POSTSCRIPT has a large selection of graphics operators that allow it to precisely describe a desired page. These operators control the placement of three types of graphics objects:

• Text in a wide variety of typefaces can be placed on a page in any position, orientation, and scale.

• Geometric figures can be constructed using POSTSCRIPT graphics operators. These describe the locations of straight lines and curves of any size, orientation, and width, as well as filled spaces of any size, shape, and color.

• Sampled Images of digitized photographs, free-hand sketches, or any other image may be placed on a page in any scale or orientation.

All graphic objects may be easily rotated, scaled, and clipped to a specified portion of the output page.

POSTSCRIPT Imaging Model

An imaging model is the set of rules that are incorporated into the design of a graphics system. The POSTSCRIPT imaging model is very similar to the model we instinctively adopt when we draw by hand.

The POSTSCRIPT model considers an image to be built up by placing ink on a page in selected areas. The ink may form letters, lines, filled shapes, or halftone representations of photographs. The ink itself may be black, white, colored, or any shade of gray. These elements may be cropped to a boundary of any shape as they are placed on the page. Once the page has been built up to the desired form, it may be printed on an output device.

Three concepts are central to the implementation of the POSTSCRIPT imaging model:

• Current Page: The current page is the "ideal page" on which POSTSCRIPT draws. It is independent of the capabilities of the printer being used.

  When a program begins, the current page is completely empty. POSTSCRIPT painting operators place marks on the current page, each of which completely obscures marks that they may overlay. Once the current page is completely described, it is sent to the printer, which reproduces the page as well as it can.

  It is important to remember that no matter what color a mark has--white, gray, black, or color--it is put onto the current page as if it were applied with opaque paint.

• Current Path: The current path is a set of connected and dis- connected points, lines, and curves that together describe shapes and their positions. There is no restriction to the shapes that may be defined by the current path; they may be convex or concave, even self-intersecting. The elements of the current path are specified in terms of their positions on the current page. The resolution of the printer in use in no way constrains the definition of the path.

  The current path is not itself a mark on the current page. POSTSCRIPT path operators define the current path, but do not mark the page. Once a path has been defined, it can be stroked onto the current page (resulting in a line drawn along the path), filled (yielding solid regions of ink), or used as a clipping boundary.

• Clipping Path: The current clipping path is the boundary of the area that may be drawn upon. Initially, the clipping path matches the printer's default paper size. The clipping path may be changed to any size and shape desired. If an imaging operator tries to mark the current page outside of the current clipping path, only those parts of the mark that fall within the clipping path will actually be drawn onto the current page.

Coordinate Systems

Positions on a page are described as x and y pairs in a coordinate system imposed on the page.

Every output device has a built-in coordinate system by which it addresses points on a page. We call this built-in coordinate system, idiosyncratic to each device, device space. Device space varies widely from printer to printer; there is no uniformity in the placement of coordinate origins or in horizontal and vertical scaling.

Positions on the POSTSCRIPT current page are described in terms of a user coordinate system or user space. This coordinate system is independent of the printer's device space. Coordinates in a POSTSCRIPT program are automatically transformed from user space into the printer's device space before printing the current page. User space thus provides a coordinate system within which a page may be described without regard for the particular machine on which the page is to be printed.

The POSTSCRIPT user space can be altered in three ways. The coordinate system's origin may be translated, moved to any point in user space. The axes may be rotated to any orientation. The axes may be scaled to any degree desired; the scaling may be different in the x and y directions. A sophisticated user may specify any linear transformation from user space to device space. Thus, coordinates in a POSTSCRIPT program are changeable with respect to the current page, since they are described from within a coordinate system that may slide around, turn, shrink, or expand.

About one-third of the POSTSCRIPT language is devoted to graphics. The remainder makes up an entirely general computer programming language. The POSTSCRIPT language contains elements of many other programming languages, but most closely resembles the FORTH language.

POSTSCRIPT Stack

POSTSCRIPT reserves a piece of memory called a stack for the data with which it is working. The stack behaves like a stack of books. The last book placed on the stack is the first book that will later be removed. Similarly, numbers, strings, and other pieces of data placed on the stack will be removed in reverse order, the last item added to the stack being the first retrieved.

Postfix Notation

POSTSCRIPT operators that require numbers or other data, such as add and sub, retrieve that data from the stack. To use an operator, one must first place the data it requires, its operands, on the stack, and then call the operator. The operator will place its own results on the stack. This style of programming, in which the operands are specified before the operator, is referred to as postfix notation.

POSTSCRIPT Data Types

POSTSCRIPT supports many data types common to other languages, including reals, booleans, arrays, and strings. The POSTSCRIPT language also defines object types such as dictionary and mark. For descriptions of all the POSTSCRIPT data and object types, refer to the POSTSCRIPT Language Reference Manual.

POSTSCRIPT Flexibility

POSTSCRIPT is an extremely flexible language. Functions that do not exist, but which would be useful for an application, can be defined and then used like other POSTSCRIPT operators. Thus, POSTSCRIPT is not a fixed tool within whose limits an application must be written, but is an environment that can be changed to match the task at hand. Pieces of one page description can be used to compose other, more complicated pages. Such pieces can be used in their original form or translated, rotated, and scaled to form a myriad of new composite pages.

Printable Programs

POSTSCRIPT programs are written entirely in printable ASCII characters. This allows them to be handled as ordinary text files by the vast majority of communication and computer file systems. In addition, it ensures that a POSTSCRIPT program will be as easy for a person to read as the structure of the program allows.