Visual Variables

Information may not have a direct visual manifestation. Therefore one needs to map this information to a visual representation. This is done by visual varibles.

Definition

Marks

A mark is made to represent some information other than itself. It is also referred to as a sign.

Marks can be

• Points are dimensionless locations on the plane, represented by signs that obviously need to have some size, shape or color for visualisation.
• Lines represent information with a certain length, but no area and therefore no width. Again lines are visualised by signs of some thickness.
• Areas hava a length and a width and therfore a two-dimensional size.
• Surfaces are areas in a three-dimensional space, but with no thickness.
• Volumes have a length, a width and a depth. They are thus truly three-dimensional.

Visual Variables

Jaques Bertin defined seven Visual Variables [Bertin, 1967] consisting of:

Since nowadays information is presented by computers, the addition of motion as a new visual variable becomes important.

Characteristics

The choice of the variable, which would be most appropriate to present each aspect of information depends on the characteristics.

• Selective: If a mark changes in this variable and it can be selected by the other marks easily the visual variable is said to be selective.
• Associative: Several marks can be grouped across changes in other visual variables.
• Quantitative: If the difference between two marks in this variable can be interpreted numerically, the visual variable is quantitative.
• Order: If the variable supports ordered reading it is an ordered visual variable. This means that a change could be read as more or less. e.g. in size you can order according to the area of the marks.
• Length: The length defines how many values the variable features. For example how many shades of grey can be recognised.

References

• Proceedings

[Bertin, 1967] Bertin, Jaques: "Sémiologie Graphique". Paris: Editions Gauthier-Villars. Deutsche Übersetzung von Jensch, G.; Schade, D.; Scharfe, W.: Graphische Semiologie.Diagramme – Netze - Karten. Berlin: Walter de Gruyter, 1974.

[Skupin, 2000] Skupin, A: "From Metaphor to Method: Cartographic Perspectives on Information Visualization" IEEE Symposium on Information Visualization. October 9-10, 2000, Salt Lake City, Utah.

[Carpendale, 2003] M. S. T. Carpendale: "Considering Visual Variables as a Basis for Information Visualisation", University of Calgary, Department of Computer Science, 2001-693-16, 2003