To begin with, there are two basic image formats used in print: vector and raster (also called bitmap). The vector format is used for graphics and text, and is composed of lines and fills. Each element that makes up a vector image is made up of text instructions written in the PostScript language. PostScript allows us to start with a point on a page and describe a line or curve to another point. When we have at least two points, we can fill the object or stroke the line with a color. Since text is used to describe points and fills, we can easily scale a vector graphic to any size.
Raster files are made up of pixels of color. Like dots of ink on a printed page, when viewed from a distance, these pixels (or picture elements) appear to us as a continuous tone image. If we scale a raster image up in size, we start to see the individual pixels and the illusion of continuous tone is spoiled. When we begin to see the individual pixels, we refer to the stepping as “jaggies”. The resolution of the image is based on the number of pixels per inch (“ppi” also referred to as “dpi”,) so raster are larger in size than vector graphics.
In electronic pre-press all pages are made up of vector and raster graphics – and our file formats are combinations of these graphics. Photoshop is the most frequently used program to edit raster graphics (although it can import vector graphics.) Vector graphics are generally created within programs like Adobe Illustrator, Macromedia Freehand and CorelDraw. Fonts and typefaces are usually created with vector graphics; they can be scaled and colored and remain crisp.
Two of the popular raster formats that came out of the early desktop publishing days are TIFF (tagged image file format) and EPS (Encapsulated PostScript.) The TIFF format was created by Aldus (makers of PageMaker) in 1987 and reached its final format in 1992 as TIFF 6. A TIFF file can be created in many color formats; line art (black and white), grayscale, RGB, CMYK, as well as other formats. The line art and grayscale formats can be colorized which is useful in many graphic effects.
The Encapsulated PostScript format (EPS) can describe the entire page – because it is actually a PostScript program, the original page description language pioneered by Adobe. An EPS by design can contain any combination of text, graphics and images. As PostScript, the EPS format (sometimes called EPSF) is the most versatile file format. EPS files also contain a 72 dpi preview file, so that they don’t require a PostScript interpreter to preview the content. To get the best quality out of an EPS, we need to have a PostScript interpreter (also called a “raster image processor” – RIP) to transform the “program” into dots on film or paper.
In order to come up with a reliable format for exchanging digital advertisements and pages the TIFF/IT was finalized in 1996. TIFF/IT based on the TIFF format contains only raster data. TIFF/IT P1 was developed specifically for use on CMYK jobs. TIFF/IT P1 is what people are actually talking about when they mention TIFF/IT. The fact that a TIFF/IT P1 file doesn’t contain vector data implies that it is fixed. Like the “final film,” we used until recently, it is always going to be consistent as it is distributed. The TIFF/IT P2 format will support additional color formats, but at the rate that PDF is being adopted it may already be too late.
Out of PostScript, Adobe came up with a file format that could be independent of computer platform, or even output devices. This desire evolved into their “portable document format”. Adobe’s PDF format can describe all of the information on a printed page – but with some limitations. A PDF file can be created for use in a number of applications. This can introduce a problem for use in pre-press because the images may not have enough resolution. PDFs created for email are extremely compressed and down-sampled so that they are unusable on press. PDFs that are created for the press can be quite large. Images in a PDF are not compressed for press application.
PDFs can also support spot colors and can contain other elements that will create problems in press application (such as movies, annotations). Normally, PDFs are created by sending PostScript to Adobe’s Distiller using “Press Optimized” settings. QuarkXPress can export to PDF using the Jaws PDFcreator. On Mac OS X Apple has added their own technology called Quartz, which allows Mac users to create PDFs from any file. There is also a version of PDF used by high-end workflows called Extreme. Adobe Extreme is used by Apogee and Prinergy, to PDFs for Press. Other than using Extreme, using Distiller for press-ready PDFs or exporting from Adobe’s InDesign is your best bet.
In order to simplify the PDF format for press a subset was created – PDF/X format. Since there are two many variables in a PDF format, CGATS (Committee for Graphic Arts Technology Standards) began working on PDF/X-1 in 1999 so that PDF would be more consistent and predictable in press applications.
PDF/X-1 is designed for CMYK workflows and can be created directly in Adobe’s Distiller 6 as well. PDF/X-1 files will have all its resources embedded, so there won’t be missing images or fonts. PDF/X-1a (released in 2001) is ISO certified and adds better support for named spot colors. The file format can/will ignore music, movies and non-printable annotations. PDF/X-1 will only support certain raster formats; TIFF/IT, DCS 1 and 2 and EPS. PDF/X1 also indicates whether the PDF has been trapped or not.
Another item that came out of Adobe’s PDF workflow is the latest term JDF. While developing a strategy for PDF workflows in their OEM products, Adobe came up with the portable job ticket format or PJTF to compliment the PDF format. The PJTF file contained information of how to handle a job that contained multiple PDFs.
The PJTF was relatively limited so it has evolved into the JDF or job description format. JDF is starting to appear in a lot of articles and brochures because it reaches into pre-press, press and post-press systems. In layman’s terms, it is an electronic docket bag. The docket bag contained instructions for several departments and could contain all of the elements that make up a job. It used to contain art boards, transparencies and galley type. Now dockets contain a CD-ROM or DVD.
The JDF is a electronic file that contains information and instructions for pre-press, press, bindery, MIS and even accounting. JDF is the final piece attempting to tie all the elements together.