Postscript is designed to be printed and published as a de facto industry standard for digital prepress technology: Multimedia applications have been developed to a very high degree when designing PDF documents, so it is suitable for print publishing and electronic publishing: after the emergence of web publishing , PDF still found its place. The PDF has features that meet the requirements of cross-media publishing because it is rooted in Postscript technology, inheriting all the advantages of the technology, and taking into account the needs and technical characteristics of electronic publishing. In addition, display Postscript technology is also integrated in the PDF to achieve consistency in hard copy output and screen display.

After cross-media publishing requirements have been designed and produced, the results of the work can be used for a variety of publishing purposes, namely one production and multiple uses. Cross-media publishing also requires that the production content has an adaptive processing feature that is consistent with the purpose of the publication, that is, the relevant parameters can be set for a specific publication purpose to suit the recording technology and media used by the publication.

First, cross-media publishing and its basic requirements

The basic task of cross-media publishing can be summarized as "the processing, use and storage of data files can adapt to any kind of output". This basic task can also be used as a definition of the concept of cross-media.

From the definition of cross-media, we can see that it not only sums up and summarizes the relationship and technical characteristics of digital prepress and digital printing, but also reflects the new requirements put forward by the ever-developing social needs for print publishing and electronic publishing: It also expresses the broad applicability of this concept, that is, the cross-media concept is the basic requirement for the cross-use of traditional publishing, electronic publishing and web publishing content. Cross-media publishing requires user-designed content to have certain basic characteristics.

1. Content adaptive processing features

After the content is produced, it needs to be finished. With the advent of electronic publishing systems, the professional knowledge requirements of operators have been reduced, and more and more people are engaged in the production of publications. However, there are misunderstandings on the fundamental issue of significant differences between production and finalization. Design and production are only the preliminary work for the completion of the dissemination of content, during which it may not be necessary to take into account the use of the content and the recording medium, but it is therefore not appropriate to ignore the importance of the final draft. After the design and production are completed, it should be targeted at a specific dissemination method and according to the recording technology and recording medium to be used for targeted processing. This process is called finalization. After the concept of cross-media is proposed, the final draft becomes more important.

Taking the production content for a print job as an example, the page should have adaptive processing features for recording information using a combination of paper, ink, printing press, and a specific printing process after being processed. For example, the current page should be converted to the CMYK color space in other color spaces (this process is also referred to as color separation in the digital prepress process), to compensate for the lack of accuracy of the printing press and plate to produce overprinting. The content is used for trapping. Different screening parameters need to be selected for the quality requirements and copying requirements of the printed matter. On the relevant pages, bite marks, cutting marks, signal bars, etc. are provided to facilitate the printing plate installation, cutting and controlling the printing quality, etc. .

2. modify

Everything in the world is in development and changes, and customer requirements may change. In addition, the design and production of good content can not be without errors, such as text input errors, graphical expression errors. For this reason, cross-media publishing requires that the produced content be edited and corrected before output to reduce losses and create favorable conditions for cross-media publishing.

3. Data format conversion

With respect to the publication content production process that uses digital devices and digital technologies in its entirety, there are different requirements for digital file formats for publishing methods, depending on the nature of the recording media and the mode of dissemination. Therefore, after the content is produced, the data format conversion should be performed as required. For example, the documents used for printing and publishing are converted into Postscript, EPS or PDF formats, and the documents used for network publishing are converted into HTML formats.

From the basic requirements for cross-media mentioned above, effective implementation of cross-media is not enough due to traditional output technologies. It calls for new output technologies, such as Adobe's successful development based on Postscript Level 3 and PDF technologies. SUPRA structure.

Second, PDF is suitable for printing and publishing features

The characteristics of PDF for printing and publishing are mainly divided into two aspects: first, it continues to support prepress and printing workflows, and second, each PDF description page is independent of each other.

1. The importance of page independence

Physically, a Pootscript file describes pages that are connected to each other. They are arranged in the order of the contents of the publication. After being output on a hard copy device, a sequence of consecutive pages is obtained. These pages have a sequential relationship, a logical relationship, or a causal relationship. In the file structure, each Postscript file is actually a collection of Postscript instructions, which are divided into two parts: object definition and description. The description is about the description of the object. Because of this, it is possible to generate correlation between the object and the object. , and the relevance of the object to the object causes the relevance of the page because the related object may be in a different physical page.

The biggest drawback of page correlation is that it is very error-prone when outputting. For example, when RIP interprets an object on page 5, it finds that it is related to the object on page 2, then RIP will jump to the 2nd page. Page, reinterpret the object, and return to page 5. If we only need to re-interpret it once, the problem is not big. It is a waste of time. But the problem is that a related object may be interpreted again after being reinterpreted, so that the result of repeated interpretation is the multiple nesting of the interpretation, which causes the Postscript interpreter to issue a limit error message or an overflow error when it is output.

The fundamental difference between PDF and Postscript is that RDF is a document format, not a page description language. Although it also describes objects, because there is no programming structure, pages and pages are not related. Therefore, after the application Acrobat Distiller, which generates the PDF file, "distills", the relevance of the page to the page is cut off. From this it can be seen that the page-independent nature of PDF enables it to meet the basic functions of cross-media publishing and can be used for print publishing, electronic publishing or web publishing without additional processing.

2. PDF print publishing function

Since PDF is based on Postscript technology, there should be no doubt that it should be published in print. However, in the early days of its introduction, the PDF format provided only a limited degree of print publishing capabilities until the release of Acrobat 3.0. Considering that there have been many articles on the printing and publishing function of PDF, there is no intention to discuss it in more space here. To put it together, PDF's support for print publishing manifests itself in:

(1) PDF, like Postscript, generates objects using a method similar to the printing process (template coloring), which is the most fundamental attribute of PDF:

(2) Beginning with version 1.2, PDF has begun to support the CMYK color space, which is necessary for the realization of print reproduction:

(3) Each Postscript file or EPS file is always related to the page. This is the main reason why a work flow based on PS or EPS file often generates a limit check error or overflow error, but when PS file or EPS file is converted into After the PDF file, the relevance of the page is cut off to ensure that each PDF page can be stably output:

(4) Open prepress interface QPI (Open Prepress interface) is supported to achieve high resolution image substitution because digital images used for printing purposes usually have high spatial resolution and tonal resolution, and thus image files The amount of storage is quite large. In order to reduce the burden of "carriage", it is necessary to adopt a low-resolution representative image at the time of typesetting, and to replace it with a high resolution image at the time of output:

(5) Support for composite font technology, all required fonts in the publication can be displayed on the page;

(6) Support trapping to solve the problem that existing printing equipment may appear "white" at the edge of the subject due to overprinting;

(7) Built-in digital screening algorithm, which allows to specify parameters such as dot shape, screen number and screen angle for different publishing requirements and printing quality;

(8) It supports the clipping path, so that the clipping path output by the image processing software can be used as the boundary of the character winding;

(9) Support for color tone images to achieve monochromatic prints with more layers of secondary copies, suitable for printing high-quality prints;

(10) Support separation color space to facilitate conversion from other image modes to CMYK mode;

(11) Providing bite marks, cutting marks, signal strips, or gray scales on certain pages, which is conducive to printing plate installation before printing, inspection overprint accuracy, and inspection of print quality.

The functions of PDF support printing and publishing can also be mentioned, but the above list is the main content.

Third, PDF electronic publishing and online publishing features

The PDF file format also takes into account the need for electronic publishing in the design, and the functionality applicable to electronic publishing is equally important for online publishing because there is no principle difference between electronic publishing and web publishing.

The support of PDF for electronic publishing is manifested in many aspects. The main contents are selected below.

1. Compression technology is essential for electronic publishing and web publishing

PDF uses industry-standard compression algorithms such as JPEG, LZW, CCITTGroup3, CCITTGroup4, Flate, and run-length encoding, and can even compress embedded audio data. When users use Distiller to generate PDF files, they can specify different compression methods for different objects on the page. This ensures that PDF files are always lightweight, flexible, and portable. PDF's reduced file storage requirements reflect its suitability for burning on optical disks and for transmission over the Internet.

2. Hypertext link function

PDF supports hypertext and hypermedia links, and an object on the page can be linked to another object as needed. We know that hypertext links are one of the basic features of multimedia electronic publications and therefore are the main indicators for measuring whether PDF has electronic publishing capabilities. Hypertext links or hypermedia links ensure that PDF files can be read in a non-linear way when they are displayed on a computer monitor screen. Users can access related content simply by clicking on text or other objects marked with hypertext links or hyperlinks. We can return to the place at any time.

3. Object description has nothing to do with the device

Each PDF file is described in a device-independent way. In this way, each PDF file can be displayed on different computer platforms (such as personal computers and workstations) and different computer operating systems (such as Windows, Mac QS, and UNIX operating systems), and the display results have a consistent appearance. Electronic publishing is a fundamental requirement.

4. Bookmarks and annotations

You can bookmark, annotate, and annotate PDF files, which is convenient for users who read PDFs on a computer screen, meaning that readers can stop reading at any time, and they can return in time if needed.