Imposition

Imposition is a term used in the printing industry. Print operators will print books using large sheets of paper which will be folded later. This allows for faster printing, simplified binding, and lower production costs. Imposition is the process of arranging pages correctly prior to printing so that they fold in the correct order. To someone unfamiliar with the imposition process, the pages may seem to be arranged randomly; but after printing, the paper is folded, bound and trimmed. If correctly imposed, the pages should all appear in the correct orientation and readable sequence.

In the example above, a 16 page book is prepared for printing. There are eight pages on the front of the sheet, and the corresponding eight pages on the back. After printing, the paper will be folded in half vertically (page two falls on page three). Then it will be folded again horizontally (page four meets page five). A third fold completes this process (page nine meets page eight). The example below shows the final result prior to binding and trimming.

Non-digital Techniques

Imposition has been a requirement since the earliest days of printing. When pages were set using movable type, pages were assembled in a metal frame called a chase, and locked into place using wedges called quoins.

By the late twentieth century, most typesetting was onto photographic film. These sheets were combined manually on a light table, in a process called stripping. Skilled workers would spend many hours stripping pieces of film together in the correct sequence and orientation. The term stripping was also used for other changes to a prepared page, such as a spelling correction, or a "stop press" story in a newspaper. Digital techniques rendered stripping less necessary, but what has forced increasing numbers to abandon it completely is the introduction of "platesetters", which put pages directly onto printing plates; these plates cannot be adjusted with a sharp knife. In addition, an extremely high precision would be needed for stripping of colour work, as each ink colour is on a separate piece of film.

Digital Techniques

In recent years, the process of imposition has been automated by computers and is sometimes called digital stripping. Digital imposition was invented in 1988 by Ultimate Technographics Inc . An entire book may be imposed and many complex functions applied in an instant. Binding options may be changed on the fly and impositions produced to multiple output devices at once, often with no user-intervention at all. There are several different approaches to digital imposition.

• In the design application. If a software package can be used to design single pages, it can often be used to design entire printed sheets, sometimes by a process as simple as copy/paste onto a larger sheet. This is still in use, especially for low volumes of work, but a popular alternative is an imposition function built in, or added in, to the design tool. This would typically take a document prepared as single pages, and create a new document with much larger pages containing full sheet layouts. This larger layout could then be printed to film or plate, as normal.

• Post-design imposition might take a PostScript or PDF file in single pages, and produce a new PostScript or PDF file with sheet layouts, which could then be printed. A variation of this would take a large number of files as input, each containing a single page. This is especially suitable for a magazine or newspaper where pages may be worked on by different groups at the same time.

• Print driver imposition would add functions to a printer driver so that the application program printed single pages, but what was sent to the printer was full sheets. This is not often found in professional production, but is popular for such things as booklet printing on office laser printers. A variation of this offers the ability to print layouts as an option in the application.

• Imposition could be placed into the output device. This is sometimes called "in-RIP imposition". This allows regular pages to be printed by any suitable means, and the output device does the work of imposition. This has the advantage that the imposition can be specifically tuned for each different output device. However, it may for some have a corresponding disadvantage that there is no preview until the output is produced, which may be a costly printing plate that takes some time to produce, or even (with a digital press) finished copies; expensive mistakes are possible.

Where an imposition layout is viewed on screen, it may be referred to as a printer's spread. This is used to contrast with reader's spread, which shows a finished printed piece on screen as it will appear to the reader, rather than the printer; specifically, in a reader's spread for a typical book, pairs of facing pages will be shown side by side (e.g. pages 2 and 3 together).