Graphic synthesis follows a visual and sculptural analogy for sound specification. The composer creates sound images by drawing on a tablet or screen; these drawings are translated into sound. In addition, images of sampled sounds can be transformed using graphic tools. (Source - Curtis Roads (1996). The Computer Music Tutorial. Boston: MIT.)
A notable implementation of a system for graphic synthesis is UPIC (Unité Polyagogique Informatique du CEMAMu).
The UPIC is a composing system which combines a graphic score editor, a voice editor and a powerful "performance" (or playback) system, all sharing the same data. Therefore all drawing and editing operations are available while the music plays. All the commands are mouse driven. A menu command allows one to switch the drawing input device from the mouse to the digitizer and vice versa. The new and final version of the system runs on a computer connected to a real-time synthesis unit. The new software offers a mouse controlled, "user-friendly" window style graphical interface and allows real-time drawing, editing and playing of a musical page as well as the recording of a "performance". (Source - Iannis Xenakis (1992). Formalised Music, Revised Edition. New York: Pendragon Press.)
See also:Synthesis and Resynthesis Techniques, Visual Representation
UNESCO DigiArts- Introduction à l’histoire et à l’esthétique des musiques électroacoustiques: Session 2 - Une nouvelle lutherie ( French )