Display PostScript

Display PostScript. It is a graphics system that extends the PostScript page description language into a comprehensive, device-independent protocol for rendering graphical user interfaces on computer displays. Developed by Adobe Systems, it integrates a complete imaging model with powerful programming capabilities, allowing applications to describe complex visual elements using the same language employed for high-quality printing. This unification of screen and print graphics aimed to provide a consistent, high-fidelity visual experience across different output devices, influencing several major operating systems and graphical environments.

Overview

The core innovation was its use of the full PostScript interpreter as the fundamental engine for all on-screen drawing, managed by a central display server. This architecture meant that application programs could send compact PostScript code to describe windows, menus, and other graphical primitives, which the server would then rasterize for the specific display hardware. This approach contrasted sharply with systems that used simpler, device-dependent drawing commands. Key proponents of the technology, including Steve Jobs at NeXT, championed its ability to enable WYSIWYG (What You See Is What You Get) publishing and sophisticated graphics. The system provided a standardized set of extensions for handling interactive elements like bitmap images, fonts, and color management, creating a powerful foundation for application development.

Technical architecture

At its heart, the system relies on a client-server model where the display server houses the PostScript interpreter and maintains the state of the entire screen. Clients communicate with this server via an inter-process communication protocol, sending programs written in the extended PostScript language. These programs can define and manipulate graphical objects, known as display lists, which the server retains for efficient redrawing. The imaging model includes advanced features such as coordinate system transformations, clipping paths, and compositing operations, all described with the same precision used for laser printer output. Security and resource management were handled through a protected environment within the server, preventing errant client code from crashing the entire graphical session. This design allowed for resolution independence and sophisticated typography, as the same font outlines could be used for both display and print.

History and development

The concept originated at Adobe Systems in the mid-1980s, with key figures like John Warnock and Charles Geschke seeking to bring the power of their PostScript language to the computer screen. A major catalyst for its development was the partnership with NeXT, founded by Steve Jobs, which adopted it as the exclusive graphics system for its NeXTSTEP operating environment, launched in 1989. This implementation demonstrated the system's capabilities for object-oriented programming and advanced user interfaces. Around the same time, Sun Microsystems licensed the technology for its NeWS windowing system, and Apple Inc. later integrated a version into its Classic Mac OS as part of the QuickDraw GX technology in the mid-1990s. Despite its technical merits, the system faced challenges due to its computational demands and licensing costs from Adobe.

Implementations and adoption

The most famous and complete implementation was within the NeXTSTEP (and its successor, OPENSTEP) operating system, where it was integral to the Objective-C application framework. Sun Microsystems created the NeWS window server, which competed with the X Window System by using a PostScript-based protocol. Apple Inc.'s incorporation into Mac OS was less pervasive, primarily appearing in the printing and graphics architecture of System 7. IBM also implemented a version for its AIX operating system. However, widespread adoption was limited by the rising dominance of the X Window System on Unix workstations and the less resource-intensive Win32 API on Microsoft Windows. The technology's legacy is most directly seen in macOS, which inherited the display model and PDF-based imaging from NeXTSTEP.

Comparison with other systems

Unlike the X Window System, which uses a simple protocol for drawing primitives and delegates complex rendering to clients, this system placed a full graphics interpreter in the server, offering more powerful and consistent imaging at the cost of server complexity and processing load. Compared to Microsoft Windows' GDI (Graphics Device Interface), it provided a device-independent, high-level programming model versus GDI's more device-dependent and lower-level approach. The Java 2D API and modern systems like Quartz 2D in macOS share its philosophical commitment to a rich, retained imaging model, though they typically use PDF as a basis rather than the PostScript language itself. Its integration of graphics and text handling was more seamless than in many contemporary systems, influencing later standards for vector graphics and document presentation.

Category:Adobe software Category:PostScript Category:Windowing systems Category:NeXT Category:1987 software