The TV Turtle: A LOGO Graphics System for Raster Displays. AI Memo 361

Abstract

This discussion of the advantages and limitations of raster graphics systems points out that until recently, most computer graphics systems have been oriented toward the display of line drawings, continually refreshing the screen from a display list of vectors. Developments such as plasma panel displays and rapidly declining memory prices have now made feasible raster graphics systems, which instead associate some memory with each point on the screen and display points according to the contents of the memory. It is explained that raster systems permit operations which are not feasible on vector displays, such as reading directly from the screen as well as writing on it, and manipulating two-dimensional areas and vectors. Conceptual differences between programming for raster and vector systems are illustrated with a description of the TV Turtle, a graphics system for raster scan video display terminals. This system is imbedded in LOGO, a Lisp-like interactive programming language designed for use by children, and is based on LOGO's turtle geometry approach to graphics. LOGO, it is noted, provides powerful ideas for using graphics which are easy for children to learn, yet generalize naturally when advanced capabilities such as primitives for animation and color are added to the system. (Author/LLS)