Prepress is the term used in the printing and publishing industries for the processes and procedures that occur between the procurement of a written manuscript and original artwork, and the manufacture of a printing plate, image carrier, or (traditionally) forme, ready for mounting on a printing press.
In today's prepress shop, the form of delivery from the customer is usually electronic, either a PDF or application files created from such programs as InDesign or QuarkXPress.
The following items have each been considered part of prepress at one time or another:
- Typesetting involves the presentation of textual material in graphic form on paper or some other medium. Before the advent of desktop publishing, typesetting of printed material was produced in print shops by compositors or typesetters working by hand, and later with machines.
- Copyediting (also copy-editing) is the work that an editor does to improve the formatting, style, and accuracy of a manuscript. Copy (as a noun) refers to written or typewritten text for typesetting, printing, or publication. Copy-editing is done prior to the work of proofreaders, who handle documents before final publication.
- Markup A markup language is an artificial language using a set of annotations to text that give instructions regarding the structure of text or how it is to be displayed. Markup languages have been in use for centuries, and in recent years have also been used in computer typesetting and word-processing systems.
- Proofreading traditionally means reading a proof copy of a text in order to detect and correct any errors. Modern proofreading often requires reading copy at earlier stages as well.
- Page layout is the part of graphic design that handles the arrangement and style treatment of elements (content) on a page.
- Screening of continuous-tone images such as photographs
- Page assembly or stripping
- Imposition, or the combination of many pages into a single signature form
- Trapping, also known as spreading and choking
- Separation, or specifying images or text to be put on plates applying individual printing media (inks, varnishes, etc.) to a common print.
- Platemaking, the photomechanical exposure and processing of light-sensitive emulsion on a printing plate.
In most modern publishing environments, the tasks related to content generation and refinement are carried out separately from other prepress tasks, and are commonly characterized as part of graphic design. Some companies combine the roles of graphic design and prepress production into desktop publishing.
The set of procedures used in any particular prepress environment is known as a workflow. Workflows vary, depending on the printing process (e.g., letterpress, offset, digital printing, screen printing), the final product (books, newspapers, product packaging), and the implementation of specific prepress technologies. For example, it is not uncommon to use a computer and imagesetter to generate film which is then stripped and used to expose the plate in a vacuum frame; this workflow is hybrid because separation and halftoning are carried out via digital processes while the exposure of the plate is an analog one.
In a common digital prepress workflow, a collection of computer files provided by clients will be translated from an application-specific format such as Adobe InDesign or QuarkXPress to a format that the raster image processor (RIP) can interpret. But before this rasterization occurs, workers in the prepress department confirm the incoming materials to make sure they are ready to be sent to the RIP. This is an important step because it prevents production delays caused by missing materials or improperly prepared materials. Once the incoming materials have passed the pre-flight check, they are ready to be put into production and sent to the RIP.
These intermediate formats are commonly called page description languages (two common PDLs are Adobe PostScript and Hewlett-Packard PCL). The RIP prepares the final raster image that will be printed directly (as in desktop inkjet or laser printing), set to photographic film or paper (using an imagesetter), or transferred direct-to-plate.
Depending on the hardware and software components and configurations, RIPs have unique problems rasterizing the image data contained in a PDL file. If there is a failure in rasterizing the image, it can be costly, as imagesetters, direct-to-plate systems, and high-end inkjet printers can consume expensive supplies, can require extensive amounts of time to process complex image data, and require skilled labor to operate.
The pre-flight process
The process of pre-flighting a print job helps reduce the likelihood of rasterization problems that cause production delays. Page layout software applications, (which allow users to combine images, graphics, and text from a variety of formats,) automate portions of the pre-flight process. Typically, client provided materials are verified by a pre-flight operator for completeness and to confirm the incoming materials meet the production requirements. The pre-flight process checks for:
- images and graphics embedded by the client have been provided and are available to the application
- fonts are accessible to the system
- fonts are not corrupt
- fonts are in a compatible format
- image files are of formats that the application can process
- image files are of the correct color format (some RIPs have problems processing RGB images, for example)
- image files are of the correct resolution
- required color profiles are included
- image files are not corrupt
- confirm that the page layout document size, margins, bleeds, marks and page information all fit within the constraints of the output device and match the client specifications
- confirm that the correct colour separations or ink plates are being output
Other, more advanced pre-flight steps might also include:
- removing non-printing data, such as non-printing objects, hidden objects, objects outside the printable area and objects on layers below
- flattening transparent objects into a single opaque object
- converting fonts to paths
- gathering embedded image and graphic files to one location accessible to the system
- compressing files into an archive format
The specifics of what checks are made is governed by the features of the pre-flight application, the formats of the client provided files, and the targeted output device as well as the printing specifications.
A purpose-built software application is not required to pre-flight a file, although several commercial applications are available. Small shops may use an inexpensive laser printer to test whether or not their file will print. The conversion to Portable Document Format (PDF) can reveal problems, and can in some circumstances be considered a pre-flight process.