Technology

Early interactive molecular computer graphics systems were vector graphics machines, which used stroke-writing vector monitors, sometimes even oscilloscopes. The electron beam does not sweep left-and-right as in a raster display. The display hardware followed a sequential list of digital drawing instructions (the display list), directly drawing at an angle one stroke for each molecular bond. When the list was complete, drawing would begin again from the top of the list, so if the list was long (a large number of molecular bonds), the display would flicker heavily. Later vector displays could rotate complex structures with smooth motion, since the orientation of all of the coordinates in the display list could be changed by loading just a few numbers into rotation registers in the display unit, and the display unit would multiply all coordinates in the display list by the contents of these registers as the picture was drawn. The early black-and white vector displays could somewhat distinguish for example a molecular structure from its surrounding electron density map for crystallographic structure solution work by drawing the molecule brighter than the map. Color display makes them easier to tell apart. During the 1970s two-color stroke-writing Penetron tubes were available, but not used in molecular computer graphics systems. In about 1980 Evans & Sutherland made the first practical full-color vector displays for molecular graphics, typically attached to an E&S PS-300 display. This early color tube was expensive, because its was originally engineered to withstand the shaking of a flight-simulator motion base.