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PDF Spec 14 Web Capture
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The E (expiration) entry specifies an expiration date for each source location associated with a content set. If the current date and time are later than those specified, the contents of the content set shall be considered out of date with respect to the original source.
When a URL is accessed via HTTP, a response header may be returned indicating that the requested data is at a different URL. This redirection process may be repeated in turn at the new URL and can potentially continue indefinitely. It is not uncommon to find multiple URLs that all lead eventually to the same destination through one or more redirections. A URL alias dictionary represents such a set of URL chains leading to a common destination. Table 356 shows the contents of this type of dictionary.
Table 356 - Entries in a URL alias dictionary
| Key | Type | Value |
|---|---|---|
| U | ASCII string | (Required) The destination URL to which all of the chains specified by the C entry lead. |
| C | array | (Optional) An array of one or more arrays of strings, each representing a chain of URLs leading to the common destination specified by U . |
The C (chains) entry may be omitted if the URL alias dictionary contains only one URL. If C is present, its value shall be an array of arrays, each representing a chain of URLs leading to the common destination. Within each chain, the URLs shall be stored as ASCII strings in the order in which they occur in the redirection sequence. The common destination (the last URL in a chain) may be omitted, since it is already identified by the U entry.
A Web Capture command dictionary represents a command executed by Web Capture to retrieve one or more pieces of source data that were used to create new pages or modify existing pages. The entries in this dictionary represent parameters that were originally specified interactively by the user who requested that the Web content be captured. This information is recorded so that the command can subsequently be repeated to update the captured content. Table 357 shows the contents of this type of dictionary.
Table 357 - Entries in a Web Capture command dictionary
| Key | Type | Value |
|---|---|---|
| URL | ASCII string | (Required) The initial URL from which source data was requested. |
| L | integer | (Optional) The number of levels of pages retrieved from the initial URL. Default value: 1. |
| F | integer | (Optional) A set of flags specifying various characteristics of the command (see Table 357). Defaut value: 0. |
| P | string or stream | (Optional) Data that was posted to the URL. |
| CT | ASCII string | (Optional) A content type describing the data posted to the URL. Default value: application/x-www-form-urlencoded. |
| H | string | (Optional) Additional HTTP request headers sent to the URL. |
Table 357 - Entries in a Web Capture command dictionary (continued)
| Key | Type | Value |
|---|---|---|
| S | dictionary | (Optional) A command settings dictionary containing settings used in the conversion process (see 14.10.5.4, 'Command Settings'). |
The URL entry shall contain the initial URL for the retrieval command. The L (levels) entry shall contain the number of levels of the hyperlinked URL hierarchy to follow from this URL, creating PDF pages from the retrieved material. If the L entry is omitted, its value shall be assumed to be 1, denoting retrieval of the initial URL only.
The value of the command dictionary's F entry shall be an integer that shall be interpreted as an array of flags specifying various characteristics of the command. The flags shall be interpreted as defined in Table 358. Only those flags defined in Table 358 may be set to 1; all other flags shall be 0. Flags not defined in Table 358 are reserved for future use, and shall not be used by a conforming reader.
NOTE 3 The low-order bit of the flags value is referred to as being at bit-position 1.
Table 358 - Web Capture command flags
| Bit position | Name | Meaning |
|---|---|---|
| 1 | SameSite | If set, pages were retrieved only from the host specified in the initial URL. |
| 2 | SamePath | If set, pages were retrieved only from the path specified in the initial URL. |
| 3 | Submit | If set, the command represents a form submission. |
The SamePath flag shall be set if the retrieval of source content was restricted to source content in the same path as specified in the initial URL. Source content shall be considered to be in the same path if its scheme and network location components (as defined in Internet RFC 1808, Relative Uniform Resource Locators ) match those of the initial URL and its path component matches up to and including the last forward slash ( / ) character in the initial URL.
EXAMPLE 1 the URL
http : / / www . adobe . com / fiddle / faddle / foo . html is considered to be in the same path as the initial URL
http : / / www . adobe . com / fiddle / initial . html
The comparison shall be case-insensitive for the scheme and network location components and case-sensitive for the path component.
The Submit flag shall be set when the command represents a form submission. If no P (posted data) entry is present, the submitted data shall be encoded in the URL (an HTTP GET request). If P is present, the command shall be an HTTP POST request. In this case, the value of the Submit flag shall be ignored.
NOTE 4 If the posted data is small enough, it may be represented by a string. For large amounts of data, a stream should be used because it can be compressed.
The CT (content type) entry shall only be present for POST requests. It shall describe the content type of the posted data, as described in Internet RFC 2045, Multipurpose Internet Mail Extensions (MIME), Part One: Format of Internet Message Bodies (see the Bibliography).
The H (headers) entry, if present, shall specify additional HTTP request headers that were sent in the request for the URL. Each header line in the string shall be terminated with a CARRIAGE RETURN and a LINE FEED, as in this example:
EXAMPLE 2 ( Referer : http : / / frumble . com \ 015 \ 012From : veeble @ frotz . com \ 015 \ 012 )
The HTTP request header format is specified in Internet RFC 2616, Hypertext Transfer Protocol-HTTP/1.1 (see the Bibliography).
The S (settings) entry specifies a command settings dictionary (see 14.10.5.4, 'Command Settings'). Holding settings specific to the conversion engines.
The S (settings) entry in a command dictionary, if present, shall contain a command settings dictionary , which holds settings for conversion engines that shall be used in converting the results of the command to PDF. Table 359 shows the contents of this type of dictionary. If this entry is omitted, default values are assumed. Command settings dictionaries may be shared by any command dictionaries that use the same settings.
Table 359 - Entries in a Web Capture command settings dictionary
| Key | Type | Value |
|---|---|---|
| G | dictionary | (Optional) A dictionary containing global conversion engine settings relevant to all conversion engines. If this entry is absent, default settings shall be used. |
| C | dictionary | (Optional) Settings for specific conversion engines. Each key in this dictionary is the internal name of a conversion engine. The associated value is a dictionary containing the settings associated with that conversion engine. If the settings for a particular conversion engine are not found in the dictionary, default settings shall be used. |
Each key in the C dictionary represents the internal name of a conversion engine, which shall be a name object of the following form:
/ company : product : version : contentType
company denotes the name (or abbreviation) of the company that created the conversion engine.
product denotes the name of the conversion engine. This field may be left blank, but the trailing COLON character (3Ah) is still required.
version denotes the version of the conversion engine.
contentType denotes an identifier for the content type the associated settings. shall be used because some converters may handle multiple content types.
EXAMPLE / ADBE : H2PDF : 1 . 0 : HTML
All fields in the internal name are case-sensitive. The company field shall conform to the naming guidelines described in Annex E. The values of the other fields shall be unrestricted, except that they shall not contain a COLON.
The directed graph of PDF objects rooted by the command settings dictionary shall be entirely self-contained; that is, it shall not contain any object referred to from elsewhere in the PDF file.
NOTE This facilitates the operation of making a deep copy of a command settings dictionary without explicit knowledge of the settings it may contain.
A given page object or image XObject may belong to at most one Web Capture content set, called its parent content set . However, the object shall not have direct pointer to its parent content set. Such a pointer may present problems for an application that traces all pointers from an object to determine what resources the object depends on. Instead, the object's ID entry (see Table 30 and Table 89) contains the digital identifier of the parent content set, which shall be used to locate the parent content set via the IDS name tree in the document's name dictionary. (If the IDS entry for the identifier contains an array of content sets, the parent may be found by searching the array for the content set whose O entry includes the child object.)
In the course of creating PDF pages from HTML files, Web Capture frequently scales the contents down to fit on fixed-sized pages. The PZ (preferred zoom) entry in a page object (see 7.7.3.3, 'Page Objects') specifies a magnification factor by which the page may be scaled to undo the downscaling and view the page at its original size. That is, when the page is viewed at the preferred magnification factor, one unit in default user space corresponds to one original source pixel.
This sub-clause describes features of PDF that support prepress production workflows:
- The specification of page boundaries governing various aspects of the prepress process, such as cropping, bleed, and trimming (14.11.2, 'Page Boundaries')
- Facilities for including printer's marks , such as registration targets, gray ramps, colour bars, and cut marks to assist in the production process (14.11.3, 'Printer's Marks')
- Information for generating colour separations for pages in a document (14.11.4, 'Separation Dictionaries')
- Output intents for matching the colour characteristics of a document with those of a target output device or production environment in which it will be printed (14.11.5, 'Output Intents')
- Support for the generation of traps to minimize the visual effects of misregistration between multiple colorants (14.11.6, 'Trapping Support')
- The Open Prepress Interface (OPI) for creating low-resolution proxies for high-resolution images (14.11.7, 'Open Prepress Interface (OPI)')
A PDF page may be prepared either for a finished medium, such as a sheet of paper, or as part of a prepress process in which the content of the page is placed on an intermediate medium, such as film or an imposed reproduction plate. In the latter case, it is important to distinguish between the intermediate page and the finished page. The intermediate page may often include additional production-related content, such as bleeds or printer marks, that falls outside the boundaries of the finished page. To handle such cases, a PDF page may define as many as five separate boundaries to control various aspects of the imaging process:
-
The media box defines the boundaries of the physical medium on which the page is to be printed. It may include any extended area surrounding the finished page for bleed, printing marks, or other such purposes. It may also include areas close to the edges of the medium that cannot be marked because of physical limitations of the output device. Content falling outside this boundary may safely be discarded without affecting the meaning of the PDF file.
-
The crop box defines the region to which the contents of the page shall be clipped (cropped) when displayed or printed. Unlike the other boxes, the crop box has no defined meaning in terms of physical page geometry or intended use; it merely imposes clipping on the page contents. However, in the absence of additional information (such as imposition instructions specified in a JDF or PJTF job ticket), the crop box determines how the page's contents shall be positioned on the output medium. The default value is the page's media box.
-
The bleed box (PDF 1.3) defines the region to which the contents of the page shall be clipped when output in a production environment. This may include any extra bleed area needed to accommodate the physical limitations of cutting, folding, and trimming equipment. The actual printed page may include printing marks that fall outside the bleed box. The default value is the page's crop box.
-
The trim box (PDF 1.3) defines the intended dimensions of the finished page after trimming. It may be smaller than the media box to allow for production-related content, such as printing instructions, cut marks, or colour bars. The default value is the page's crop box.
-
The art box (PDF 1.3) defines the extent of the page's meaningful content (including potential white space) as intended by the page's creator. The default value is the page's crop box.
The page object dictionary specifies these boundaries in the MediaBox , CropBox , BleedBox , TrimBox , and ArtBox entries, respectively (see Table 30). All of them are rectangles expressed in default user space units. The crop, bleed, trim, and art boxes shall not ordinarily extend beyond the boundaries of the media box. If they do, they are effectively reduced to their intersection with the media box. Figure 86 illustrates the relationships among these boundaries. (The crop box is not shown in the figure because it has no defined relationship with any of the other boundaries.)
Figure 86 - Page boundaries
- NOTE 1 How the various boundaries are used depends on the purpose to which the page is being put. The following are typical purposes:
Placing the content of a page in another application. The art box determines the boundary of the content that is to be placed in the application. Depending on the applicable usage conventions, the placed content may be clipped to either the art box or the bleed box. For example, a quarter-page advertisement to be placed on a magazine page might be clipped to the art box on the two sides of the ad that face into the middle of the page and to the bleed box on the two sides that bleed over the edge of the page. The media box and trim box are ignored.
Printing a finished page. This case is typical of desktop or shared page printers, in which the page content is positioned directly on the final output medium. The art box and bleed box are ignored. The media box may be used as advice for selecting media of the appropriate size. The crop box and trim box, if present, should be the same as the media box.
Printing an intermediate page for use in a prepress process. The art box is ignored. The bleed box defines the boundary of the content to be imaged. The trim box specifies the positioning of the content on the medium; it may also be used to generate cut or fold marks outside the bleed box. Content falling within the media box but outside the bleed box may or may not be imaged, depending on the specific production process being used.
Building an imposition of multiple pages on a press sheet. The art box is ignored. The bleed box defines the clipping boundary of the content to be imaged; content outside the bleed box is ignored. The trim box specifies the positioning of the page's content within the imposition. Cut and fold marks are typically generated for the imposition as a whole.
- NOTE 2 In the preceding scenarios, an application that interprets the bleed, trim, and art boxes for some purpose typically alters the crop box so as to impose the clipping that those boxes prescribe.
Conforming readers may offer the ability to display guidelines on the screen for the various page boundaries. The optional BoxColorInfo entry in a page object (see 7.7.3.3, 'Page Objects') holds a box colour information dictionary (PDF 1.4) specifying the colours and other visual characteristics to be used for such display. Conforming readers typically provide a user interface to allow the user to set these characteristics interactively.
NOTE This information is page-specific and may vary from one page to another.
As shown in Table 360, the box colour information dictionary contains an optional entry for each of the possible page boundaries other than the media box. The value of each entry is a box style dictionary , whose contents are shown in Table 361. If a given entry is absent, the conforming reader shall use its own current default settings instead.
Printer's marks are graphic symbols or text added to a page to assist production personnel in identifying components of a multiple-plate job and maintaining consistent output during production. Examples commonly used in the printing industry include:
- Registration targets for aligning plates
- Gray ramps and colour bars for measuring colours and ink densities
- Cut marks showing where the output medium is to be trimmed
Although conforming writers traditionally include such marks in the content stream of a document, they are logically separate from the content of the page itself and typically appear outside the boundaries (the crop box, trim box, and art box) defining the extent of that content (see 14.11.2, 'Page Boundaries').
Printer's mark annotations (PDF 1.4) provide a mechanism for incorporating printer's marks into the PDF representation of a page, while keeping them separate from the actual page content. Each page in a PDF document may contain any number of such annotations, each of which represents a single printer's mark.
- NOTE 1 Because printer's marks typically fall outside the page's content boundaries, each mark is represented as a separate annotation. Otherwise-if, for example, the cut marks at the four corners of the page were defined in a single annotation-the annotation rectangle would encompass the entire contents of the page and could interfere with the user's ability to select content or interact with other annotations on the page. Defining printer's marks in separate annotations also facilitates the implementation of a drag-and-drop user interface for specifying them.
| Key | Type | Value |
|---|---|---|
| CropBox | dictionary | (Optional) A box style dictionary (see Table 361) specifying the visual characteristics for displaying guidelines for the page's crop box. This entry shall be ignored if no crop box is defined in the page object. |
| BleedBox | dictionary | (Optional) A box style dictionary (see Table 361) specifying the visual characteristics for displaying guidelines for the page's bleed box. This entry shall be ignored if no bleed box is defined in the page object. |
Table 360 - Entries in a box colour information dictionary (continued)
| Key | Type | Value |
|---|---|---|
| TrimBox | dictionary | (Optional) A box style dictionary (see Table 361) specifying the visual characteristics for displaying guidelines for the page's trim box. This entry shall be ignored if no trim box is defined in the page object. |
| ArtBox | dictionary | (Optional) A box style dictionary (see Table 361) specifying the visual characteristics for displaying guidelines for the page's art box. This entry shall be ignored if no art box is defined in the page object. |
Table 361 - Entries in a box style dictionary
| Key | Type | Value |
|---|---|---|
| C | array | (Optional) An array of three numbers in the range 0.0 to 1.0, representing the components in the DeviceRGB colour space of the colour to be used for displaying the guidelines. Default value: [0.0 0.0 0.0]. |
| W | number | (Optional) The guideline width in default user space units. Default value: 1. |
| S | name | (Optional) The guideline style: S (Solid) A solid rectangle. D (Dashed) A dashed rectangle. The dash pattern shall be specified by the D entry. Other guideline styles may be defined in the future. Default value: S. |
| D | array | (Optional) A dash array defining a pattern of dashes and gaps to be used in drawing dashed guidelines (guideline style D). The dash array shall be specified in default user space units, in the same format as in the line dash pattern parameter of the graphics state (see 8.4.3.6, 'Line Dash Pattern'). The dash phase shall not be specified and shall be assumed to be 0. EXAMPLE A D entry of [3 2] specifies guidelines drawn with 3-point dashes alternating with 2-point gaps. Default value: [3]. |
The visual presentation of a printer's mark shall be defined by a form XObject specified as an appearance stream in the N (normal) entry of the printer's mark annotation's appearance dictionary (see 12.5.5, 'Appearance Streams'). More than one appearance may be defined for the same printer's mark to meet the requirements of different regions or production facilities. In this case, the appearance dictionary's N entry holds a subdictionary containing the alternate appearances, each identified by an arbitrary key. The AS (appearance state) entry in the annotation dictionary designates one of them to be displayed or printed.
NOTE 2 The printer's mark annotation's appearance dictionary may include R (rollover) or D (down) entries, but appearances defined in either of these entries are never displayed or printed.
Like all annotations, a printer's mark annotation shall be defined by an annotation dictionary (see 12.5.2, 'Annotation Dictionaries'); its annotation type is PrinterMark . The AP (appearances) and F (flags) entries (which ordinarily are optional) shall be present, as shall the AS (appearance state) entry if the appearance dictionary AP contains more than one appearance stream. The Print and ReadOnly flags in the F entry shall be set and all others clear (see 12.5.3, 'Annotation Flags'). Table 362 shows an additional annotation dictionary entry specific to this type of annotation.
Table 362 - Additional entries specific to a printer's mark annotation
| Key | Type | Value |
|---|---|---|
| Subtype | name | (Required) The type of annotation that this dictionary describes; shall be PrinterMark for a printer's mark annotation. |
| MN | name | (Optional) An arbitrary name identifying the type of printer's mark, such as ColorBar or RegistrationTarget. |
The form dictionary defining a printer's mark may contain the optional entries shown in Table 363 in addition to the standard ones common to all form dictionaries (see 8.10.2, 'Form Dictionaries').
Table 363 - Additional entries specific to a printer's mark form dictionary
| Key | Type | Value |
|---|---|---|
| MarkStyle | text string | (Optional; PDF 1.4) A text string representing the printer's mark in human-readable form and suitable for presentation to the user. |
| Colorants | dictionary | (Optional; PDF 1.4) A dictionary identifying the individual colorants associated with a printer's mark, such as a colour bar. For each entry in this dictionary, the key is a colorant name and the value is 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. |
In high-end printing workflows, pages are ultimately produced as sets of separations , one per colorant (see 8.6.6.4, 'Separation Colour Spaces'). Ordinarily, each page in a PDF file shall be treated as a composite page that paints graphics objects using all the process colorants and perhaps some spot colorants as well. In other words, all separations for a page shall be generated from a single PDF description of that page.
In some workflows, however, pages are preseparated before generating the PDF file. In a preseparated PDF file, the separations for a page shall be described as separate page objects, each painting only a single colorant (usually specified in the DeviceGray colour space). In this case, additional information is needed to identify the actual colorant associated with each separation and to group together the page objects representing all the separations for a given page. This information shall be contained in a separation dictionary (PDF 1.3) in the SeparationInfo entry of each page object (see 7.7.3.3, 'Page Objects'). Table 364 shows the contents of this type of dictionary.
Table 364 - Entries in a separation dictionary
| Key | Type | Value |
|---|---|---|
| Pages | array | (Required ) An array of indirect references to page objects representing separations of the same document page. One of the page objects in the array shall be the one with which this separation dictionary is associated, and all of them shall have separation dictionaries ( SeparationInfo entries) containing Pages arrays identical to this one. |
| DeviceColorant | name or string | (Required) The name of the device colorant to be used in rendering this separation, such as Cyan or PANTONE 35 CV. |
Table 364 - Entries in a separation dictionary (continued)
| Key | Type | Value |
|---|---|---|
| ColorSpace | array | (Optional) An array defining a Separation or DeviceN colour space (see 8.6.6.4, 'Separation Colour Spaces' and 8.6.6.5, 'DeviceN Colour Spaces'). It provides additional information about the colour specified by DeviceColorant -in particular, the alternate colour space and tint transformation function that shall be used to represent the colorant as a process colour. This information enables a conforming reader to preview the separation in a colour that approximates the device colorant. The value of DeviceColorant shall match the space's colorant name (if it is a Separation space) or be one of the space's colorant names (if it is a DeviceN space). |
Output intents (PDF 1.4) provide a means for matching the colour characteristics of a PDF document with those of a target output device or production environment in which the document will be printed. The optional OutputIntents entry in the document catalogue (see 7.7.2, 'Document Catalog') holds an array of output intent dictionaries , each describing the colour reproduction characteristics of a possible output device or production condition. The contents of these dictionaries may vary for different devices and conditions. The dictionary's S entry specifies an output intent subtype that determines the format and meaning of the remaining entries.
- NOTE 1 This use of multiple output intents allows the production process to be customized to the expected workflow and the specific tools available. For example, one production facility might process files conforming to a recognized standard such as PDF/X-1, while another uses the PDF/A standard to produce RGB output for document distribution on the Web. Each of these workflows would require different sets of output intent information. Multiple output intents also allow the same PDF file to be distributed unmodified to multiple production facilities. The choice of which output intent to use in a given production environment is a matter for agreement between the purchaser and provider of production services. PDF intentionally does not include a selector for choosing a particular output intent from within the PDF file.
At the time of publication, three output intent subtypes have been defined: GTS_PDFX corresponding to the PDF/X format standard specified in ISO 15930, GTS_PDFA1 corresponding to the PDF/A standard as defined by ISO 19005, and ISO_PDFE1 corresponding to the PDF/E standard as defined by ISO 24517. Table 365 shows the contents of this type of output intent dictionary. Other subtypes may be added in the future; the names of any such additional subtypes shall conform to the naming guidelines described in Annex E.
Table 365 - Entries in an output intent dictionary
| Key | Type | Value |
|---|---|---|
| Type | name | (Optional) The type of PDF object that this dictionary describes; if present, shall be OutputIntent for an output intent dictionary. |
| S | name | (Required) The output intent subtype; shall be either one of GTS_PDFX , GTS_PDFA1 , ISO_PDFE1 or a key defined by an ISO 32000 extension. |
| OutputCondition | text string | (Optional) A text string concisely identifying the intended output device or production condition in human-readable form. This is the preferred method of defining such a string for presentation to the user. |
Table 365 - Entries in an output intent dictionary (continued)
| Key | Type | Value |
|---|---|---|
| OutputConditionIdentifier | text string | (Required) A text string identifying the intended output device or production condition in human- or machine- readable form. If human-readable, this string may be used in lieu of an OutputCondition string for presentation to the user. A typical value for this entry may be the name of a production condition maintained in an industry-standard registry such as the ICC Characterization Data Registry (see the Bibliography). If the designated condition matches that in effect at production time, the production software is responsible for providing the corresponding ICC profile as defined in the registry. If the intended production condition is not a recognized standard, the value of this entry may be Custom or an application-specific, machine-readable name. The DestOutputProfile entry defines the ICC profile, and the Info entry shall be used for further human-readable identification. |
| RegistryName | text string | (Optional) An text string (conventionally a uniform resource identifier, or URI) identifying the registry in which the condition designated by OutputConditionIdentifier is defined. |
| Info | text string | (Required if OutputConditionIdentifier does not specify a standard production condition; optional otherwise) A human-readable text string containing additional information or comments about the intended target device or production condition. |
| DestOutputProfile | stream | (Required if OutputConditionIdentifier does not specify a standard production condition; optional otherwise) An ICC profile stream defining the transformation from the PDF document's source colours to output device colorants. The format of the profile stream is the same as that used in specifying an ICCBased colour space (see 8.6.5.5, 'ICCBased Colour Spaces'). The output transformation uses the profile's 'from CIE' information (BToA in ICC terminology); the 'to CIE' (AToB) information may optionally be used to remap source colour values to some other destination colour space, such as for screen preview or hardcopy proofing. |
NOTE 2 PDF/X is actually a family of standards representing varying levels of conformance. The standard for a given conformance level may prescribe further restrictions on the usage and meaning of entries in the output intent dictionary. Any such restrictions take precedence over the descriptions given in Table 364.
The ICC profile information in an output intent dictionary supplements rather than replaces that in an ICCBased or default colour space (see 8.6.5.5, 'ICCBased Colour Spaces,' and 8.6.5.6, 'Default Colour Spaces'). Those mechanisms are specifically intended for describing the characteristics of source colour component values. An output intent can be used in conjunction with them to convert source colours to those required for a specific production condition or to enable the display or proofing of the intended output.
The data in an output intent dictionary shall be provided for informational purposes only, and conforming readers are free to disregard it. In particular, there is no expectation that PDF production tools automatically convert colours expressed in the same source colour space to the specified target space before generating output. (In some workflows, such conversion may, in fact, be undesirable).
- NOTE When working with CMYK source colours tagged with a source ICC profile solely for purposes of characterization, converting such colours from four components to three and back is unnecessary and will result in a loss of fidelity in the values of the black component; see 8.6.5.7, 'Implicit Conversion of CIE-Based Colour Spaces' for further discussion.) On the other hand, when source colours are expressed in different base colour spaces-for example, when combining separately generated images on the same PDF page-it is possible (though not required) to use the destination profile specified in the output intent dictionary to convert source colours to the same target colour space.
- EXAMPLE 1 This Example shows a PDF/X output intent dictionary based on an industry-standard production condition (CGATS TR 001) from the ICC Characterization Data Registry . Example 2 shows one for a custom production condition.
<< /Type /OutputIntent % Output intent dictionary /S /GTS_PDFX /OutputCondition ( CGATS TR 001 ( SWOP ) ) /OutputConditionIdentifier ( CGATS TR 001 ) /RegistryName ( http : / / www . color . org ) /DestOutputProfile 100 0 R >> 100 0 obj % ICC profile stream << /N 4 /Length 1605 /Filter /ASCIIHexDecode >> stream 00 00 02 0C 61 70 … > endstream endobj << /Type /OutputIntent % Output intent dictionary /S /GTS_PDFX /OutputCondition ( Coated ) /OutputConditionIdentifier ( Custom ) /Info ( Coated 150lpi ) /DestOutputProfile 100 0 R >> 100 0 obj % ICC profile stream << /N 4 /Length 1605 /Filter /ASCIIHexDecode >> stream 00 00 02 0C 61 70 … > endstream endobj
On devices such as offset printing presses, which mark multiple colorants on a single sheet of physical medium, mechanical limitations of the device can cause imprecise alignment, or misregistration , between colorants. This can produce unwanted visual artifacts such as brightly coloured gaps or bands around the edges of printed objects. In high-quality reproduction of colour documents, such artifacts are commonly avoided by creating an overlap, called a trap , between areas of adjacent colour.
NOTE
Figure 87 shows an example of trapping. The light and medium grays represent two different colorants, which are used to paint the background and the glyph denoting the letter A. The first figure shows the intended result, with the two colorants properly registered. The second figure shows what happens when the colorants are
misregistered. In the third figure, traps have been overprinted along the boundaries, obscuring the artifacts caused by the misregistration. (For emphasis, the traps are shown here in dark gray; in actual practice, their colour will be similar to one of the adjoining colours.)
Figure 87 - Trapping example
Trapping may be implemented by the application generating a PDF file, by some intermediate application that adds traps to a PDF document, or by the raster image processor (RIP) that produces final output. In the last two cases, the trapping process is controlled by a set of trapping instructions , which define two kinds of information:
- Trapping zones within which traps should be created
- Trapping parameters specifying the nature of the traps within each zone
Trapping zones and trapping parameters are discussed fully in Sections 6.3.2 and 6.3.3, respectively, of the PostScript Language Reference , Third Edition. Trapping instructions are not directly specified in a PDF file (as they are in a PostScript file). Instead, they shall be specified in a job ticket that accompanies the PDF file or is embedded within it. Various standards exist for the format of job tickets; two of them, JDF (Job Definition Format) and PJTF (Portable Job Ticket Format), are described in the CIP4 document JDF Specification and in Adobe Technical Note #5620, Portable Job Ticket Format (see the Bibliography).
When trapping is performed before the production of final output, the resulting traps shall be placed in the PDF file for subsequent use. The traps themselves shall be described as a content stream in a trap network annotation (see 14.11.6.2, 'Trap Network Annotations'). The stream dictionary may include additional entries describing the method that was used to produce the traps and other information about their appearance.
A complete set of traps generated for a given page under a specified set of trapping instructions is called a trap network (PDF 1.3) . It is a form XObject containing graphics objects for painting the required traps on the page. A page may have more than one trap network based on different trapping instructions, presumably intended for different output devices. All of the trap networks for a given page shall be contained in a single trap network annotation (see 12.5, 'Annotations'). There may be at most one trap network annotation per page, which shall be the last element in the page's Annots array (see 7.7.3.3, 'Page Objects'). This ensures that the trap network shall be printed after all of the page's other contents.
The form XObject defining a trap network shall be specified as an appearance stream in the N (normal) entry of the trap network annotation's appearance dictionary (see 12.5.5, 'Appearance Streams'). If more than one trap network is defined for the same page, the N entry holds a subdictionary containing the alternate trap networks, each identified by an arbitrary key. The AS (appearance state) entry in the annotation dictionary designates one of them as the current trap network to be displayed or printed.
- NOTE 1 The trap network annotation's appearance dictionary may include R (rollover) or D (down) entries, but appearances defined in either of these entries are never printed.
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