Vector General

Content sourced from Wikipedia, licensed under CC BY-SA 3.0.

Vector General (VG) was a family of graphics terminals made by a California company. Introduced in 1969, these terminals were used in computer labs through the early 1980s. They worked by taking vector data from a host minicomputer and transforming it inside the terminal itself before drawing the image on a built‑in vector monitor. This approach let users rotate, zoom, and otherwise manipulate 2D or 3D shapes much faster than if every operation had to be done on the host computer.

A key idea was that the VG terminals carried very little internal memory. Instead, most of the vector data stayed in the host computer’s memory, and the VG hardware fetched it over a direct memory access (DMA) link. This avoided slow data transfers over serial connections and sped up display updates. The trade‑off was that DMA required compatible host machines and sometimes a specialized adaptor.

VG terminals came in several models. The basic 2D version handled panning and zooming of 2D images. The 2DR model added rotation around an arbitrary point. The top model was the 3D version, offering rotation, panning, and zooming for 3D vectors. An optional character generator could be added to produce text and simple symbols. The displays used square cathode‑ray tubes driven directly by the transformation hardware, a setup known at the time as a “random scan” vector monitor.

Two CRT sizes were available: 17 inches and 21 inches. The 21‑inch version also had a high‑speed option for faster drawing. The displays used electromagnetic deflection (not the same as TV ferrites) to achieve high‑speed vector scanning. Users could connect a variety of input devices, including a 70‑key keyboard, pushbutton switches with lights, a graphics tablet, a light pen, a dial box, and a joystick. The whole system was large—roughly the size of a small refrigerator.

In operation, a 2D or 3D model was defined as a series of points stored in the host memory as 12‑bit values (typically packed into 16‑bit words with extra status bits). The terminal would interrupt the host 30 to 60 times per second to fetch and display the data. Each point was loaded into local registers, transformed (scaled, translated, and possibly rotated), and then sent to the vector monitor for display. The system also supported different ways of describing vectors to save memory: absolute (two end points per vector), relative (offsets from the previous point, needing only one point), incremental (packed 6‑bit points for compact storage), and autoincrementing vectors (only one value stored, with the rest implied by a preset increment). The VG could also draw circular arcs with a dedicated circuit.

The display supported 32 intensity levels, which could be controlled by hardware or used automatically to give depth cues—objects closer to the viewer appeared brighter. A zoom effect was controlled by a scale register, and the system could also scroll by translating coordinates. Optional text was drawn with a small built‑in character generator that used a few basic shapes to form letters and symbols, including Greek letters and mathematical symbols.

Communications with the host were handled through a single bidirectional I/O port, with an interrupt request mechanism for commands. The host could place data into and read data from one of 85 terminal registers. The base address and data offset for the vector list were stored in specific registers, allowing a form of page flipping by switching to a new base address.

The VG3D model is well remembered for its role in Star Wars. A VG3D terminal connected to a PDP‑11/45 helped create the famous Death Star trench run scenes. The animation was produced by feeding 3D points into the terminal, applying rotations and zooms via the terminal’s registers, and shooting individual film frames frame by frame using a camera trigger connected to a panel light. Each frame took about two minutes to render. The early demonstrations demonstrated the potential of real-time 3D graphics and helped spur further work in computer graphics.

VG terminals were also notable beyond cinema. They were used in the development of early computer‑aided design (CAD) tools. In one famous anecdote, Mike Muuss and others demonstrated rotating 3D objects on VG hardware, which inspired further graphics research. This work contributed to later graphics systems and tools, including the BRL‑CAD project.

In short, Vector General terminals brought fast, hardware‑assisted vector transformations to the host’s data, using DMA to access memory directly and delivering interactive 2D and 3D graphics a step ahead of many contemporaries. Their influence echoes in the way many early graphics systems balanced host computation with capable display hardware to deliver responsive, real‑time visuals.