

ing operations in the alternate color space specified by this parameter.

Note: For NChannel color spaces, the components are evaluated individually; that

is, only the ones not present on the output device use the alternate color space.

The tintTransform parameter specifies a function (see Section 3.9, “Functions”)

that is used to transform the tint values into the alternate color space. It is called

with n tint values and returns m color component values, where n is the number

of components needed to specify a color in the DeviceN color space and m is the

number required by the alternate color space.

Note: Painting in the alternate color space may produce a good approximation of

the intended color when only opaque objects are painted. However, it does not cor-

rectly represent the interactions between an object and its backdrop when the object

is painted with transparency or when overprinting (see Section 4.5.6, “Overprint

Control”) is enabled.

The color component name None, which may be present only for DeviceN color

spaces that do not have the NChannel subtype, indicates that the corresponding

color component is never painted on the page, as in a Separation color space for

the None colorant. (However, see implementation note 48 in Appendix H.) When

a DeviceN color space is painting the named device colorants directly, color com-

ponents corresponding to None colorants are discarded. However, when the

DeviceN color space reverts to its alternate color space, those components are

passed to the tint transformation function, which can use them as desired.

Note: A DeviceN color space whose component colorant names are all None always

discards its output, just the same as a Separation color space for None; it never

reverts to the alternate color space. Reversion occurs only if at least one color com-

ponent (other than None) is specified and is not available on the device.

The optional attributes parameter is a dictionary (see Table 4.21) containing addi-

tional information about the components of color space that PDF consumer ap-

plications may use. PDF consumers are not required to use the alternateSpace and

tintTransform parameters, and may instead use custom blending algorithms, along

with other information provided in the attributes dictionary if present. (If the val-

ue of the Subtype entry in the attributes dictionary is NChannel, such informa-

tion must be present.) However, alternateSpace and tintTransform must always be

provided for applications that want to use them or do not support PDF 1.6.

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TABLE 4.21 Entries in a DeviceN color space attributes dictionary

KEY

TYPE

VALUE

Subtype

name

(Optional; PDF 1.6) A name specifying the preferred treatment for the color

space. Possible values are DeviceN and NChannel. Default value: DeviceN.

Colorants

dictionary

(Required if Subtype is NChannel and the color space includes spot colorants; other-

wise optional) A dictionary describing the individual colorants used in the

DeviceN color space. For each entry in this dictionary, the key is a colorant name

and the value is an array defining a Separation color space for that colorant (see

“Separation Color Spaces” on page 264). The key must match the colorant name

given in that color space.

This dictionary provides information about the individual colorants that may be

useful to some applications. In particular, the alternate color space and tint trans-

formation function of a Separation color space describe the appearance of that

colorant alone, whereas those of a DeviceN color space describe only the appear-

ance of its colorants in combination.

If Subtype is NChannel, this dictionary must have entries for all spot colorants in

this color space. This dictionary may also include additional colorants not used

by this color space.

Process

dictionary

(Required if Subtype is NChannel and the color space includes components of a pro-

cess color space, otherwise optional; PDF 1.6) A dictionary (see Table 4.22) that de-

scribes the process color space whose components are included in this color

space.

MixingHints

dictionary

(Optional; PDF 1.6) A dictionary (see Table 4.23) that specifies optional attributes

of the inks to be used in blending calculations when used as an alternative to the

tint transformation function.

A value of NChannel for the Subtype entry indicates that some of the other en-

tries in this dictionary are required rather than optional. The Colorants entry

specifies a colorants dictionary that contains entries for all the spot colorants in

the color space; they are defined using individual Separation color spaces. The

Process entry specifies a process dictionary (see Table 4.22) that identifies the pro-

cess color space that is used by this color space and the names of its components.

It must be present if Subtype is NChannel and the color space has process color

components. (An NChannel color space may contain components from at most

one process color space.)

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Color Spaces

For color spaces that have a value of NChannel for the Subtype entry in the at-

tributes dictionary (see Table 4.21), the following restrictions apply to process

colors:

• There can be color components from at most one process color space, which

can be any device or CIE-based color space.

• For a non-CMYK color space, the names of the process components must ap-

pear sequentially in the names array, in the normal color space order (for exam-

ple, Red, Green, and Blue). However, the names in the names array need not

match the actual color space names (for example, a Red component need not be

named Red).The mapping of names is specified in the process dictionary (see

Table 4.22 and discussion below), which is required to be present.

• Definitions for process colorants should not appear in the colorants dictionary.

Any such definition should be ignored if the colorant is also present in the pro-

cess dictionary. Any component not specified in the process dictionary is con-

sidered to be a spot colorant.

• For a CMYK color space, a subset of the components may be present, and they

may appear in any order in the names array. The reserved names Cyan,

Magenta, Yellow, and Black are always considered to be process colors, which

do not necessarily correspond to the colorants of a specific device; they are not

required to have entries in the process dictionary.

• The values associated with the process components must be stored in their nat-

ural form (that is, subtractive color values for CMYK and additive color values

for RGB), since they are interpreted directly as process values by consumers

making use of the process dictionary. (For additive color spaces, this is the re-

verse of how color values are specified for DeviceN, as described above in the

discussion of the names parameter.)

The MixingHints entry in the attributes dictionary specifies a mixing hints dictio-

nary (see Table 4.23) that provides information about the characteristics of colo-

rants that can be used in blending calculations when the actual colorants are not

available on the target device. Applications are not required to use this informa-

tion.

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TABLE 4.22 Entries in a DeviceN process dictionary

KEY

TYPE

VALUE

ColorSpace name or

(Required) A name or array identifying the process color space, which may be any

array

device or CIE-based color space. If an ICCBased color space is specified, it must

provide calibration information appropriate for the process color components

specified in the names array of the DeviceN color space.

Components array

(Required) An array of component names that correspond, in order, to the com-

ponents of the process color space specified in ColorSpace. For example, an RGB

color space must have three names corresponding to red, green, and blue. The

names may be arbitrary (that is, not the same as the standard names for the color

space components) and must match those specified in the names array of the

DeviceN color space, even if all components are not present in the names array.

TABLE 4.23 Entries in a DeviceN mixing hints dictionary

KEY

TYPE

VALUE

Solidities

dictionary

(Optional) A dictionary specifying the solidity of inks to be used in blending

calculations when used as an alternative to the tint transformation function.

For each entry, the key is a colorant name, and the value is a number between

0.0 and 1.0. This dictionary need not contain entries for all colorants used in

this color space; it may also include additional colorants not used by this color

space.

A value of 1.0 simulates an ink that completely covers the inks beneath; a value

of 0.0 simulates a transparent ink that completely reveals the inks beneath. An

entry with a key of Default specifies a value to be used by all components in the

associated DeviceN color space for which a solidity value is not explicitly pro-

vided. If Default is not present, the default value for unspecified colorants is

0.0; applications may choose to use other values.

If this entry is present, PrintingOrder must also be present.

PrintingOrder

array

(Required if Solidities is present) An array of colorant names, specifying the or-

der in which inks are laid down. Each component in the names array of the

DeviceN color space must appear in this array (although the order is unrelated

to the order specified in the names array). This entry may also list colorants

unused by this specific DeviceN instance.

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SECTION 4.5

Color Spaces

KEY

TYPE

VALUE

DotGain

dictionary

(Optional) A dictionary specifying the dot gain of inks to be used in blending

calculations when used as an alternative to the tint transformation function.

Dot gain (or loss) represents the amount by which a printer’s halftone dots

change as the ink spreads and is absorbed by paper.

For each entry, the key is a colorant name, and the value is a function that maps

values in the range 0 to 1 to values in the range 0 to 1. The dictionary may list

colorants unused by this specific DeviceN instance and need not list all colo-

rants. An entry with a key of Default specifies a function to be used by all colo-

rants for which a dot gain function is not explicitly specified.

PDF consumer applications may ignore values in this dictionary when other

sources of dot gain information are available, such as ICC profiles associated

with the process color space or tint transformation functions associated with

individual colorants.

Each entry in the mixing hints dictionary refers to colorant names, which include

spot colorants referenced by the Colorants dictionary. Under some circumstanc-

es, they may also refer to one or more individual process components called

Cyan, Magenta, Yellow, or Black when DeviceCMYK is specified as the process col-

or space in the process dictionary. However, applications should ignore these pro-

cess component entries if they can obtain the information from an ICC profile.

Note: The mixing hints subdictionaries (as well as the colorants dictionary) may

specify colorants that are not used in any given instance of a DeviceN color space.

This allows them to be referenced from multiple DeviceN color spaces, which can

produce smaller file sizes as well as consistent color definitions across instances.

For consistency of color, PDF consumers should follow these guidelines:

• The consumer should apply either the specified tint transformation function or

invoke the same alternative blending algorithm for all DeviceN instances in the

document.

Note: When the tint transformation function is used, the burden is on the produc-

er to guarantee that the individual function definitions chosen for all DeviceN in-

stances produce similar color appearances throughout the document.

• Blending algorithms should produce a similar appearance for colors when they

are used as separation colors or as a component of a DeviceN color space.

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Example 4.12 shows a DeviceN color space consisting of three color components

named Orange, Green, and None. In this example, the DeviceN color space,

object 30, has an attributes dictionary whose Colorants entry is an indirect refer-

ence to object 45 (which might also be referenced by attributes dictionaries of

other DeviceN color spaces). tintTransform1, whose definition is not shown, maps

three color components (tints of the colorants Orange, Green, and None) to four

color components in the alternate color space, DeviceCMYK. tintTransform2 maps

a single color component (an orange tint) to four components in DeviceCMYK.

Likewise, tintTransform3 maps a green tint to DeviceCMYK, and tintTransform4

maps a tint of PANTONE 131 to DeviceCMYK.

Example 4.12

30 0 obj

% Color space

[

/DeviceN

[ /Orange /Green /None ]

/DeviceCMYK

tintTransform1

<< /Colorants 45 0 R >>

]

endobj

45 0 obj

% Colorants dictionary

<< /Orange [ /Separation

/Orange

/DeviceCMYK

tintTransform2

]

/Green [ /Separation

/Green

/DeviceCMYK

tintTransform3

]

/PANTONE#20131 [ /Separation

/PANTONE#20131

/DeviceCMYK

tintTransform4

]

endobj

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SECTION 4.5

Color Spaces

Examples 4.13 through 4.16 show the use of NChannel color spaces. Example 4.13

shows the use of calibrated CMYK process components. Example 4.14 shows the

use of Lab process components.

Example 4.13

10 0 obj

% Color space

[

/DeviceN

[/Magenta /Spot1 /Yellow /Spot2]

alternateSpace

tintTransform1

% Attributes dictionary

/Subtype /NChannel

/Process

<< /ColorSpace [/ICCBased CMYK_ICC profile ]

/Components [/Cyan /Magenta /Yellow /Black]

/Colorants

<< /Spot1 [/Separation /Spot1 alternateSpace tintTransform2]

/Spot2 [/Separation /Spot2 alternateSpace tintTransform3]

]

endobj

Example 4.14

10 0 obj

% Color space

[ /DeviceN

[/L /a /b /Spot1 /Spot2]

alternateSpace

tintTransform1

% Attributes dictionary

/Subtype /NChannel

/Process

<< /ColorSpace [ /Lab << /WhitePoint ... /Range ... >> ]

/Components [/L /a /b]

/Colorants

<< /Spot1 [/Separation /Spot1 alternateSpace tintTransform2 ]

/Spot2 [/Separation /Spot2 alternateSpace tintTransform3]

]

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Example 4.15 shows the recommended convention for dealing with situations

where a spot colorant and a process color component have the same name. Since

the names array may not have duplicate names, the process colors should be given

different names, which are mapped to process components in the Components

entry of the process dictionary. In this case, Red refers to a spot colorant;

ProcessRed, ProcessGreen, and ProcessBlue are mapped to the components of an

RGB color space.

Example 4.15

10 0 obj

% Color space

[ /DeviceN

[/ProcessRed /ProcessGreen /ProcessBlue /Red]

alternateSpace

tintTransform1

% Attributes dictionary

/Subtype /NChannel

/Process

<< /ColorSpace [ /ICCBased RGB_ICC profile ]

/Components [/ProcessRed /ProcessGreen /ProcessBlue]

/Colorants

<< /Red [/Separation /Red alternateSpace tintTransform2 ] >>

]

Example 4.16 shows the use of a mixing hints dictionary.

Example 4.16

10 0 obj

% Color space

[/DeviceN

[/Magenta /Spot1 /Yellow /Spot2]

alternateSpace

tintTransform1

/Subtype /NChannel

/Process

<< /ColorSpace [ /ICCBased CMYK_ICC profile ]

/Components [/Cyan /Magenta /Yellow /Black]

/Colorants

<< /Spot1 [/Separation /Spot1 alternateSpace tintTransform2 ]

/Spot2 [/Separation /Spot2 alternateSpace tintTransform2 ]

279

SECTION 4.5

Color Spaces

/MixingHints

/Solidities

<< /Spot1 1.0

/Spot2 0.0

/DotGain

<< /Spot1 function1

/Spot2 function2

/Magenta function3

/Yellow function4

/PrintingOrder [/Magenta /Yellow /Spot1 /Spot2]

]

See Section 7.6.2, “Spot Colors and Transparency,” for further discussion of the

role of DeviceN color spaces in the transparent imaging model.

Multitone Examples

The following examples illustrate various interesting and useful special cases of

the use of Indexed and DeviceN color spaces in combination to produce multi-

tone colors.

Examples 4.17 and 4.18 illustrate the use of DeviceN to create duotone color spac-

es. In Example 4.17, an Indexed color space maps index values in the range 0 to

255 to a duotone DeviceN space in cyan and black. In effect, the index values are

treated as if they were tints of the duotone space, which are then mapped into

tints of the two underlying colorants. Only the beginning of the lookup table

string for the Indexed color space is shown; the full table would contain 256 two-

byte entries, each specifying a tint value for cyan and black, for a total of 512

bytes. If the alternate color space of the DeviceN space is selected, the tint trans-

formation function (object 15 in the example) maps the two tint components for

cyan and black to the four components for a DeviceCMYK color space by supply-

ing zero values for the other two components. Example 4.18 shows the definition

of another duotone color space, this time using black and gold colorants (where

gold is a spot colorant) and using a CalRGB space as the alternate color space. This

could be defined in the same way as in the preceding example, with a tint trans-

280

CHAPTER 4

Graphics

formation function that converts from the two tint components to colors in the

alternate CalRGB color space.

Example 4.17

10 0 obj

% Color space

[

/Indexed

[ /DeviceN

[ /Cyan /Black ]

/DeviceCMYK

15 0 R

]

255

< 6605 6806 6907 6B09 6C0A … >

]

endobj

15 0 obj

% Tint transformation function

<< /FunctionType 4

/Domain [ 0.0 1.0

0.0 1.0 ]

/Range [ 0.0 1.0

0.0

1.0

0.0

1.0

0.0 1.0 ]

/Length 16

stream

{ 0

0 3 −1 roll }

endstream

endobj

Example 4.18

30 0 obj

% Color space

[

/Indexed

[

/DeviceN

[ /Black

/Gold ]

[

/CalRGB

<< /WhitePoint [ 1.0 1.0 1.0 ]

/Gamma [ 2.2 2.2 2.2 ]

]

35 0 R

% Tint transformation function

]

255

… Lookup table…

]

endobj

281

SECTION 4.5

Color Spaces

Given a formula for converting any combination of black and gold tints to cali-

brated RGB, a 2-in, 3-out type 4 (PostScript calculator) function could be used for

the tint transformation. Alternatively, a type 0 (sampled) function could be used,

but this would require a large number of sample points to represent the function

accurately; for example, sampling each input variable for 256 tint values between

0.0 and 1.0 would require 256 ² = 65,536 samples. But since the DeviceN color

space is being used as the base of an Indexed color space, there are actually only

256 possible combinations of black and gold tint values. A more compact way to

represent this information is to put the alternate color values directly into the

lookup table alongside the DeviceN color values, as in Example 4.19.

Example 4.19

10 0 obj

% Color space

[

/Indexed

[

/DeviceN

[ /Black

/Gold /None /None /None ]

[

/CalRGB

<< /WhitePoint [ 1.0 1.0 1.0 ]

/Gamma [ 2.2 2.2 2.2 ]

]

20 0 R

% Tint transformation function

]

255

… Lookup table…

]

endobj

In this example, each entry in the lookup table has five components: two for the

black and gold colorants and three more (specified as None) for the equivalent

CalRGB color components. If the black and gold colorants are available on the

output device, the None components are ignored; if black and gold are not

available, the tint transformation function is used to convert a five-component

color into a three-component equivalent in the alternate CalRGB color space.

But because, by construction, the third, fourth, and fifth components are the

CalRGB components, the tint transformation function can merely discard the

first two components and return the last three. This can be easily expressed

with a type 4 (PostScript calculator) function, as shown in Example 4.20.

282

CHAPTER 4

Graphics

Example 4.20

20 0 obj

% Tint transformation function

<< /FunctionType 4

/Domain [ 0.0 1.0

0.0

1.0

0.0

1.0

0.0

1.0

0.0 1.0 ]

/Range [ 0.0 1.0

0.0

1.0

0.0 1.0 ]

/Length 27

stream

{ 5

3 roll pop pop }

endstream

endobj

Example 4.21 uses an extension of the techniques described above to produce the

quadtone (four-component) image shown in Plate 7.

Example 4.21

5 0 obj

% Image XObject

<< /Type /XObject

/Subtype /Image

/Width 288

/Height 288

/ColorSpace 10 0 R

/BitsPerComponent 8

/Length 105278

/Filter

/ASCII85Decode

stream

… Data for grayscale image…

endstream

endobj

10 0 obj

% Indexed color space for image

[

/Indexed

15 0 R

% Base color space

255

% Table has 256 entries

30 0 R

% Lookup table

]

endobj

283

SECTION 4.5

Color Spaces

15 0 obj

% Base color space ( DeviceN ) for Indexed space

[

/DeviceN

[

/Black

% Four colorants (black plus three spot colors)

/PANTONE#20216#20CVC

/PANTONE#20409#20CVC

/PANTONE#202985#20CVC

/None

% Three components for alternate space

/None

]

16 0 R

% Alternate color space

20 0 R

% Tint transformation function

]

endobj

16 0 obj

% Alternate color space for DeviceN space

[

/CalRGB

<< /WhitePoint [ 1.0 1.0 1.0 ] >>

]

endobj

20 0 obj

% Tint transformation function for DeviceN space

<< /FunctionType 4

/Domain [ 0.0 1.0

0.0

1.0

0.0

1.0

0.0

1.0

0.0

1.0

0.0

1.0

0.0 1.0 ]

/Range [ 0.0 1.0

0.0

1.0

0.0 1.0 ]

/Length 44

stream

{

7 3 roll

% Just discard first four values

pop pop pop pop

}

endstream

endobj

30 0 obj

% Lookup table for Indexed color space

<< /Length 1975

/Filter

[ /ASCII85Decode /FlateDecode ]

stream

8;T1BB2"M7*!"psYBt1k\gY1T<D&tO]r*F7Hga*

… Additional data ( seven components for each table entry )…

endstream

endobj

284

CHAPTER 4

Graphics

As in the preceding examples, an Indexed color space based on a DeviceN space is

used to paint the grayscale image shown on the left in the plate with four colo-

rants: black and three PANTONE spot colors. The alternate color space is a sim-

ple calibrated RGB. Thus, the DeviceN color space has seven components: the

four desired colorants plus the three components of the alternate space. The ex-

ample shows the image XObject (see Section 4.8.4, “Image Dictionaries”) repre-

senting the quadtone image, followed by the color space used to interpret the

image data. (See implementation note 49 in Appendix H.)

4.5.6

Overprint Control

The graphics state contains an overprint parameter, controlled by the OP and op

entries in a graphics state parameter dictionary. Overprint control is useful main-

ly on devices that produce true physical separations, but it is available on some

composite devices as well. Although the operation of this parameter is device-

dependent, it is described here rather than in the chapter on color rendering,

because it pertains to an aspect of painting in device color spaces that is impor-

tant to many applications.

Any painting operation marks some specific set of device colorants, depending

on the color space in which the painting takes place. In a Separation or DeviceN

color space, the colorants to be marked are specified explicitly; in a device or

CIE-based color space, they are implied by the process color model of the output

device (see Chapter 6). The overprint parameter is a boolean flag that determines

how painting operations affect colorants other than those explicitly or implicitly

specified by the current color space.

If the overprint parameter is false (the default value), painting a color in any color

space causes the corresponding areas of unspecified colorants to be erased (paint-

ed with a tint value of 0.0). The effect is that the color at any position on the page

is whatever was painted there last, which is consistent with the normal painting

behavior of the opaque imaging model.

If the overprint parameter is true and the output device supports overprinting, no

such erasing actions are performed; anything previously painted in other colo-

rants is left undisturbed. Consequently, the color at a given position on the page

may be a combined result of several painting operations in different colorants.

The effect produced by such overprinting is device-dependent and is not defined

by the PDF language.

285

SECTION 4.5

Color Spaces

Note: Not all devices support overprinting. Furthermore, many PostScript printers

support it only when separations are being produced, and not for composite output.

If overprinting is not supported, the value of the overprint parameter is ignored.

An additional graphics state parameter, the overprint mode (PDF 1.3), affects the

interpretation of a tint value of 0.0 for a color component in a DeviceCMYK color

space when overprinting is enabled. This parameter is controlled by the OPM

entry in a graphics state parameter dictionary; it has an effect only when the over-

print parameter is true, as described above.

When colors are specified in a DeviceCMYK color space and the native color space

of the output device is also DeviceCMYK, each of the source color components

controls the corresponding device colorant directly. Ordinarily, each source color

component value replaces the value previously painted for the corresponding de-

vice colorant, no matter what the new value is; this is the default behavior, speci-

fied by overprint mode 0.

When the overprint mode is 1 (also called nonzero overprint mode), a tint value of

0.0 for a source color component leaves the corresponding component of the

previously painted color unchanged. The effect is equivalent to painting in a

DeviceN color space that includes only those components whose values are non-

zero. For example, if the overprint parameter is true and the overprint mode is 1,

the operation

0. 2

0. 3

0.0

1.0 k

is equivalent to

0. 2

0. 3

1.0 scn

in the color space shown in Example 4.22.

Example 4.22

10 0 obj

% Color space

[

/DeviceN

[ /Cyan /Magenta /Black ]

/DeviceCMYK

15 0 R

]

endobj

286

CHAPTER 4

Graphics

15 0 obj

% Tint transformation function

<< /FunctionType 4

/Domain [ 0.0 1.0

0.0

1.0

0.0 1.0 ]

/Range [ 0.0 1.0

0.0

1.0

0.0

1.0

0.0 1.0 ]

/Length 13

stream

{ 0 exch }

endstream

endobj

Nonzero overprint mode applies only to painting operations that use the current

color in the graphics state when the current color space is DeviceCMYK (or is im-

plicitly converted to DeviceCMYK; see “Implicit Conversion of CIE-Based Color

Spaces” on page 259). It does not apply to the painting of images or to any colors

that are the result of a computation, such as those in a shading pattern or conver-

sions from some other color space. It also does not apply if the device’s native col-

or space is not DeviceCMYK; in that case, source colors must be converted to the

device’s native color space, and all components participate in the conversion,

whatever their values. (This is shown explicitly in the alternate color space and

tint transformation function of the DeviceN color space in Example 4.22.)

See Section 7.6.3, “Overprinting and Transparency,” for further discussion of the

role of overprinting in the transparent imaging model.

4.5.7

Color Operators

Table 4.24 lists the PDF operators that control color spaces and color values. (Also

color-related is the graphics state operator ri, listed in Table 4.7 on page 219 and

discussed under “Rendering Intents” on page 260.) Color operators may appear at

the page description level or inside text objects (see Figure 4.1 on page 197).

287

SECTION 4.5

Color Spaces

TABLE 4.24 Color operators

OPERANDS

OPERATOR

DESCRIPTION

name

(PDF 1.1) Set the current color space to use for stroking operations. The operand

name must be a name object. If the color space is one that can be specified by a

name and no additional parameters (DeviceGray, DeviceRGB, DeviceCMYK, and

certain cases of Pattern), the name may be specified directly. Otherwise, it must

be a name defined in the ColorSpace subdictionary of the current resource dic-

tionary (see Section 3.7.2, “Resource Dictionaries”); the associated value is an

array describing the color space (see Section 4.5.2, “Color Space Families”).

Note: The names DeviceGray, DeviceRGB, DeviceCMYK, and Pattern always iden-

tify the corresponding color spaces directly; they never refer to resources in the

ColorSpace subdictionary.

The CS operator also sets the current stroking color to its initial value, which de-

pends on the color space:

• In a DeviceGray, DeviceRGB, CalGray, or CalRGB color space, the initial color

has all components equal to 0.0.

• In a DeviceCMYK color space, the initial color is [ 0.0 0.0 0.0 1.0 ].

• In a Lab or ICCBased color space, the initial color has all components equal to

0.0 unless that falls outside the intervals specified by the space’s Range entry,

in which case the nearest valid value is substituted.

• In an Indexed color space, the initial color value is 0.

• In a Separation or DeviceN color space, the initial tint value is 1.0 for all colo-

rants.

• In a Pattern color space, the initial color is a pattern object that causes nothing

to be painted.

name

(PDF 1.1) Same as CS but used for nonstroking operations.

c₁… c

(PDF 1.1) Set the color to use for stroking operations in a device, CIE-based

(other than ICCBased), or Indexed color space. The number of operands re-

quired and their interpretation depends on the current stroking color space:

• For DeviceGray, CalGray, and Indexed color spaces, one operand is required

(n = 1).

• For DeviceRGB, CalRGB, and Lab color spaces, three operands are required

(n = 3).

• For DeviceCMYK, four operands are required (n = 4).

288

CHAPTER 4

Graphics

OPERANDS

OPERATOR

DESCRIPTION

SCN

(PDF 1.2) Same as SC but also supports Pattern, Separation, DeviceN, and

1… cn

name

SCN

ICCBased color spaces.

1… cn

If the current stroking color space is a Separation, DeviceN, or ICCBased color

space, the operands c

1… cn are numbers. The number of operands and their in-

terpretation depends on the color space.

If the current stroking color space is a Pattern color space, name is the name of

an entry in the Pattern subdictionary of the current resource dictionary (see

Section

3.7.2,

“Resource Dictionaries”). For an uncolored tiling pattern

(PatternType = 1 and PaintType = 2), c

1 … cn are component values specifying a

color in the pattern’s underlying color space. For other types of patterns, these

operands must not be specified.

(PDF 1.1) Same as SC but used for nonstroking operations.

1… cn

scn

(PDF 1.2) Same as SCN but used for nonstroking operations.

1… cn

name

scn

1… cn

gray

Set the stroking color space to DeviceGray (or the DefaultGray color space; see

“Default Color Spaces” on page 257) and set the gray level to use for stroking op-

erations. gray is a number between 0.0 (black) and 1.0 (white).

gray

Same as G but used for nonstroking operations.

r g b

Set the stroking color space to DeviceRGB (or the DefaultRGB color space; see

“Default Color Spaces” on page 257) and set the color to use for stroking opera-

tions. Each operand must be a number between 0.0 (minimum intensity) and 1.0

(maximum intensity).

r g b

Same as RG but used for nonstroking operations.

c m y k

Set the stroking color space to DeviceCMYK (or the DefaultCMYK color space; see

“Default Color Spaces” on page 257) and set the color to use for stroking opera-

tions. Each operand must be a number between 0.0 (zero concentration) and 1.0

(maximum concentration). The behavior of this operator is affected by the over-

print mode (see Section 4.5.6, “Overprint Control”).

c m y k

Same as K but used for nonstroking operations.

Invoking operators that specify colors or other color-related parameters in the

graphics state is restricted in certain circumstances. This restriction occurs when

289

SECTION 4.6

Patterns

defining graphical figures whose colors are to be specified separately each time

they are used. Specifically, the restriction applies in these circumstances:

• In any glyph description that uses the d1 operator (see Section 5.5.4, “Type 3

Fonts”)

• In the content stream of an uncolored tiling pattern (see “Uncolored Tiling Pat-

terns” on page 298)

In these circumstances, the following actions cause an error:

• Invoking any of the following operators:

scn

SCN

• Invoking the gs operator with any of the following entries in the graphics state

parameter dictionary:

UCR

TR2

BG2

UCR2

• Painting an image. However, painting an image mask (see “Stencil Masking” on

page 350) is permitted because it does not specify colors; instead, it designates

places where the current color is to be painted.

4.6

Patterns

When operators such as S (stroke), f (fill), and Tj (show text) paint an area of the

page with the current color, they ordinarily apply a single color that covers the

area uniformly. However, it is also possible to apply “paint” that consists of a re-

peating graphical figure or a smoothly varying color gradient instead of a simple

color. Such a repeating figure or smooth gradient is called a pattern. Patterns are

quite general, and have many uses; for example, they can be used to create various

graphical textures, such as weaves, brick walls, sunbursts, and similar geometrical

and chromatic effects. (See implementation note 50 in Appendix H.)

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