This paper aims at contributing to a better understanding of essential concepts of visual design aided by computer. Towards this end, we first present an ontology of conceptual visual design. Then, we define particular components of the formal model paying attention to the role of different kinds of classification during the design process. Moreover, we present two types of logic models used in computer tools supporting the design process. Finally, a formal model of computer-aided visual design is defined. The model is illustrated on examples of designing floor layouts based on a graph-based data structure gathering information for design knowledge and describing two categories of thinking during design process.