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PDF Spec 08 Color Spaces
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- NOTE 4 The operation of a Separation colour 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 colour 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 colour (such as orange) for which no specific colorant exists on the device. A Separation colour space provides consistent, predictable behaviour, even on devices that cannot directly generate the requested colour.
[ /Separation name alternateSpace tintTransform ]
It shall be a four-element array whose first element shall be the colour space family name Separation . The remaining elements are parameters that a Separation colour space requires; their meanings are discussed below.
A colour value in a Separation colour space shall consist of a single tint component in the range 0.0 to 1.0. The value 0.0 shall represent the minimum amount of colorant that can be applied; 1.0 shall represent the maximum. Tints shall always be treated as subtractive colours, even if the device produces output for the designated component by an additive method. Thus, a tint value of 0.0 denotes the lightest colour that can be
achieved with the given colorant, and 1.0 is the darkest. The initial value for both the stroking and nonstroking colour in the graphics state shall be 1.0. The SCN and scn operators respectively shall set the current stroking and nonstroking colour to a tint value. A sampled image with single-component samples may also be used as a source of tint values.
- NOTE 5 This convention is the same as for DeviceCMYK colour components but opposite to the one for DeviceGray and DeviceRGB .
The name parameter is a name object that shall specify the name of the colorant that this Separation colour space is intended to represent (or one of the special names All or None ; see below). Such colorant names are arbitrary, and there may be any number of them, subject to implementation limits.
The special colorant name All shall refer 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 colour space, painting operators shall apply tint values to all available colorants at once.
- NOTE 6 This is useful for purposes 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 shall not produce any visible output. Painting operations in a Separation space with this colorant name shall have no effect on the current page.
A conforming reader shall support Separation colour spaces with the colorant names All and None on all devices, even if the devices are not capable of supporting any others. When processing Separation spaces with either of these colorant names conforming readers shall ignore the alternateSpace and tintTransform parameters (discussed below), although valid values shall still be provided.
At the moment the colour space is set to a Separation space, the conforming reader shall determine whether the device has an available colorant corresponding to the name of the requested space. If so, the conforming reader shall ignore the alternateSpace and tintTransform parameters; subsequent painting operations within the space shall apply the designated colorant directly, according to the tint values supplied.
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 colour space never applies a process colorant directly; it always reverts to the alternate colour space as described below. This is because the model of applying process colorants independently does not work as intended on an additive device.
EXAMPLE 1 Painting tints of the Red component on a white background produces a result that varies from white to cyan.
This exception applies only to colorants for additive devices, not to the specific names Red , Green , and Blue . In contrast, a printer might have a (subtractive) ink named Red , which should work as a Separation colour space just the same as any other supported colorant.
If the colorant name associated with a Separation colour space does not correspond to a colorant available on the device, the conforming reader shall arrange for subsequent painting operations to be performed in an alternate colour space. The intended colours may be approximated by colours in a device or CIE-based colour space, which shall then be rendered with the usual primary or process colorants:
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The alternateSpace parameter shall be an array or name object that identifies the alternate colour space, which may be any device or CIE-based colour space but may not be another special colour space ( Pattern , Indexed , Separation , or DeviceN ).
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The tintTransform parameter shall be a function (see 7.10, "Functions"). During subsequent painting operations, a conforming reader calls this function to transform a tint value into colour component values in the alternate colour space. The function shall be called with the tint value and shall return the corresponding colour component values. That is, the number of components and the interpretation of their values shall depend on the alternate colour space.
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NOTE 7 Painting in the alternate colour space may produce a good approximation of the intended colour when only opaque objects are painted. However, it does not correctly represent the interactions between an object and its backdrop when the object is painted with transparency or when overprinting (see 8.6.7, "Overprint Control") is enabled.
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EXAMPLE 2 The following illustrates the specification of a Separation colour space (object 5) that is intended to produce a colour named LogoGreen. If the output device has no colorant corresponding to this colour, DeviceCMYK is used as the alternate colour space, and the tint transformation function (object 12) maps tint values linearly into shades of a CMYK colour value approximating the LogoGreen colour.
5 0 obj % Colour 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 11.7.3, "Spot Colours and Transparency", for further discussion of the role of Separation colour spaces in the transparent imaging model.
DeviceN colour spaces (PDF 1.3) may contain an arbitrary number of colour components.
- NOTE 1 They provide greater flexibility than is possible with standard device colour spaces such as DeviceCMYK or with individual Separation colour spaces.
- EXAMPLE 1 It is possible to create a DeviceN colour space consisting of only the cyan, magenta, and yellow colour components, with the black component excluded.
- NOTE 2 DeviceN colour spaces are used in applications such as these:
High-fidelity colour is the use of more than the standard CMYK process colorants to produce an extended gamut, or range of colours. A popular example is the PANTONE Hexachrome system, which uses six colorants: the usual cyan, magenta, yellow, and black, plus orange and green.
Multitone colour systems use a single-component image to specify multiple colour components. In a duotone, for example, a single-component image can be used to specify both the black component and a spot colour 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 single-component image; the spot colour component might be generated as a nonlinear function of the image data in a manner that emphasizes the shadows. Figure L.6 in Annex L shows an example that uses black and magenta colour components. In Figure L.7 in Annex L, a single-component grayscale image is used to generate a quadtone result that uses four colorants: black and three PANTONE spot colours. See EXAMPLE 5 in this sub-clause for the code used to generate this image.
DeviceN shall be used to represent colour spaces containing multiple components that correspond to colorants of some target device. As with Separation colour spaces, conforming readers shall be able to approximate the colorants if they are not available on the current output device, such as a display. To accomplish this, the colour space definition provides a tint transformation function that shall be used to convert all the components to an alternate colour space.
PDF 1.6 extended the meaning of DeviceN to include colour spaces that are referred to as NChannel colour spaces . Such colour spaces may contain an arbitrary number of spot and process components, which may or may not correspond to specific device colorants (the process components shall be from a single process colour space). They provide information about each component that allows conforming readers more flexibility in converting colours. These colour spaces shall be identified by a value of NChannel for the Subtype entry of the attributes dictionary (see Table 71). A value of DeviceN for the Subtype entry, or no value, shall mean that only the previous features shall be supported. Conforming readers that do not support PDF 1.6 shall treat these colour spaces as normal DeviceN colour spaces and shall use the tint transformation function as appropriate. Conforming writers using the NChannel features should follow certain guidelines, as noted throughout this sub-clause, to achieve good backward compatibility.
EXAMPLE 2 They may use their own blending algorithms for on-screen viewing and composite printing, rather than being required to use a specified tint transformation function.
DeviceN colour spaces shall be defined in a similar way to Separation colour spaces-in fact, a Separation colour space can be defined as a DeviceN colour space with only one component.
- A DeviceN colour space shall be specified as follows:
[ /DeviceN names alternateSpace tintTransform ]
or
[ /DeviceN names alternateSpace tintTransform attributes ]
It is a four- or five-element array whose first element shall be the colour space family name DeviceN . The remaining elements shall be parameters that a DeviceN colour space requires.
The names parameter shall be an array of name objects specifying the individual colour components. The length of the array shall determine the number of components in the DeviceN colour space, which is subject to an implementation limit; see Annex C.The component names shall all be different from one another, except for the name None , which may be repeated as described later in this sub-clause. The special name All , used by Separation colour spaces, shall not be used.
Colour values shall be tint components in the range 0.0 to 1.0:
- For DeviceN colour spaces that do not have a subtype of NChannel , 0.0 shall represent the minimum amount of colorant; 1.0 shall represent the maximum. Tints shall always be treated as subtractive colours, even if the device produces output for the designated component by an additive method. Thus, a tint value of 0.0 shall denote the lightest colour that can be achieved with the given colorant, and 1.0 the darkest.
- NOTE 3 This convention is the same one as for DeviceCMYK colour components but opposite to the one for DeviceGray and DeviceRGB .
- For NChannel colour spaces, values for additive process colours (such as RGB ) shall be specified in their natural form, where 1.0 shall represent maximum intensity of colour.
When this space is set to the current colour space (using the CS or cs operators), each component shall be given an initial value of 1.0. The SCN and scn operators respectively shall set the current stroking and nonstroking colour. Operand values supplied to SCN or scn shall be interpreted as colour component values in the order in which the colours are given in the names array, as are the values in a sampled image that uses a DeviceN colour space.
The alternateSpace parameter shall be an array or name object that can be any device or CIE-based colour space but shall not be another special colour space ( Pattern , Indexed , Separation , or DeviceN ). When the colour space is set to a DeviceN space, if any of the component names in the colour space do not correspond to a colorant available on the device, the conforming reader shall perform subsequent painting operations in the alternate colour space specified by this parameter.
For NChannel colour spaces, the components shall be evaluated individually; that is, only the ones not present on the output device shall use the alternate colour space.
The tintTransform parameter shall specify a function (see 7.10, "Functions") that is used to transform the tint values into the alternate colour space. It shall be called with n tint values and returns m colour component values, where n is the number of components needed to specify a colour in the DeviceN colour space and m is the number required by the alternate colour space.
- NOTE 4 Painting in the alternate colour space may produce a good approximation of the intended colour when only opaque objects are painted. However, it does not correctly represent the interactions between an object and its backdrop when the object is painted with transparency or when overprinting (see 8.6.7, "Overprint Control") is enabled.
The colour component name None , which may be present only for DeviceN colour spaces that do not have the NChannel subtype, indicates that the corresponding colour component shall never be painted on the page, as in a Separation colour space for the None colorant. When a DeviceN colour space is painting the named device colorants directly, colour components corresponding to None colorants shall be discarded. However, when the DeviceN colour space reverts to its alternate colour space, those components shall be passed to the tint transformation function, which may use them as desired.
A DeviceN colour space whose component colorant names are all None shall always discard its output, just the same as a Separation colour space for None ; it shall never revert to the alternate colour space. Reversion shall occur only if at least one colour component (other than None ) is specified and is not available on the device.
The optional attributes parameter shall be a dictionary (see Table 71) containing additional information about the components of colour space that conforming readers may use. Conforming readers need not 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 value of the Subtype entry in the attributes dictionary is NChannel , such information shall be present.) However, alternateSpace and tintTransform shall always be provided for conforming readers that want to use them or do not support PDF 1.6.
Table 71 - Entries in a DeviceN Colour Space Attributes Dictionary
| Key | Type | Value |
|---|---|---|
| Subtype | name | (Optional; PDF 1.6) A name specifying the preferred treatment for the colour space. Values shall be DeviceN or NChannel . Default value: DeviceN . |
Table 71 - Entries in a DeviceN Colour Space Attributes Dictionary (continued)
| Key | Type | Value |
|---|---|---|
| Colorants | dictionary | (Required if Subtype is NChannel and the colour space includes spot colorants; otherwise optional) A dictionary describing the individual colorants that shall be used in the DeviceN colour space. For each entry in this dictionary, the key shall be a colorant name and the value shall be an array defining a Separation colour space for that colorant (see 8.6.6.4, "Separation Colour Spaces"). The key shall match the colorant name given in that colour space. This dictionary provides information about the individual colorants that may be useful to some conforming readers. In particular, the alternate colour space and tint transformation function of a Separation colour space describe the appearance of that colorant alone, whereas those of a DeviceN colour space describe only the appearance of its colorants in combination. If Subtype is NChannel , this dictionary shall have entries for all spot colorants in this colour space. This dictionary may also include additional colorants not used by this colour space. |
| Process | dictionary | (Required if Subtype is NChannel and the colour space includes components of a process colour space, otherwise optional; PDF 1.6) A dictionary (see Table 72) that describes the process colour space whose components are included in this colour space. |
| MixingHints | dictionary | (Optional; PDF 1.6) A dictionary (see Table 73) that specifies optional attributes of the inks that shall 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 entries 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 colour space; they shall be defined using individual Separation colour spaces. The Process entry specifies a process dictionary (see Table 72) that identifies the process colour space that is used by this colour space and the names of its components. It shall be present if Subtype is NChannel and the colour space has process colour components. An NChannel colour space shall contain components from at most one process colour space.
For colour spaces that have a value of NChannel for the Subtype entry in the attributes dictionary (see Table 71), the following restrictions apply to process colours:
- There may be colour components from at most one process colour space, which may be any device or CIE-based colour space.
- For a nonCMYK colour space, the names of the process components shall appear sequentially in the names array, in the normal colour space order (for example, Red , Green , and Blue ). However, the names in the names array need not match the actual colour space names (for example, a Red component need not be named Red ).The mapping of names is specified in the process dictionary (see Table 72 and discussion below), which shall be present.
- Definitions for process colorants should not appear in the colorants dictionary. Any such definition shall be ignored if the colorant is also present in the process dictionary. Any component not specified in the process dictionary shall be considered to be a spot colorant.
- For a CMYK colour 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 shall always be considered to be process colours, which do not necessarily correspond to the colorants of a specific device; they need not have entries in the process dictionary.
- The values associated with the process components shall be stored in their natural form (that is, subtractive colour values for CMYK and additive colour values for RGB ), since they shall be interpreted
directly as process values by consumers making use of the process dictionary. (For additive colour spaces, this is the reverse of how colour 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 dictionary (see Table 73) that provides information about the characteristics of colorants that may be used in blending calculations when the actual colorants are not available on the target device. Conforming readers need not use this information.
Table 72 - Entries in a DeviceN Process Dictionary
| KEY | TYPE | VALUE |
|---|---|---|
| ColorSpace | name or array | (Required) A name or array identifying the process colour space, which may be any device or CIE-based colour space. If an ICCBased colour space is specified, it shall provide calibration information appropriate for the process colour components specified in the names array of the DeviceN colour space. |
| Components | array | (Required) An array of component names that correspond, in order, to the components of the process colour space specified in ColorSpace . For example, an RGB colour space shall 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 colour space components) and shall match those specified in the names array of the DeviceN colour space, even if all components are not present in the names array. |
Table 73 - Entries in a DeviceN Mixing Hints Dictionary
| Key | Type | Value |
|---|---|---|
| Solidities | dictionary | (Optional) A dictionary specifying the solidity of inks that shall be used in blending calculations when used as an alternative to the tint transformation function. For each entry, the key shall be a colorant name, and the value shall be a number between 0.0 and 1.0. This dictionary need not contain entries for all colorants used in this colour space; it may also include additional colorants not used by this colour 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 that shall be used by all components in the associated DeviceN colour space for which a solidity value is not explicitly provided. If Default is not present, the default value for unspecified colorants shall be 0.0; conforming readers may choose to use other values. If this entry is present, PrintingOrder shall also be present. |
| PrintingOrd er | array | (Required if Solidities is present) An array of colorant names, specifying the order in which inks shall be laid down. Each component in the names array of the DeviceN colour space shall 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. |
Table 73 - Entries in a DeviceN Mixing Hints Dictionary (continued)
| Key | Type | Value |
|---|---|---|
| DotGain | dictionary | (Optional) A dictionary specifying the dot gain of inks that shall 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 shall be a colorant name, and the value shall be 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 colorants. An entry with a key of Default shall specify a function to be used by all colorants for which a dot gain function is not explicitly specified. Conforming readers may ignore values in this dictionary when other sources of dot gain information are available, such as ICC profiles associated with the process colour 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 circumstances, they may also refer to one or more individual process components called Cyan , Magenta , Yellow , or Black when DeviceCMYK is specified as the process colour space in the process dictionary. However, applications shall ignore these process component entries if they can obtain the information from an ICC profile.
NOTE 5 The mixing hints subdictionaries (as well as the colorants dictionary) may specify colorants that are not used in any given instance of a DeviceN colour space. This allows them to be referenced from multiple DeviceN colour spaces, which can produce smaller file sizes as well as consistent colour definitions across instances.
For consistency of colour, conforming readers should follow these guidelines:
- The conforming reader shall apply either the specified tint transformation function or invoke the same alternative blending algorithm for all DeviceN instances in the document.
NOTE 6 When the tint transformation function is used, the burden is on the conforming writer to guarantee that the individual function definitions chosen for all DeviceN instances produce similar colour appearances throughout the document.
- Blending algorithms should produce a similar appearance for colours when they are used as separation colours or as a component of a DeviceN colour space.
EXAMPLE 3 This example shows a DeviceN colour space consisting of three colour components named Orange , Green , and None . In this example, the DeviceN colour space, object 30, has an attributes dictionary whose Colorants entry is an indirect reference to object 45 (which might also be referenced by attributes dictionaries of other DeviceN colour spaces). tintTransform1 , whose definition is not shown, maps three colour components (tints of the colorants Orange , Green , and None ) to four colour components in the alternate colour space, DeviceCMYK . tintTransform2 maps a single colour 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 .
30 0 obj % Colour space [ /DeviceN [ /Orange /Green /None ] /DeviceCMYK tintTransform1 << /Colorants 45 0 R >> ] endobj
EXAMPLE 4 45 0 obj % Colorants dictionary << /Orange [ /Separation /Orange /DeviceCMYK tintTransform2 ] /Green [ /Separation /Green /DeviceCMYK tintTransform3 ] /PANTONE#20131 [ /Separation /PANTONE#20131 /DeviceCMYK tintTransform4 ] >> endobj NOTE 7 EXAMPLE 5 through EXAMPLE 8 show the use of NChannel colour spaces. EXAMPLE 5 This example shows the use of calibrated CMYK process components. EXAMPLE 6 shows the use of Lab process components. 10 0 obj % Colour 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 6 10 0 obj %Colour 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 ] >> >> ]
EXAMPLE 7 This example shows the recommended convention for dealing with situations where a spot colorant and a process colour component have the same name. Since the names array may not have duplicate names,
the process colours 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 colour space.
% Attributes dictionary
10 0 obj % Colour space [ /DeviceN [/ProcessRed /ProcessGreen /ProcessBlue /Red] alternateSpace tintTransform1 << /Subtype /NChannel /Process << /ColorSpace [ /ICCBased RGB_ICC profile ] /Components [/ProcessRed /ProcessGreen /ProcessBlue] >> /Colorants << /Red [/Separation /Red alternateSpace tintTransform2 ] >> >> ] EXAMPLE 8 This example shows the use of a mixing hints dictionary. 10 0 obj % Colour 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 ] >> /MixingHints << /Solidities << /Spot1 1.0 /Spot2 0.0 >> /DotGain << /Spot1 function1 /Spot2 function2 /Magenta function3 /Yellow function4 >> /PrintingOrder [/Magenta /Yellow /Spot1 /Spot2] >> >>
See 11.7.3, "Spot Colours and Transparency", for further discussion of the role of DeviceN colour spaces in the transparent imaging model.
]
- NOTE 1 The following examples illustrate various interesting and useful special cases of the use of Indexed and DeviceN colour spaces in combination to produce multitone colours.
- NOTE 2 EXAMPLE 1 and EXAMPLE 2 in this sub-clause illustrate the use of DeviceN to create duotone colour spaces.
EXAMPLE 1
EXAMPLE 2
In this example, an Indexed colour 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 colour 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 colour space of the DeviceN space is selected, the tint transformation function (object 15 in the example) maps the two tint components for cyan and black to the four components for a DeviceCMYK colour space by supplying zero values for the other two components.
10 0 obj %Colour 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
This example shows the definition of another duotone colour space, this time using black and gold colorants (where gold is a spot colorant) and using a CalRGB space as the alternate colour space. This could be defined in the same way as in the preceding example, with a tint transformation function that converts from the two tint components to colours in the alternate CalRGB colour space.
30 0 obj % Colour 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
NOTE 3 Given a formula for converting any combination of black and gold tints to calibrated 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 colour space is being used as the base of an Indexed colour space, there are actually only 256 possible combinations of black and gold tint values.
- EXAMPLE 3 This example shows a more compact way to represent this information is to put the alternate colour values directly into the lookup table alongside the DeviceN colour values.
10 0 obj % Colour 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
NOTE 4 In EXAMPLE 3 in this sub-clause, 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 colour 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 colour into a threecomponent equivalent in the alternate CalRGB colour 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 readily expressed with a type 4 (PostScript calculator) function (see EXAMPLE 4 in this sub-clause).
- EXAMPLE 4 This example shows a type 4 (PostScript calculator) function.
- EXAMPLE 5 This example uses an extension of the techniques described above to produce the quadtone (fourcomponent) image shown in Figure L.7 in Annex L.
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
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 colour space for image [ /Indexed 15 0 R % Base colour space
255 % Table has 256 entries 30 0 R % Lookup table ] endobj 15 0 obj % Base colour space ( DeviceN ) for Indexed space [ /DeviceN [ /Black % Four colorants (black plus three spot colours) /PANTONE#20216#20CVC /PANTONE#20409#20CVC /PANTONE#202985#20CVC /None % Three components for alternate space /None /None ] 16 0 R % Alternate colour space 20 0 R % Tint transformation function ] endobj 16 0 obj % Alternate colour 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 colour 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
NOTE 5 As in the preceding examples, an Indexed colour space based on a DeviceN space is used to paint the grayscale image shown on the left in the plate with four colorants: black and three PANTONE spot colours. The alternate colour space is a simple calibrated RGB . Thus, the DeviceN colour space has seven components: the four desired colorants plus the three components of the alternate space. The example shows the image XObject (see 8.9.5, "Image Dictionaries") representing the quadtone image, followed by the colour space used to interpret the image data.
The graphics state contains an overprint parameter , controlled by the OP and op entries in a graphics state parameter dictionary. Overprint control is useful mainly 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 sub-clause on colour rendering, because it pertains to an aspect of painting in device colour spaces that is important to many applications.
Any painting operation marks some specific set of device colorants, depending on the colour space in which the painting takes place. In a Separation or DeviceN colour space, the colorants to be marked shall be specified explicitly; in a device or CIE-based colour space, they shall be implied by the process colour model of the output device (see clause 10, "Rendering"). The overprint parameter is a boolean flag that determines how painting operations affect colorants other than those explicitly or implicitly specified by the current colour space.
If the overprint parameter is false (the default value), painting a colour in any colour space shall cause the corresponding areas of unspecified colorants to be erased (painted with a tint value of 0.0). The effect is that the colour at any position on the page is whatever was painted there last, which is consistent with the normal painting behaviour of the opaque imaging model.
If the overprint parameter is true and the output device supports overprinting, erasing actions shall not be performed; anything previously painted in other colorants is left undisturbed. Consequently, the colour 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 here.
NOTE 1 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 shall be ignored.
An additional graphics state parameter, the overprint mode (PDF 1.3) , shall affect the interpretation of a tint value of 0.0 for a colour component in a DeviceCMYK colour space when overprinting is enabled. This parameter is controlled by the OPM entry in a graphics state parameter dictionary; it shall have an effect only when the overprint parameter is true , as described above.
When colours are specified in a DeviceCMYK colour space and the native colour space of the output device is also DeviceCMYK , each of the source colour components controls the corresponding device colorant directly. Ordinarily, each source colour component value replaces the value previously painted for the corresponding device colorant, no matter what the new value is; this is the default behaviour, specified by overprint mode 0.
When the overprint mode is 1 (also called nonzero overprint mode ), a tint value of 0.0 for a source colour component shall leave the corresponding component of the previously painted colour unchanged. The effect is equivalent to painting in a DeviceN colour space that includes only those components whose values are nonzero.
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 colour space shown in this example. 10 0 obj %Colour space [ /DeviceN [ /Cyan /Magenta /Black ] /DeviceCMYK 15 0 R ] endobj 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 shall apply only to painting operations that use the current colour in the graphics state when the current colour space is DeviceCMYK (or is implicitly converted to DeviceCMYK ; see 8.6.5.7, "Implicit Conversion of CIE-Based Colour Spaces"). It shall not apply to the painting of images or to any colours that are the result of a computation, such as those in a shading pattern or conversions from some other colour space. It also shall not apply if the device's native colour space is not DeviceCMYK ; in that case, source colours shall be converted to the device's native colour space, and all components participate in the conversion, whatever their values.
- NOTE 2 This is shown explicitly in the alternate colour space and tint transformation function of the DeviceN colour space (see EXAMPLE 3 in 8.6.6, "Special Colour Spaces").
See 11.7.4, "Overprinting and Transparency", for further discussion of the role of overprinting in the transparent imaging model.
Table 74 lists the PDF operators that control colour spaces and colour values. Also colour-related is the graphics state operator ri , listed in Table 57 and discussed under 8.6.5.8, "Rendering Intents". Colour operators may appear at the page description level or inside text objects (see Figure 9 in Annex L).
Table 74 - Colour Operators
| Operands | Operator | Description |
|---|---|---|
| name | CS | (PDF 1.1) Set the current colour space to use for stroking operations. The operand name shall be a name object. If the colour 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 shall be a name defined in the ColorSpace subdictionary of the current resource dictionary (see 7.8.3, "Resource Dictionaries"); the associated value shall be an array describing the colour space (see 8.6.3, "Colour Space Families"). The names DeviceGray , DeviceRGB , DeviceCMYK , and Pattern always identify the corresponding colour spaces directly; they never refer to resources in the ColorSpace subdictionary. The CS operator shall also set the current stroking colour to its initial value, which depends on the colour space: In a DeviceGray , DeviceRGB , CalGray , or CalRGB colour space, the initial colour shall have all components equal to 0.0. In a DeviceCMYK colour space, the initial colour shall be [0.0 0.0 0.0 1.0]. In a Lab or ICCBased colour space, the initial colour shall have 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 shall be substituted. In an Indexed colour space, the initial colour value shall be 0. In a Separation or DeviceN colour space, the initial tint value shall be 1.0 for all colorants. In a Pattern colour space, the initial colour shall be a pattern object that causes nothing to be painted. |
| name | cs | (PDF 1.1) Same as CS but used for nonstroking operations. |
Table 74 - Colour Operators (continued)
| Operands | Operator | Description |
|---|---|---|
| c 1 … c n | SC | (PDF 1.1) Set the colour to use for stroking operations in a device, CIE- based (other than ICCBased ), or Indexed colour space. The number of operands required and their interpretation depends on the current stroking colour space: For DeviceGray , CalGray , and Indexed colour spaces, one operand shall be required ( n = 1). For DeviceRGB , CalRGB , and Lab colour spaces, three operands shall be required ( n = 3). For DeviceCMYK , four operands shall be required ( n = 4). |
| c 1 … c n c 1 … c n name | SCN SCN | (PDF 1.2) Same as SC but also supports Pattern, Separation, DeviceN and ICCBased colour spaces. If the current stroking colour space is a Separation , DeviceN , or ICCBased colour space, the operands c 1 … c n shall be numbers. The number of operands and their interpretation depends on the colour space. If the current stroking colour space is a Pattern colour space, name shall be the name of an entry in the Pattern subdictionary of the current resource dictionary (see 7.8.3, "Resource Dictionaries"). For an uncoloured tiling pattern ( PatternType = 1 and PaintType = 2), c 1 … c n shall be component values specifying a colour in the pattern's underlying colour space. For other types of patterns, these operands shall not be specified. |
| c 1 … c n | sc | (PDF 1.1) Same as SC but used for nonstroking operations. |
| c 1 … c n c 1 … c n name | scn scn | (PDF 1.2) Same as SCN but used for nonstroking operations. |
| gray | G | Set the stroking colour space to DeviceGray (or the DefaultGray colour space; see 8.6.5.6, "Default Colour Spaces") and set the gray level to use for stroking operations. gray shall be a number between 0.0 (black) and 1.0 (white). |
| gray | g | Same as G but used for nonstroking operations. |
| r g b | RG | Set the stroking colour space to DeviceRGB (or the DefaultRGB colour space; see 8.6.5.6, "Default Colour Spaces") and set the colour to use for stroking operations. Each operand shall be a number between 0.0 (minimum intensity) and 1.0 (maximum intensity). |
| r g b | rg | Same as RG but used for nonstroking operations. |
| c m y k | K | Set the stroking colour space to DeviceCMYK (or the DefaultCMYK colour space; see 8.6.5.6, "Default Colour Spaces") and set the colour to use for stroking operations. Each operand shall be a number between 0.0 (zero concentration) and 1.0 (maximum concentration). The behaviour of this operator is affected by the overprint mode (see 8.6.7, "Overprint Control"). |
| c m y k | k | Same as K but used for nonstroking operations. |
Invoking operators that specify colours or other colour-related parameters in the graphics state is restricted in certain circumstances. This restriction occurs when defining graphical figures whose colours shall 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 9.6.5, "Type 3 Fonts")
- In the content stream of an uncoloured tiling pattern (see 8.7.3.3, "Uncoloured Tiling Patterns")
In these circumstances, the following actions cause an error:
- Invoking any of the following operators:
- Invoking the gs operator with any of the following entries in the graphics state parameter dictionary:
- Painting an image. However, painting an image mask (see 8.9.6.2, "Stencil Masking") shall be permitted because it does not specify colours; instead, it designates places where the current colour shall be painted.
| CS | scn | K |
|---|---|---|
| cs | G | k |
| SC | g | ri |
| SCN | RG | sh |
| sc | rg |
| TR | BG | UCR |
|---|---|---|
| TR2 | BG2 | UCR2 |
| HT |
Patterns come in two varieties:
- Tiling patterns consist of a small graphical figure (called a pattern cell ) that is replicated at fixed horizontal and vertical intervals to fill the area to be painted. The graphics objects to use for tiling shall be described by a content stream.
- Shading patterns define a gradient fill that produces a smooth transition between colours across the area. The colour to use shall be specified as a function of position using any of a variety of methods.
- NOTE 1 When operators such as S (stroke), f (fill), and Tj (show text) paint an area of the page with the current colour, they ordinarily apply a single colour that covers the area uniformly. However, it is also possible to apply 'paint' that consists of a repeating graphical figure or a smoothly varying colour gradient instead of a simple colour. 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.
- NOTE 2 Older techniques such as defining a pattern by using character glyphs in a special font and painting them repeatedly with the Tj operator should not be used. Another technique, defining patterns as halftone screens, should not be used because the effects produced are device-dependent.
Patterns shall be specified in a special family of colour spaces named Pattern . These spaces shall use pattern objects as the equivalent of colour values instead of the numeric component values used with other spaces. A pattern object shall be a dictionary or a stream, depending on the type of pattern; the term pattern dictionary is used generically throughout this sub-clause to refer to either a dictionary object or the dictionary portion of a stream object. (Those pattern objects that are streams are specifically identified as such in the descriptions of particular pattern types; unless otherwise stated, they are understood to be simple dictionaries instead.) This sub-clause describes Pattern colour spaces and the specification of colour values within them.
- NOTE 3 See 8.6, "Colour Spaces", for information about colour spaces and colour values in general and 11.6.7, "Patterns and Transparency", for further discussion of the treatment of patterns in the transparent imaging model.
A pattern dictionary contains descriptive information defining the appearance and properties of a pattern. All pattern dictionaries shall contain an entry named PatternType , whose value identifies the kind of pattern the dictionary describes: type 1 for a tiling pattern or type 2 for a shading pattern. The remaining contents of the dictionary depend on the pattern type and are detailed in the sub-clauses on individual pattern types.
All patterns shall be treated as colours; a Pattern colour space shall be established with the CS or cs operator just like other colour spaces, and a particular pattern shall be installed as the current colour with the SCN or scn operator (see Table 74).
A pattern's appearance is described with respect to its own internal coordinate system. Every pattern has a pattern matrix , a transformation matrix that maps the pattern's internal coordinate system to the default coordinate system of the pattern's parent content stream (the content stream in which the pattern is defined as a resource). The concatenation of the pattern matrix with that of the parent content stream establishes the pattern coordinate space , within which all graphics objects in the pattern shall be interpreted.
- NOTE 1 If a pattern is used on a page, the pattern appears in the Pattern subdictionary of that page's resource dictionary, and the pattern matrix maps pattern space to the default (initial) coordinate space of the page. Changes to the page's transformation matrix that occur within the page's content stream, such as rotation and scaling, have no effect on the pattern; it maintains its original relationship to the page no matter where on the page it is used. Similarly, if a pattern is used within a form XObject (see 8.10, "Form XObjects"), the pattern matrix maps pattern space to the form's default user space (that is, the form coordinate space at the time the form is painted with the Do operator). A pattern may be used within another pattern; the inner pattern's matrix defines its relationship to the pattern space of the outer pattern.
- NOTE 2 PostScript allows a pattern to be defined in one context but used in another. For example, a pattern might be defined on a page (that is, its pattern matrix maps the pattern coordinate space to the user space of the page) but be used in a form on that page, so that its relationship to the page is independent of each individual placement of the form. PDF does not support this feature; in PDF, all patterns shall be local to the context in which they are defined.
A tiling pattern consists of a small graphical figure called a pattern cell . Painting with the pattern replicates the cell at fixed horizontal and vertical intervals to fill an area. The effect is as if the figure were painted on the surface of a clear glass tile, identical copies of which were then laid down in an array covering the area and trimmed to its boundaries. This process is called tiling the area.
The pattern cell can include graphical elements such as filled areas, text, and sampled images. Its shape need not be rectangular, and the spacing of tiles can differ from the dimensions of the cell itself. When performing painting operations such as S (stroke) or f (fill), the conforming reader shall paint the cell on the current page as many times as necessary to fill an area. The order in which individual tiles (instances of the cell) are painted is unspecified and unpredictable; figures on adjacent tiles should not overlap.
The appearance of the pattern cell shall be defined by a content stream containing the painting operators needed to paint one instance of the cell. Besides the usual entries common to all streams (see Table 5), this stream's dictionary may contain the additional entries listed in Table 75.
Table 75 - Additional Entries Specific to a Type 1 Pattern Dictionary
| Key | Type | Value |
|---|---|---|
| Type | name | (Optional) The type of PDF object that this dictionary describes; if present, shall be Pattern for a pattern dictionary. |
| PatternType | integer | (Required) A code identifying the type of pattern that this dictionary describes; shall be 1 for a tiling pattern. |
Table 75 - Additional Entries Specific to a Type 1 Pattern Dictionary (continued)
| Key | Type | Value |
|---|---|---|
| PaintType | integer | (Required) A code that determines how the colour of the pattern cell shall be specified: a) Coloured tiling pattern . The pattern's content stream shall specify the colours used to paint the pattern cell. When the content stream begins execution, the current colour is the one that was initially in effect in the pattern's parent content stream. This is similar to the definition of the pattern matrix; see 8.7.2, "General Properties of Patterns". b) Uncoloured tiling pattern . The pattern's content stream shall not specify any colour information. Instead, the entire pattern cell is painted with a separately specified colour each time the pattern is used. Essentially, the content stream describes a stencil through which the current colour shall be poured. The content stream shall not invoke operators that specify colours or other colour- related parameters in the graphics state; otherwise, an error occurs (see 8.6.8, "Colour Operators"). The content stream may paint an image mask, however, since it does not specify any colour information (see 8.9.6.2, "Stencil Masking"). |
| TilingType | integer | (Required) A code that controls adjustments to the spacing of tiles relative to the device pixel grid: a) Constant spacing . Pattern cells shall be spaced consistently-that is, by a multiple of a device pixel. To achieve this, the conforming reader may need to distort the pattern cell slightly by making small adjustments to XStep , YStep , and the transformation matrix. The amount of distortion shall not exceed 1 device pixel. b) No distortion . The pattern cell shall not be distorted, but the spacing between pattern cells may vary by as much as 1 device pixel, both horizontally and vertically, when the pattern is painted. This achieves the spacing requested by XStep and YStep on average but not necessarily for each individual pattern cell. c) Constant spacing and faster tiling . Pattern cells shall be spaced consistently as in tiling type 1 but with additional distortion permitted to enable a more efficient implementation. |
| BBox | rectangle | (Required) An array of four numbers in the pattern coordinate system giving the coordinates of the left, bottom, right, and top edges, respectively, of the pattern cell's bounding box. These boundaries shall be used to clip the pattern cell. |
| XStep | number | (Required) The desired horizontal spacing between pattern cells, measured in the pattern coordinate system. |
| YStep | number | (Required) The desired vertical spacing between pattern cells, measured in the pattern coordinate system. NOTE XStep and YStep may differ from the dimensions of the pattern cell implied by the BBox entry. This allows tiling with irregularly shaped figures. XStep and YStep may be either positive or negative but shall not be zero. |
| Resources | dictionary | (Required) A resource dictionary that shall contain all of the named resources required by the pattern's content stream (see 7.8.3, "Resource Dictionaries"). |
| Matrix | array | (Optional) An array of six numbers specifying the pattern matrix (see 8.7.2, "General Properties of Patterns"). Default value: the identity matrix [1 0 0 1 0 0]. |
The pattern dictionary's BBox , XStep , and YStep values shall be interpreted in the pattern coordinate system, and the graphics objects in the pattern's content stream shall be defined with respect to that coordinate system. The placement of pattern cells in the tiling is based on the location of one key pattern cell , which is then displaced by multiples of XStep and YStep to replicate the pattern. The origin of the key pattern cell coincides with the origin of the pattern coordinate system. The phase of the tiling can be controlled by the translation components of the Matrix entry in the pattern dictionary.
Prior to painting with a tiling pattern, the conforming writer shall establish the pattern as the current colour in the graphics state. Subsequent painting operations tile the painted areas with the pattern cell described by the pattern's content stream. To obtain the pattern cell, the conforming reader shall perform these steps:
- a) Saves the current graphics state (as if by invoking the q operator)
- b) Installs the graphics state that was in effect at the beginning of the pattern's parent content stream, with the current transformation matrix altered by the pattern matrix as described in 8.7.2, "General Properties of Patterns"
- c) Paints the graphics objects specified in the pattern's content stream
- d) Restores the saved graphics state (as if by invoking the Q operator)
NOTE The pattern's content stream should not set any of the device-dependent parameters in the graphics state (see Table 53) because it may result in incorrect output.
A coloured tiling pattern is a pattern whose colour is self-contained. In the course of painting the pattern cell, the pattern's content stream explicitly sets the colour of each graphical element it paints. A single pattern cell may contain elements that are painted different colours; it may also contain sampled grayscale or colour images. This type of pattern is identified by a pattern type of 1 and a paint type of 1 in the pattern dictionary.
When the current colour space is a Pattern space, a coloured tiling pattern shall be selected as the current colour by supplying its name as the single operand to the SCN or scn operator. This name shall be the key of an entry in the Pattern subdictionary of the current resource dictionary (see 7.8.3, "Resource Dictionaries"), whose value shall be the stream object representing the pattern. Since the pattern defines its own colour information, no additional operands representing colour components shall be specified to SCN or scn .
- EXAMPLE 1 If P1 is the name of a pattern resource in the current resource dictionary, the following code establishes it as the current nonstroking colour:
/Pattern cs /P1 scn
- NOTE 1 Subsequent executions of nonstroking painting operators, such as f (fill), Tj (show text), or Do (paint external object) with an image mask, use the designated pattern to tile the areas to be painted.
- NOTE 2 The following defines a page (object 5) that paints three circles and a triangle using a coloured tiling pattern (object 15) over a yellow background. The pattern consists of the symbols for the four suits of playing cards (spades, hearts, diamonds, and clubs), which are character glyphs taken from the ZapfDingbats font (see D.6, "ZapfDingbats Set and Encoding"); the pattern's content stream specifies the colour of each glyph. Figure L.8 in Annex L shows the results.
EXAMPLE 2 5 0 obj % Page object << /Type /Page /Parent 2 0 R /Resources 10 0 R /Contents 30 0 R /CropBox [ 0 0 225 225 ] >> endobj 10 0 obj % Resource dictionary for page << /Pattern << /P1 15 0 R >>
>> endobj 15 0 obj % Pattern definition << /Type /Pattern /PatternType 1 % Tiling pattern /PaintType 1 % Coloured /TilingType 2 /BBox [ 0 0 100 100 ] /XStep 100 /YStep 100 /Resources 16 0 R /Matrix [ 0.4 0.0 0.0 0.4 0.0 0.0 ] /Length 183 >> stream BT % Begin text object /F1 1 Tf % Set text font and size 64 0 0 64 7.1771 2.4414 Tm % Set text matrix 0 Tc % Set character spacing 0 Tw % Set word spacing 1.0 0.0 0.0 rg % Set nonstroking colour to red ( \001 ) Tj % Show spade glyph 0.7478 -0.007 TD % Move text position 0.0 1.0 0.0 rg % Set nonstroking colour to green ( \002 ) Tj % Show heart glyph -0.7323 0.7813 TD % Move text position 0.0 0.0 1.0 rg % Set nonstroking colour to blue ( \003 ) Tj % Show diamond glyph 0.6913 0.007 TD % Move text position 0.0 0.0 0.0 rg % Set nonstroking colour to black ( \004 ) Tj % Show club glyph ET % End text object endstream endobj 16 0 obj % Resource dictionary for pattern << /Font << /F1 20 0 R >> >> endobj 20 0 obj % Font for pattern << /Type /Font /Subtype /Type1 /Encoding 21 0 R /BaseFont /ZapfDingbats >> endobj 21 0 obj % Font encoding << /Type /Encoding /Differences [ 1 /a109 /a110 /a111 /a112 ] >> endobj 30 0 obj % Contents of page << /Length 1252 >> stream 0.0 G % Set stroking colour to black
1.0 1.0 0.0 rg % Set nonstroking colour to yellow 25 175 175 -150 re % Construct rectangular path f % Fill path /Pattern cs % Set pattern colour space /P1 scn % Set pattern as nonstroking colour 99.92 49.92 m % Start new path 99.92 77.52 77.52 99.92 49.92 99.92 c % Construct lower-left circle 22.32 99.92 -0.08 77.52 -0.08 49.92 c -0.08 22.32 22.32 -0.08 49.92 -0.08 c 77.52 -0.08 99.92 22.32 99.92 49.92 c B % Fill and stroke path 224.96 49.92 m % Start new path 224.96 77.52 202.56 99.92 174.96 99.92 c % Construct lower-right circle 147.36 99.92 124.96 77.52 124.96 49.92 c 124.96 22.32 147.36 -0.08 174.96 -0.08 c 202.56 -0.08 224.96 22.32 224.96 49.92 c B % Fill and stroke path 87.56 201.70 m % Start new path 63.66 187.90 55.46 157.32 69.26 133.40 c % Construct upper circle 83.06 109.50 113.66 101.30 137.56 115.10 c 161.46 128.90 169.66 159.50 155.86 183.40 c 142.06 207.30 111.46 215.50 87.56 201.70 c B % Fill and stroke path 50 50 m % Start new path 175 50 l % Construct triangular path 112.5 158.253 l b % Close, fill, and stroke path
endstream endobj
- NOTE 3 Several features of EXAMPLE 2 in this sub-clause are noteworthy:
The three circles and the triangle are painted with the same pattern. The pattern cells align, even though the circles and triangle are not aligned with respect to the pattern cell. For example, the position of the blue diamonds varies relative to the three circles.
The pattern cell does not completely cover the tile: it leaves the spaces between the glyphs unpainted. When the tiling pattern is used as a colour, the existing background (the yellow rectangle) shows through these unpainted areas.
An uncoloured tiling pattern is a pattern that has no inherent colour: the colour shall be specified separately whenever the pattern is used. It provides a way to tile different regions of the page with pattern cells having the same shape but different colours. This type of pattern shall be identified by a pattern type of 1 and a paint type of 2 in the pattern dictionary. The pattern's content stream shall not explicitly specify any colours; it may paint an image mask (see 8.9.6.2, "Stencil Masking") but no other kind of image.
A Pattern colour space representing an uncoloured tiling pattern shall have a parameter: an object identifying the underlying colour space in which the actual colour of the pattern shall be specified. The underlying colour space shall be given as the second element of the array that defines the Pattern colour space.
as its underlying colour space.
EXAMPLE 1 The array [ /Pattern /DeviceRGB ] defines a Pattern colour space with DeviceRGB
NOTE The underlying colour space cannot be another Pattern colour space.
Operands supplied to the SCN or scn operator in such a colour space shall include a colour value in the underlying colour space, specified by one or more numeric colour components, as well as the name of a pattern object representing an uncoloured tiling pattern.
- EXAMPLE 2 If the current resource dictionary (see 7.8.3, "Resource Dictionaries") defines Cs3 as the name of a ColorSpace resource whose value is the Pattern colour space shown above and P2 as a Pattern resource denoting an uncoloured tiling pattern, the code
/Cs3 cs 0.30 0.75 0.21 /P2 scn establishes Cs3 as the current nonstroking colour space and P2 as the current nonstroking colour, to be painted in the colour represented by the specified components in the DeviceRGB colour space. Subsequent executions of nonstroking painting operators, such as f (fill), Tj (show text), and Do (paint external object) with an image mask, use the designated pattern and colour to tile the areas to be painted. The same pattern can be used repeatedly with a different colour each time.
- EXAMPLE 3 This example is similar to EXAMPLE 2 in 8.7.3.2, except that it uses an uncoloured tiling pattern to paint the three circles and the triangle, each in a different colour (see Figure L.9 in Annex L). To do so, it supplies four operands each time it invokes the scn operator: three numbers denoting the colour components in the underlying DeviceRGB colour space, along with the name of the pattern.
5 0 obj % Page object << /Type /Page /Parent 2 0 R /Resources 10 0 R /Contents 30 0 R /CropBox [ 0 0 225 225 ] >> endobj 10 0 obj % Resource dictionary for page << /ColorSpace << /Cs12 12 0 R >> /Pattern << /P1 15 0 R >> >> endobj 12 0 obj %Colour space [ /Pattern /DeviceRGB ] endobj 15 0 obj % Pattern definition << /Type /Pattern /PatternType 1 % Tiling pattern /PaintType 2 % Uncoloured /TilingType 2 /BBox [ 0 0 100 100 ] /XStep 100 /YStep 100 /Resources 16 0 R /Matrix [ 0.4 0.0 0.0 0.4 0.0 0.0 ] /Length 127 >> stream BT % Begin text object /F1 1 Tf % Set text font and size 64 0 0 64 7.1771 2.4414 Tm % Set text matrix 0 Tc % Set character spacing 0 Tw % Set word spacing ( \001 ) Tj % Show spade glyph
0.7478 -0.007 TD % Move text position ( \002 ) Tj % Show heart glyph -0.7323 0.7813 TD % Move text position ( \003 ) Tj % Show diamond glyph 0.6913 0.007 TD % Move text position ( \004 ) Tj % Show club glyph ET % End text object endstream endobj 16 0 obj % Resource dictionary for pattern << /Font << /F1 20 0 R >> >> endobj 20 0 obj % Font for pattern << /Type /Font /Subtype /Type1 /Encoding 21 0 R /BaseFont /ZapfDingbats >> endobj 21 0 obj % Font encoding << /Type /Encoding /Differences [ 1 /a109 /a110 /a111 /a112 ] >> endobj 30 0 obj % Contents of page << /Length 1316 >> stream 0.0 G % Set stroking colour to black 1.0 1.0 0.0 rg % Set nonstroking colour to yellow 25 175 175 -150 re % Construct rectangular path f % Fill path /Cs12 cs % Set pattern colour space 0.77 0.20 0.00 /P1 scn % Set nonstroking colour and pattern 99.92 49.92 m % Start new path 99.92 77.52 77.52 99.92 49.92 99.92 c % Construct lower-left circle 22.32 99.92 -0.08 77.52 -0.08 49.92 c -0.08 22.32 22.32 -0.08 49.92 -0.08 c 77.52 -0.08 99.92 22.32 99.92 49.92 c B % Fill and stroke path 0.2 0.8 0.4 /P1 scn % Change nonstroking colour 224.96 49.92 m % Start new path 224.96 77.52 202.56 99.92 174.96 99.92 c % Construct lower-right circle 147.36 99.92 124.96 77.52 124.96 49.92 c 124.96 22.32 147.36 -0.08 174.96 -0.08 c 202.56 -0.08 224.96 22.32 224.96 49.92 c B % Fill and stroke path 0.3 0.7 1.0 /P1 scn % Change nonstroking colour 87.56 201.70 m % Start new path 63.66 187.90 55.46 157.30 69.26 133.40 c % Construct upper circle 83.06 109.50 113.66 101.30 137.56 115.10 c 161.46 128.90 169.66 159.50 155.86 183.40 c 142.06 207.30 111.46 215.50 87.56 201.70 c B % Fill and stroke path
0.5 0.2 1.0 /P1 scn 50 50 m 175 50 l 112.5 158.253 l b endstream endobj
Shading patterns (PDF 1.3) provide a smooth transition between colours across an area to be painted, independent of the resolution of any particular output device and without specifying the number of steps in the colour transition. Patterns of this type shall be described by pattern dictionaries with a pattern type of 2. Table 76 shows the contents of this type of dictionary.
Table 76 - Entries in a Type 2 Pattern Dictionary
| Key | Type | Value |
|---|---|---|
| Type | name | (Optional) The type of PDF object that this dictionary describes; if present, shall be Pattern for a pattern dictionary. |
| PatternType | integer | (Required) A code identifying the type of pattern that this dictionary describes; shall be 2 for a shading pattern. |
| Shading | dictionary or stream | (Required) A shading object (see below) defining the shading pattern's gradient fill. The contents of the dictionary shall consist of the entries in Table 78 and those in one of Tables 79 to 84. |
| Matrix | array | (Optional) An array of six numbers specifying the pattern matrix (see 8.7.2, "General Properties of Patterns"). Default value: the identity matrix [1 0 0 1 0 0]. |
| ExtGState | dictionary | (Optional) A graphics state parameter dictionary (see 8.4.5, "Graphics State Parameter Dictionaries") containing graphics state parameters to be put into effect temporarily while the shading pattern is painted. Any parameters that are so specified shall be inherited from the graphics state that was in effect at the beginning of the content stream in which the pattern is defined as a resource. |
The most significant entry is Shading , whose value shall be a shading object defining the properties of the shading pattern's gradient fill . This is a complex 'paint' that determines the type of colour transition the shading pattern produces when painted across an area. A shading object shall be a dictionary or a stream, depending on the type of shading; the term shading dictionary is used generically throughout this sub-clause to refer to either a dictionary object or the dictionary portion of a stream object. (Those shading objects that are streams are specifically identified as such in the descriptions of particular shading types; unless otherwise stated, they are understood to be simple dictionaries instead.)
By setting a shading pattern as the current colour in the graphics state, a PDF content stream may use it with painting operators such as f (fill), S (stroke), Tj (show text), or Do (paint external object) with an image mask to paint a path, character glyph, or mask with a smooth colour transition. When a shading is used in this way, the geometry of the gradient fill is independent of that of the object being painted.
When the area to be painted is a relatively simple shape whose geometry is the same as that of the gradient fill itself, the sh operator may be used instead of the usual painting operators. sh accepts a shading dictionary as an operand and applies the corresponding gradient fill directly to current user space. This operator does not
% Change nonstroking colour % Start new path % Construct triangular path
% Close, fill, and stroke path
require the creation of a pattern dictionary or a path and works without reference to the current colour in the graphics state. Table 77 describes the sh operator.
- NOTE Patterns defined by type 2 pattern dictionaries do not tile. To create a tiling pattern containing a gradient fill, invoke the sh operator from within the content stream of a type 1 (tiling) pattern.
Table 77 - Shading Operator
| Operands | Operator | Description |
|---|---|---|
| name | sh | (PDF 1.3) Paint the shape and colour shading described by a shading dictionary, subject to the current clipping path. The current colour in the graphics state is neither used nor altered. The effect is different from that of painting a path using a shading pattern as the current colour. name is the name of a shading dictionary resource in the Shading subdictionary of the current resource dictionary (see 7.8.3, "Resource Dictionaries"). All coordinates in the shading dictionary are interpreted relative to the current user space. (By contrast, when a shading dictionary is used in a type 2 pattern, the coordinates are expressed in pattern space.) All colours are interpreted in the colour space identified by the shading dictionary's ColorSpace entry (see Table 78). The Background entry, if present, is ignored. This operator should be applied only to bounded or geometrically defined shadings. If applied to an unbounded shading, it paints the shading's gradient fill across the entire clipping region, which may be time-consuming. |
A shading dictionary specifies details of a particular gradient fill, including the type of shading to be used, the geometry of the area to be shaded, and the geometry of the gradient fill. Various shading types are available, depending on the value of the dictionary's ShadingType entry:
- Function-based shadings (type 1) define the colour of every point in the domain using a mathematical function (not necessarily smooth or continuous).
- Axial shadings (type 2) define a colour blend along a line between two points, optionally extended beyond the boundary points by continuing the boundary colours.
- Radial shadings (type 3) define a blend between two circles, optionally extended beyond the boundary circles by continuing the boundary colours. This type of shading is commonly used to represent threedimensional spheres and cones.
- Free-form Gouraud-shaded triangle meshes (type 4) define a common construct used by many threedimensional applications to represent complex coloured and shaded shapes. Vertices are specified in freeform geometry.
- Lattice-form Gouraud-shaded triangle meshes (type 5) are based on the same geometrical construct as type 4 but with vertices specified as a pseudorectangular lattice.
- Coons patch meshes (type 6) construct a shading from one or more colour patches, each bounded by four cubic Bézier curves.
- Tensor-product patch meshes (type 7) are similar to type 6 but with additional control points in each patch, affording greater control over colour mapping.
- NOTE 1 Table 78 shows the entries that all shading dictionaries share in common; entries specific to particular shading types are described in the relevant sub-clause.
- NOTE 2 The term target coordinate space , used in many of the following descriptions, refers to the coordinate space into which a shading is painted. For shadings used with a type 2 pattern dictionary, this is the pattern
coordinate space, discussed in 8.7.2, "General Properties of Patterns". For shadings used directly with the sh operator, it is the current user space.
Table 78 - Entries Common to All Shading Dictionaries
| Key | Type | Value |
|---|---|---|
| ShadingType | integer | (Required) The shading type: 1 Function-based shading 2 Axial shading 3 Radial shading 4 Free-form Gouraud-shaded triangle mesh 5 Lattice-form Gouraud-shaded triangle mesh 6 Coons patch mesh 7 Tensor-product patch mesh |
| ColorSpace | name or array | (Required) The colour space in which colour values shall be expressed. This may be any device, CIE-based, or special colour space except a Pattern space. See 8.7.4.4, "Colour Space: Special Considerations" for further information. |
| Background | array | (Optional) An array of colour components appropriate to the colour space, specifying a single background colour value. If present, this colour shall be used, before any painting operation involving the shading, to fill those portions of the area to be painted that lie outside the bounds of the shading object. NOTE In the opaque imaging model, the effect is as if the painting operation were performed twice: first with the background colour and then with the shading. NOTE The background colour is applied only when the shading is used as part of a shading pattern, not when it is painted directly with the sh operator. |
| BBox | rectangle | (Optional) An array of four numbers giving the left, bottom, right, and top coordinates, respectively, of the shading's bounding box. The coordinates shall be interpreted in the shading's target coordinate space. If present, this bounding box shall be applied as a temporary clipping boundary when the shading is painted, in addition to the current clipping path and any other clipping boundaries in effect at that time. |
| AntiAlias | boolean | (Optional) A flag indicating whether to filter the shading function to prevent aliasing artifacts. NOTE The shading operators sample shading functions at a rate determined by the resolution of the output device. Aliasing can occur if the function is not smooth-that is, if it has a high spatial frequency relative to the sampling rate. Anti-aliasing can be computationally expensive and is usually unnecessary, since most shading functions are smooth enough or are sampled at a high enough frequency to avoid aliasing effects. Anti-aliasing may not be implemented on some output devices, in which case this flag is ignored. Default value: false . |
Shading types 4 to 7 shall be defined by a stream containing descriptive data characterizing the shading's gradient fill. In these cases, the shading dictionary is also a stream dictionary and may contain any of the standard entries common to all streams (see Table 5). In particular, shall include a Length entry.
In addition, some shading dictionaries also include a Function entry whose value shall be a function object (dictionary or stream) defining how colours vary across the area to be shaded. In such cases, the shading
dictionary usually defines the geometry of the shading, and the function defines the colour transitions across that geometry. The function is required for some types of shading and optional for others. Functions are described in detail in 7.10, "Functions".
- NOTE 3 Discontinuous colour transitions, or those with high spatial frequency, may exhibit aliasing effects when painted at low effective resolutions.
Conceptually, a shading determines a colour value for each individual point within the area to be painted. In practice, however, the shading may actually be used to compute colour values only for some subset of the points in the target area, with the colours of the intervening points determined by interpolation between the ones computed. Conforming readers are free to use this strategy as long as the interpolated colour values approximate those defined by the shading to within the smoothness tolerance specified in the graphics state (see 10.6.3, "Smoothness Tolerance"). The ColorSpace entry common to all shading dictionaries not only defines the colour space in which the shading specifies its colour values but also determines the colour space in which colour interpolation is performed.
- NOTE 1 Some types of shading (4 to 7) perform interpolation on a parametric value supplied as input to the shading's colour function, as described in the relevant sub-clause. This form of interpolation is conceptually distinct from the interpolation described here, which operates on the output colour values produced by the colour function and takes place within the shading's target colour space.
Gradient fills between colours defined by most shadings may be implemented using a variety of interpolation algorithms, and these algorithms may be sensitive to the characteristics of the colour space.
- NOTE 2 Linear interpolation, for example, may have observably different results when applied in a DeviceCMYK colour space than in a Lab colour space, even if the starting and ending colours are perceptually identical. The difference arises because the two colour spaces are not linear relative to each other.
Shadings shall be rendered according to the following rules:
- If ColorSpace is a device colour space different from the native colour space of the output device, colour values in the shading shall be converted to the native colour space using the standard conversion formulas described in 10.3, "Conversions among Device Colour Spaces". To optimize performance, these conversions may take place at any time (before or after any interpolation on the colour values in the shading). Thus, shadings defined with device colour spaces may have colour gradient fills that are less accurate and somewhat device-dependent. (This does not apply to axial and radial shadings-shading types 2 and 3-because those shading types perform gradient fill calculations on a single variable and then convert to parametric colours.)
- If ColorSpace is a CIE-based colour space, all gradient fill calculations shall be performed in that space. Conversion to device colours shall occur only after all interpolation calculations have been performed. Thus, the colour gradients are device-independent for the colours generated at each point.
- If ColorSpace is a Separation or DeviceN colour space, a colour conversion (to the alternate colour space) occurs only if one or more of the specified colorants is not supported by the device. In that case, gradient fill calculations shall be performed in the designated Separation or DeviceN colour space before conversion to the alternate space. Thus, nonlinear tint transformation functions shall be accommodated for the best possible representation of the shading.
- If ColorSpace is an Indexed colour space, all colour values specified in the shading shall be immediately converted to the base colour space. Depending on whether the base colour space is a device or CIEbased space, gradient fill calculations shall be performed as stated above. Interpolation shall never occur in an Indexed colour space, which is quantized and therefore inappropriate for calculations that assume a
continuous range of colours. For similar reasons, an Indexed colour space shall not be used in any shading whose colour values are generated by a function; this rule applies to any shading dictionary that contains a Function entry.
In addition to the entries listed in Table 78, all shading dictionaries have entries specific to the type of shading they represent, as indicated by the value of their ShadingType entry. The following sub-clauses describe the available shading types and the dictionary entries specific to each.
In Type 1 (function-based) shadings, the colour at every point in the domain is defined by a specified mathematical function. The function need not be smooth or continuous. This type is the most general of the available shading types and is useful for shadings that cannot be adequately described with any of the other types. Table 79 shows the shading dictionary entries specific to this type of shading, in addition to those common to all shading dictionaries (see Table 78).
This type of shading shall not be used with an Indexed colour space.
Table 79 - Additional Entries Specific to a Type 1 Shading Dictionary
| Key | Type | Value |
|---|---|---|
| Domain | array | (Optional) An array of four numbers [ x min x max y min y max ] specifying the rectangular domain of coordinates over which the colour function(s) are defined. Default value: [0.0 1.0 0.0 1.0]. |
| Matrix | array | (Optional) An array of six numbers specifying a transformation matrix mapping the coordinate space specified by the Domain entry into the shading's target coordinate space. NOTE To map the domain rectangle [0.0 1.0 0.0 1.0] to a 1-inch square with lower-left corner at coordinates (100, 100) in default user space, the Matrix value would be [72 0 0 72 100 100]. Default value: the identity matrix [1 0 0 1 0 0]. |
| Function | function | (Required) A 2-in, n -out function or an array of n 2-in, 1-out functions (where n is the number of colour components in the shading dictionary's colour space). Each function's domain shall be a superset of that of the shading dictionary. If the value returned by the function for a given colour component is out of range, it shall be adjusted to the nearest valid value. |
The domain rectangle ( Domain ) establishes an internal coordinate space for the shading that is independent of the target coordinate space in which it shall be painted. The colour function(s) ( Function ) specify the colour of the shading at each point within this domain rectangle. The transformation matrix ( Matrix ) then maps the domain rectangle into a corresponding rectangle or parallelogram in the target coordinate space. Points within the shading's bounding box ( BBox ) that fall outside this transformed domain rectangle shall be painted with the shading's background colour ( Background ); if the shading dictionary has no Background entry, such points shall be left unpainted. If the function is undefined at any point within the declared domain rectangle, an error may occur, even if the corresponding transformed point falls outside the shading's bounding box.
Type 2 (axial) shadings define a colour blend that varies along a linear axis between two endpoints and extends indefinitely perpendicular to that axis. The shading may optionally be extended beyond either or both endpoints
by continuing the boundary colours indefinitely. Table 80 shows the shading dictionary entries specific to this type of shading, in addition to those common to all shading dictionaries (see Table 78).
This type of shading shall not be used with an Indexed colour space.
Table 80 - Additional Entries Specific to a Type 2 Shading Dictionary
| Key | Type | Value |
|---|---|---|
| Coords | array | (Required) An array of four numbers [ x 0 y 0 x 1 y 1 ] specifying the starting and ending coordinates of the axis, expressed in the shading's target coordinate space. |
| Domain | array | (Optional) An array of two numbers [ t 0 t 1 ] specifying the limiting values of a parametric variable t. The variable is considered to vary linearly between these two values as the colour gradient varies between the starting and ending points of the axis. The variable t becomes the input argument to the colour function(s). Default value: [0.0 1.0]. |
| Function | function | (Required) A 1-in, n -out function or an array of n 1-in, 1-out functions (where n is the number of colour components in the shading dictionary's colour space). The function(s) shall be called with values of the parametric variable t in the domain defined by the Domain entry. Each function's domain shall be a superset of that of the shading dictionary. If the value returned by the function for a given colour component is out of range, it shall be adjusted to the nearest valid value. |
| Extend | array | (Optional) An array of two boolean values specifying whether to extend the shading beyond the starting and ending points of the axis, respectively. Default value: [ false false ]. |
The colour blend shall be accomplished by linearly mapping each point ( x , y ) along the axis between the endpoints ( x 0 , y 0 ) and ( x 1 , y 1 ) to a corresponding point in the domain specified by the shading dictionary's Domain entry. The points (0, 0) and (1, 0) in the domain correspond respectively to ( x 0 , y 0 ) and ( x 1 , y 1 ) on the axis. Since all points along a line in domain space perpendicular to the line from (0, 0) to (1, 0) have the same colour, only the new value of x needs to be computed:
The value of the parametric variable t is then determined from x ¢ as follows:
- For 0 £ x ¢ £ 1, t = t 0 + ( t 1 -t 0 ) ¥ x ¢.
- For x ¢ < 0, if the first element of the Extend array is true , then t = t 0 ; otherwise, t is undefined and the point shall be left unpainted.
- For x ¢ > 1, if the second element of the Extend array is true , then t = t 1 ; otherwise, t is undefined and the point shall be left unpainted.
The resulting value of t shall be passed as input to the function(s) defined by the shading dictionary's Function entry, yielding the component values of the colour with which to paint the point ( x , y ).
NOTE Figure L.10 in Annex L shows three examples of the use of an axial shading to fill a rectangle and display text. The area to be filled extends beyond the shading's bounding box. The shading is the same in all three cases, except for the values of the Background and Extend entries in the shading dictionary. In the first example, the shading is not extended at either end and no background colour is specified; therefore, the shading is clipped to its bounding box at both ends. The second example still has no background colour specified, but the shading is extended at both ends; the result is to fill the remaining portions of the filled area with the colours
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