Comparison of flexibility and reliability of prepress data formats

In the prepress digital workflow, many companies and organizations have been devoted to the development of reliable file formats for the seamless exchange of page data between devices and software. The main file formats used for prepress data exchange are: Application file format, PostScript, EPS, PDF, PDF / X, TIFF / IT, Copydot, etc., the file format is different, its flexibility and reliability are different.

1 Application file format

Application file format refers to the file format of application software such as desktop design, typesetting, and word processing, such as .PSD format, .CDR format, .PM65 format, .DOC format, etc. The application file format is very flexible. Moreover, the files created by the desktop application only emphasize the graphic content of the design and creation, and will not contain all the elements required for printed output. Some lack the ICC feature description file and color definition space. Some files use the image. Links instead of direct embedding, some do not determine the trap data, and some files cannot use the fonts etc. The reliability is very low.

2 PostScript format

Due to the great flexibility of the application file format, it cannot be applied to the prepress workflow. Adobe has developed the page description program language PostScript language, which can encode all elements (text, graphics, images) in a file. When outputting, the interpreter (RIP) interprets the encoding into a language that the output device can recognize, thereby outputting a complete page.

PostScript files cannot be edited after they are generated. If there is a problem after RIP, you must return to the original software used to modify it. This greatly enhances the reliability of the file and reduces the error rate of the file, but also reduces the flexibility of the file.

The PostScript format is versatile and can be output on any output device with PostScript function. It is widely used in the pre-press output field and is dominant.

3 EPS format

The EPS format is a packaged PostScript file format, which is a professional file version of the page description language PostScript. Use PostScript language to describe the graphics, images, and text elements on the page, and then encapsulate all the elements together to form an EPS file. The structure of the EPS file is very similar to the PS file. In addition to the main body of PostScript, you can also choose a low-resolution preview image.

The EPS format is widely used in the prepress workflow, and almost all prepress applications can generate or call EPS files. EPS files can be nested and used in PostScript files.

When the EPS file is imported into other pages, all the objects form a whole, and the content of the elements cannot be edited separately. Only the whole can be zoomed, rotated, and cut.

4 PDF format

PDF (Portable Document Format) is also a data format for describing files. It is based on PostScript and uses a method similar to PostScript to describe pages. But PDF is a file structure, which has advantages that the PostScript format does not.

PostScript files do not have page independence and cannot process a single page in the file, while PDF files have page independence and are interactive; PDF also has software and hard platform independence in different environments (different hardware Devices, different operating systems) The format and content of the PDF file seen are the same; the most important thing is that the PDF format solves the font problem in the prepress workflow. The PDF file can be embedded with the font used, such a file The output on any device can ensure the consistency of the font. It can be seen from these advantages that in the pre-press data exchange, the PDF format is more reliable than the PostScript format.

Using Adobe Acrobat, you can make simple changes to the contents of the PDF file. When large modification operations are required, it is best to convert the PDF file to the source application file and modify it in the source application.

The reliability of the PDF format makes it widely used in pre-press data exchange and becomes the main file format in digital workflows. [next]

5 PDF / X format

Although the PDF format has many advantages, in the data exchange, the flexibility of the PDF file format is too high, and there are many parameter settings, which are prone to errors. In order to further improve the reliability of pre-press data exchange, pre-press equipment manufacturers and software developers Businesses and related organizations have formulated a unified PDF / X standard to limit the flexibility of the PDF format and regulate the PDF file format.

PDF / X is a subset of PDF, and its purpose is to make pre-press data exchange more reliable. The PDF / X standard series released by ISO is divided into three categories: PDF / X-1a, PDF / X-2, and PDF / X-3.

1) PDF / X-1a
The PDF / X-1a standard only allows the complete exchange of CMYK data and spot color data, and cannot use RGB or Lab color modes. It is forbidden to use encryption and OPI objects to embed the fonts used. This standard is the most reliable and has the least flexibility.

2) PDF / X-3
PDF / X-3 is an extension of the PDF / X-1a standard. However, in addition to using CMYK and spot color data, it also allows the use of RGB color data and device-independent color data (such as CIE LAB), and can implement color management on these color data. The output target of the file may not be unique. The file is also embedded in the font used. The PDF / X-3 standard is more flexible than the PDF / X-1a standard.

3) PDF / X-2
PDF / X-2 is the most flexible format standard among PDF / X standards. It can be converted using multiple color data like PDF / X-3, and it also supports partial exchange and external objects like OPI. It allows the sending and receiving of PDF files to exchange information according to the conditions of the files. It is not required to embed the font used.

In the PDF workflow established by the PDF / X standard, data exchange becomes more reliable. On the one hand, the compatibility between hardware and software of different manufacturers based on standards has been improved; on the other hand, the versatility of PDF files based on standards has been enhanced, which greatly promoted the development of digital workflow.

6 TIFF / IT format

TIFF / IT files only contain bitmap data, no vector data, all text, graphics, and images are converted into high-resolution bitmap data, and then compressed and stored using appropriate methods. Since TIFF is a bitmap file format, there will be no font mismatch and text error codes. The image quality in TIFF format is high, suitable for the exchange of pre-press raster data files, and is a very stable file format.

7 Copydot format

Copydot means copying the dots on the existing film into a digital bitmap file, that is, digitizing the existing film and restoring it to an electronic file.

Among the pre-press data formats, the Copydot format is the data closest to the final form and the most reliable data format. Copydot format data is actually equivalent to digital film. The copied digital dots are consistent with each dot on the original film. Copydot format files can be directly used for output to film, printing plate, or direct printing, without the need for screen processing.

Although the PDF, PDF / X, and TIFF / IT formats can prevent garbled characters and image replacement errors, they cannot do anything about the problems of image screening. When the pre-press data in these formats is output at different locations, due to the difference in RIP characteristics of different manufacturers, the difference in RIP parameter settings, and the variability of RIP screening (the change of the shape and angle of the dot), the output will produce differences or errors . The Copydot format already includes outlet information, and the output will be completely consistent.

The Copydot format is the most reliable, but the flexibility is also the worst. In the Copydot file, the page cannot be changed in size or rotated and modified, and color management, overprint inspection, and color proofing cannot be performed; it is not controlled by ordinary RIP correction; the file is very large and data compression is required; The accuracy of scanning is very high in order to copy the film to the Internet.

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