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Differences between Prepress and View-on-Screen Image Files

By "prepress" we usually mean the preparation of document files that are meant to be printed on paper, or some other substrate using a printing press or large format printing device. These files can be contrasted with image files meant only for viewing on a computer monitor or TV screen. These "view-on-screen" images are not meant to be printed onto a physical substrate such as paper or vinyl. Or if they are reproduced, it is usually accepted that reproduction quality is not an important consideration.

Typically prepress applications require image files that contain a lot of detailed data because the images have to be colour-separated into the constituent "primary" colours and then imaged to printing plates. Or in the case of direct printing, the files are imaged directly to paper, vinyl, or other media. This leads to the two most important differences between prepress files and files used for view-on-screen — images used in web pages, for instance.

Prepress files are larger for number of reasons

The first difference, which has already been noted, is a technical one. Prepress files normally contain more detailed data, and are therefore larger in file size. There are several reasons for this. First, as already mentioned, greater detail is required because the process requires higher resolution.

What does it mean to say "higher resolution" is required? This is a term that applies to bitmap files where images consist of rows of dots of different colours. The higher the resolution, the more rows of dots you have making up the image. This is particularly important when dealing with images that contain fine detail, thin lines or small text. We are prepared to accept relatively poor reproduction of these elements on computer screens. But we are not normally prepared to accept it when printing to paper (although there are exceptions to this).

The second reason for prepress files being larger than view-on-screen files is more practical than technical. Because prepress image files are normally worked on locally, they are not subject to the same bandwidth restrictions that apply to web images. The result is that graphic designers usually err on the side of caution by providing files with more than enough resolution to give the reproduction quality required by their clients.

This too is changing. As the line between prepress and web design gets blurred beyond recognition, designers have brought techniques that work in web design over to prepress. For instance .jpg files — a compressed format that actually loses some detail during compression — are now often used for high resolution reproduction, where just five or six years ago only .tif or .eps files would be used.

The use of the .pdf format is another example of this convergence. This format was initially created for "cross-platform" web viewing of documents not originally created in web-friendly applications (such as Wordperfect, Word, or PowerPoint). About 10 years ago it was also adopted as a cross-platform, application-independent format for prepress documents. In the process of creating a .pdf document from, for instance, Quarkxpress, the designer makes some critical resolution decisions. The .pdf-writer than takes the original high resolution images and embeds a version in the final .pdf file that it calculates has sufficiently high resolution to achieve the desired result. In other words, it compresses the image files more than a designer would normally compress them. This process is obviously driven by the necessity to send documents over the internet to outside specialty production houses.

Prepress uses CMYK mode, images-on-screen use RGB mode

The other major difference between these two types of files is that prepress files are normally saved in CMYK mode, whereas files meant to be viewed on a computer or TV screen are saved in RGB mode. The differences between CMYK and RGB are complex, but the reasons one is required over the other are quite simple. It has everything to do with the output device.

Computer monitors and TV screens are RGB devices which create colours by combining emissions of light of the three constituent colours: Red, Green, and Blue — RGB. Devices that print digital images onto paper or other media such as sign boards, vinyl, canvas, fabric, and so on, create colours by combining inks of the four CMYK colours: Cyan, Magenta, Yellow, and Black (K).

A technical explanation of these two processes is well beyond the scope of this article. For practical purposes the most important issue is the difference in reproduction "gamut" of these two types of devices. Printing devices that use CMYK inks cannot reproduce some of the colours you see on your computer screen — the colours out on the margins where the primary RGB colours intersect: especially vibrant oranges, purples and blue/greens. There is simply no way to get these colours from a combination of the standard CMYK primaries. So viewing RGB images containing these colours — of a client's corporate logo, for instance — will be misleading if you are trying to reproduce them on paper. You (and the client) will be disappointed because these colours simply cannot be reproduced.

Traditionally, printers and graphic designers have skirted this issue by using "spot" colours to reproduce especially vibrant hues or particular colours that cannot be easily and consistently matched with CMYK combinations. The addition of gold or silver are perhaps the best examples. The printer runs the job in CMYK, leaving the gold spots blank. Then he runs the job through the press again and adds the fifth (or sixth, or seventh) colour(s).

A number of years ago alternatives to the CMYK model were developed that included fifth or sixth colours to extend the CMYK gamut. The most commonly used is Hexachrome, which uses six colours instead of four. One website, for instance, describes hexachrome this way:

This enhanced six-color process uses a different cyan, magenta, yellow and black than the standard four-color process and adds green and orange to create a vibrant palette. Hexachrome allows you to work completely in the RGB color gamut allowing exceptional reproduction of photographs and graphics. http://www.bopi.com/hexachrome.htm

For various practical and technical reasons, neither the hexachrome model, nor any other alternative to CMYK have been adopted on a widespread basis. The CMYK model has been with us since long before the digital revolution, and it is likely to be kicking around for many more years to come.

Here is another explanation of CMYK vs RBG.

Rick Hendershot has been involved in the graphic arts, prepress, and web design businesses for many years. He is founding publisher of the Linknet Creative Resource Library.

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