The TV Turtle: A Logo Graphics System for Raster Displays

Abstract

Until recently, most computer graphics systems have been oriented toward the display of line drawins, 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. This paper discusses the advantages and limitations of such systems. Raster systems permit operations which are not feasible on vector displays, such as reading directly from the screen as well as writing it, and manipulating two dimensional areas as well as vectors. Conceptual differences between programming for raster and vector systems are illustrated with a description of the author's TV Turtle, a graphics system for raster scan video display terminals. This system is embedded in Logo, a Lisp-like interactive programming language designed for use by kids, and is based on Logo's turtle geometry approach to graphics. Logo provides powerful ideas for using graphics which are easy for kids to learn, yet generalize naturally when advanced capabilities such as primitives for animation and color are added to the system.