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Since the 1980s, prepress technology has shifted from the past typing and image processing methods to DTP technology, which has greatly simplified the work and greatly reduced costs. Designers, advertisers and other customers can even use their own series of hardware and software to perform their own work. Prepress operations. However, there are many problems that often arise when using these devices and software that require the operator to solve them. This also allows the output center to exist, help customers solve problems in their production, and output films.
With the development of prepress technology, especially the appearance of large format photofinishing machines and CTP systems, the amount of data processed by the prepress system has become larger and larger, the process has become more and more complicated, and errors have become more and more likely to occur during work. . Therefore, the printing industry hopes to have a digital way to replace the film, which can reduce costs and reduce errors. At present, some companies are ready to purchase large-format image-setters or CTP systems. However, to make these devices function properly, there is no sufficient amount of data files, and managing large amounts of data files is a daunting task. In the past, film has always been the only medium between prepress and printing. Today, the PDF data format is the ideal solution. It can be generated by a variety of software and is cross-platform. It contains all the necessary information in the page. data.
Adobe's initial development of PDF was to enable files in the office to be passed between computers, and the appearance of the files would be consistent without having to install special software and fonts. The original version of PDF was used for computer screen displays and laser printers, but as an image-setter and CTP, it is difficult to cope with numerous complicated information such as color separation.
Due to the pressing demand of the printing industry, Adobe has also introduced the format of PDF 1.3 at the same time as the introduction of Acrobat 4.0, making PDF include all the information needed for prepress production and becoming an increasingly popular file format.
PDF features
Adobe provides a series of PDF generation software that allows users to generate PDF files in a number of ways.
It is also possible to convert Postscript files to PDF files. The most reliable method is to use Adobe's Distiller, which can not only generate high-quality PDFs from Postscript files, but also generate PDF files for various purposes.
Once a PDF file is created, it does not need to be read by using the hardware and software that created it, but can be read using a variety of work platforms and a variety of software. The correct PDF file should include all data such as for display, for printer output, and for imagesetter and CTP output.
Each PDF page is independent, so a PDF file can easily separate a file into separate pages. This feature is very important for spelling. The features of the PDF also make it possible to make last minute changes before exporting.
In most cases, the output speed of PDF files is much faster than the output speed of Postscript files. The distiller functions as its name, while "distilling" the important data of the Postscript file, and also deletes unnecessary instructions in the Postscript file. Therefore, the Postscript file output from Acrobat is smaller than the original Postscript file, and the RIP is faster because the PDF has been pre-interpreted.
All data in PDF files, such as color images, continuous tone images, monochrome images, and text and vector images, can be compressed using different compression methods. This also means that PDF files are usually much more than the original layout files and image files. smaller.
Due to the application of PDF, there is a possibility of standardization of file transfer. The PDF data format is evolving into a page data exchange standard for print production. PDF is the basis for using modern workflows and automatic output devices. The American Standardization Committee and the Printing Technology Standardization Committee have already recommended PDF as a national standard for the printing industry. The International Organization for Standardization is also developing an international standard based on the PDF file format.
Postscript and PDF
The page description language Postscript was introduced in the 1980s. The purpose was to help the personal computer at that time output complex pages containing text, images, etc. to laser printers and imagesetters. Due to the extremely weak processing power of personal computers. So as much as possible to be processed work and functions designed to RIP, driven by the RIP output device.
Postscript is a mature programming language. Each Postscript file is a program that is interpreted by RIP. Of course, there will be errors in this process, and it is also difficult to predict the speed of interpretation of RIP. In addition, Postscript files often contain device-specific commands, and not every output device can read these commands. Postscript is therefore not well-suited for fast, reliable page display on a computer screen, and some output devices often fail to output Postscript files.
PDF is a pure data format that contains only the page information required for output. Before the output, it is no longer necessary to perform calculations or execute other programs, so it is more reliable than the output of the Postscript file.
The biggest difference between Postscript and PDF is:
Postscript files must be processed continuously from the beginning to the end. It is very difficult to extract a separate page from a Postscript file because it is difficult to know if the characteristics of the previous page definition are still needed in the next page. This is also an important issue faced by the software, which is how to arrange separate pages in Postscript files in different layouts. The PDF data format simplifies this work to a considerable extent.
A PDF style can be thought of as a database where the user can read all the elements directly on a document page. Individual pages can be extracted from a PDF file and imported into another PDF file without any problems. Individual pages in a PDF file can be moved, copied or deleted, and various elements on the page can also be edited.
Postscript and PDF are based on the same image mode, so the conversion between them is very easy.
The processing of the Postscript file is to combine the interpretation, description and screening of the file once and then image it on the output device. The processing of Postscript 3 RIP and PDF files is to explain, describe and screen the job separately. Therefore, various tasks can be performed in parallel, which is conducive to the improvement of work efficiency.
Portable job card format
Unlike Postscript files, PDF files do not contain any instructions for device operation. Therefore, Adobe must develop a new way to store operating information that is not related to page content. To this end, Adobe has created a new data format, the Portable Job Card Format (PJTP), whose structure is very similar to PDF. PJTP stores hierarchical physical information and can be processed directly with the PDF.
The page content of the PDF can be split and added to the operating instructions in PJTP, which greatly increases the flexibility in production. If there is a change in the production process, such as a change in the type of paper, we need to open the original file in the original software, change the settings, and just correct it in the job card.
The following information can also be stored on the portable job card:
- Page processing instructions (folding format, leaking rules, etc.)
- Output parameters (network cable, screen angle, precision, etc.)
- Media tools (name, size, weight, color signal, etc.)
- Postpress (folding, cutting, binding, etc.)
- Delivery information (address, quantity, etc.)
- Schedules (such as deadlines, etc.)
- Management (eg customer name, customer order number, responsible person, etc.)
PJTP contains specific job information that can be embedded in a PDF file or saved as a separate file.
work process
If simply outputting a page is not enough, on the contrary, the page needs to go through various processes. The workflow includes the following processes:
- Preflight (full inspection of documents and finding out what is bad for the output)
- OPI function (replaces low-precision images with high-precision images)
- Output accuracy adjustment (reducing unnecessary high-resolution images to the required level)
- Optimize the image (remove unnecessary elements, trim unnecessary image edges, etc.)
- Trap (set leak parameters and rules)
Color conversion (color management, such as converting RGB to CMYK)
- Soft proofing (for checking the page on the screen)
- Proofing of individual pages (output to proofing equipment, inspection of images and text)
- Folding handprints (spelling several pages into a full page)
- Layout proofing (output to the printer, check the folding layout)
- Description (convert text and vector graphics to pixels)
- Screening (continuous tone image screening)
- Imaging (output to imagesetter or CTP)
- Archive (archive all data of the entire job and save the backup)
- Restore (Restore all data for the entire job from the backup media)
The above process basically summarizes the content contained in the prepress digital workflow and can be automated. Since the beginning of the year, many manufacturers have developed some workflow systems for this purpose, but most of them are closed systems and use their own proprietary data formats. Sometimes they are used.
Postscript format. Thanks to PDF and PJTP, a new generation of PDF-based workflow system has entered the market, which is also the user's long-awaited open prepress system.
Adobe developed a new workflow structure for this, known as Adobe Extreme. Some well-known manufacturers have developed a prepress workflow that includes their own proprietary technologies.
Adobe Extreme
This structure was originally developed for digital printing. Because Extreme can make the Renderer work in parallel at the same time, this technology just meets the requirement of digital printing to quickly print hundreds of pages in one minute. If you want to use Postscript, it is very difficult to achieve this requirement. This is because Postscript files must be processed in order, and PDF files can be divided into several files to be processed simultaneously on several descriptors. In Extreme, Postscript data is converted to PDF by the Normalizer.
Extreme is an open, modular and flexible workflow structure. It is compatible with both Postscript and PDF, and supports a wider range of workflow requirements. Some products on the market today have similar functions, but they cannot be called Extreme.
According to Adobe requirements, Extreme should meet 5 conditions:
1. Accepts input of Postscript and PDF files, but only PDF as the internal file format of the system.
2. Internally generated by the system using the Normalizer PDF file, you can view and edit the PDF file in order to facilitate the translation of the final version and the final modification.
3. Use Adobe's PJTF to control workflow parameters and data processing parameters.
4. Use Adobe's Postscript 3 RIP to describe the file and use Adobe's descriptor to manage the job card.
5. The Prinergy Workflow System co-developed by Heidelberg and Creo, Agfa's Apogee Workflow System, and Scitex's Brisque Workflow System all feature Extreme as its core product. These products are basically designed in accordance with the workflow mentioned above. The workflow before printing is also a very complicated process. Some people in foreign countries subdivide the content of the workflow into 46 departments.
PDF-based digital workflow
The emergence of PDF