|This article does not cite any sources. (October 2008)|
In printing, pre-flight is the process of confirming that the digital files required for the printing process are all present, valid, correctly formatted, and of the desired type. The term originates from the preflight checklists used by pilots. The term was first used in a presentation at the Color Connections conference in 1990 by consultant Chuck Weger.
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.
Files were originally printed as Adobe PostScript; however in modern workflows, PDF files have become the popular file type for submitting data to a RIP. The RIP generates 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 can experience problems rasterizing the image data contained in PostScript or PDF files. If there is a failure in rasterizing the image, it can be costly, as output systems (printers, plate-setters, etc.) consume expensive supplies, can require extensive amounts of time to process complex image data, and require skilled labor to operate.
The preflight process
The process of preflighting a file 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 preflight process. Typically, client provided materials are verified by a preflight 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 file 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 preflight 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 preflight 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 preflight a file, although several commercial applications are available. Many desktop publishing applications have some type of preflight capability, however they may not be as robust as commercial based applications. Many printers and publishers utilize high-end preflight and file optimization solutions versus relying those within desktop publishing apps. They will make sure the preflight settings match their specific production requirements.
Early preflight methods were largely manual, and typically relied on checklists that highly skilled prepress operators would use to verify the production readiness of each incoming job. As desktop publishing and graphics applications, PDLs, RIPs, and output devices evolved, the process became more complex. Software plug-ins and stand-alone applications that supported the major desktop publishing applications were then developed to meet that need, along with proprietary tools made by hardware manufacturers and commercial printers and service bureaus. The developers of the major applications then began to incorporate functionality in their applications, leveraging their knowledge of their own file formats.