Digital Printing First Step: Scan Separation (3)

4. Several digital printing common file formats

There are many file formats for image storage, but only a few are widely used as practical standards. In digital prepress technology, there are three commonly used data formats: TIFF, EPS, and JPEG. The EPS and TIFF formats are the two basic formats that desktop publishers are most interested in; while the JPEG format affects most of the time in the Internet or multimedia. People working on it; using other formats such as: PICT, GIF, BMP, WMF, etc., are usually converted to the above-mentioned three commonly used file formats before use.

4.1 TIFF file format

TIFF is an acronym for Tagged Image File Format. This file format was developed by Aldus and Microsoft for scanner and desktop computer publishing software and is used to store black and white images, grayscale images, and color images. The defined storage format has now become an important file format for publishing multimedia CD-ROMs. Although the history of the TIFF format is longer than other file formats, it is still the most widely used industry-standard bitmap file format. This is mainly due to the fact that the specifications of the TIFF format have been repeatedly improved. The TIFF bitmap can have any size and resolution. In theory it can have infinite depth, ie 1-8 bits, 24 bits, 32 bits (CMYK mode) or 48 bits (RGB mode) per sample point. The TIFF format can encode grayscale, CMYK mode, index color mode, or RGB mode. It can be saved in compressed and uncompressed formats. Almost any application that involves bitmaps can handle TIFF file formats—whether it's placing, printing, trimming, or editing bitmaps.

The TIFF specification allows the use of two color modes, CMYK and RGB, to divide an image into four overprint colors and save the image before separation to TIFF format. When you place a TIFF format file in a page layout or similar program, no further color separation is required. When the cyan printing plate is printed, the program simply pulls the cyan channel; when printing the red printing plate, it simply pulls the magenta channel; and so on. TIFF format can also save the index color bitmap, but few people do it. For indexed color images, the GIF format is more often used.

The TIFF format can contain both compressed and uncompressed pixel data. The compression method (LZW) is non-destructive (the data of the image is not reduced, ie, the information is not lost during processing), can produce a compression ratio of about 2:1, and the original document can be reduced to about half.

The current version of the TIFF format supports high-resolution color, which divides different parts of an image into blocks, or chunks of data. For each block, there is a flag that provides information about what the block looks like. The advantage of the block is that software packages that support the TIFF format only need to save the portion of the image that is currently displayed on the screen. Parts of the image that are not displayed on the screen are also saved on the hard disk, and are not loaded into memory until they are needed. This feature is important when editing a very large high resolution image.

In TIFF files, no tools contain screen processing instructions. Screen processing is controlled by a program that prints TIFF format files. If you want to save the screen processing instructions while saving the bitmap, you must use the EPS file format. However, the TIFF format handles the clip path. Both QuarkXPress and PageMaker can read the clip path and correctly subtract the background.

4.2 EPS file format

Encapsulated Postscript format. The Postscript language is a page description language designed by Adobe for printing files to any printer that supports the Postscript language. It is like Basic, C, or any other programming language except that it is optimized for printing text and images on paper. When you work on a Postscript printer and tell the word processor (or any other application) to print a page, the computer writes a program describing the page in Postscript and sends the program to the printer. The printer actually contains a full-featured computer and a Postscript language interpreter to execute this program, drawing graphics on virtual paper in memory and printing it on paper.

An EPS file is a Postscript file that includes file header information. Using file header information, other applications can embed this file into a document. The EPS file has some restrictions, and these restrictions do not apply to standard Postscript files. These restrictions are mainly rules to ensure that EPS files can be inserted into different files without damaging the file. For example: In Microsoft Word, you can embed an ESP file in a Word document in a Word. The most popular application of EPS files is to embed them in desktop publishing files, especially those created by PageMaker or QuarkXPress. Desktop Color Separation (DCS) was developed by Quark and used for Process color processing. The DCS image is an EPS grid or image and consists of five parts: a low-resolution screen preview, plus cyan, magenta, yellow, and black layers. The DCS version 2.0 document can include more than 4 types of overprint colors, and can also include a number of spot colors or high fidelity color separations.

The EPS file format can be used for encoding pixel images, text, and vector graphics. If the EPS is used only for pixel images (for example, Adobe Photoshop is selected as the output), the trapping information and the tone copy transfer curve may remain in the file, while TIFF does not allow such information to be included in the image file.

Since the EPS file is actually a collection of Postscript language code, it can be printed in a variety of ways on Postscript printers. Software that creates or edits EPS files can define capacity, resolution, fonts, and other formatting and printing information. This information is embedded in the EPS file and then read and processed by the printer. There are hundreds of printers that support the Postscript language, including all image typesetting systems used in the desktop publishing industry. Therefore, the EPS format is a file format used by the professional publishing and printing industry.

The EPS format is a format for printing. The Postscript language code embedded in the EPS file provides an important print definition, but this makes the file size larger. In addition, the value and memory overhead required to set up a Postscript engine in software is also high. As a result, most web browsers do not support EPS files, and most image viewing shareware and free software also do not support EPS files. For this reason, the EPS format cannot be used on the Web site's image display.

4.3 PS file

PS is an acronym for Postscript, a flexible and device-independent page description language that has become a standard printing technology for producing high-quality output. Since Adobe developed Postscript in 1985, after continuous improvement and improvement, PS2 and PS3 have been introduced, solving many problems such as double-byte and color pre-press processing, becoming an indispensable type in today's printing and publishing industry. Universal programming language can be output on any PS function print output device. In recent years, Adobe has made many improvements on the PS3, including smooth gradation nuances; improved color gradation quality; PS3 can be used to identify the gradient generated in the PS2 application software, and automatically use the PS3 has achieved better quality; Font technology can handle double-byte fonts such as Chinese characters; expanding color separation to more than 4 spaces can not only distinguish four colors of C, M, Y, and K, but also separate other spot colors.

4.4 JPEG file format

The JPEG file format - known as the "Joint Photographic Experts Group" standard for experts who want to obtain extremely dark image effects. Now, it has risen to the main format of compressed files published by print and Internet.

Images saved in the JPEG file format are actually a mixture of 2 different formats: the JPEG format specification itself, which defines the compression method for the image, and is packaged in the image data format that defines the resolution and color mode. Photoshop and other applications that can actually read and write JPEG file formats store image data in the JFIF file format (JPEG File Interchange Format) or other formats that are very similar to the JFIF format. The JFIF file format is just a simple way to compress an image grid or a surround JPEG without any more.

The original JFIF file format specification history allows 8-bit grayscale images and 24-bit RGB images; however, Adobe has modified this format to also handle 32-bit CMYK mode data. However, most layout applications cannot actually separate the CMYK-mode JPEG images, so Adobe's modification does not make much sense. The JPEG file format allows 8-bit, 24-bit, and 32-bit depth images to be stored using variable compression. For example, when saving a Photoshop image in JPEG format, Photoshop offers several save options: low compression, medium compression, high compression, and best resolution. Experiments have shown that when printing or viewing on a monitor, JPEG can generally compress the image to one-tenth of its original size without noticeable differences. The image is broken down into small squares of 8 x 8 pixel image elements. This type of JPEG distortion is sometimes found in news images, which are greatly compressed before being electronically transmitted and then printed at high magnification.

JPEG uses a lossy compression format, which makes it an ideal format for quickly displaying images and saving better resolution. It is also because the JPEG format can greatly compress scanning or natural images, which facilitates storage or transmission via a modem, so it has been widely used on the Internet.

The JPEG format has a special variant called "Progressive JPEG". When creating a Progressive JPEG file, the data is arranged such that when the image is loaded, only a blurred image is initially displayed, and as the data is loaded, the image gradually becomes clear.

The main disadvantage of the JPEG format is also its biggest advantage. That is, the lossy compression algorithm limits the JPEG to the display format, and loses some data each time the JPEG format image is saved. Therefore, it is usually only possible to save the image once in JPEG format at the final stage of creation.

4.5 PDF file format

PDF is the abbreviation of Portable Document Format, developed by Adobe Corporation. It can encapsulate characters, fonts, formats, colors, and independent graphics and images of devices and resolution into a folder. The file format can also contain electronic information such as hypertext links, sounds and motion pictures. Supports specialty files, with high integration and security. Because PDF files can rely on the language and fonts of the operating system and the display device, the original appearance of the file can be vividly presented to each reader. PDF has become a de facto standard for electronic document distribution and digital information dissemination. This is because: From the very beginning, PDF has outstanding use advantages. Its files can be used on Macs, PCs and UNIX computers. Since PDF can be used in conjunction with a color management system to solve production problems, it is also accepted by production experts. The PDF can be directly displayed so that the preview and proofreading can be done quickly and safely on the screen. For this reason, PDF filters out a large amount of data that is insignificant to the final product, making the data extremely compact. Therefore, PDF file data transmission is quite satisfactory.

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 like its name, while "distilling" important Postscript files, but also removes unnecessary instructions from Postscript files. 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.