Visual Encoding

The visual encoding is the way in which data is mapped into visual structures, upon which we build the images on a screen. It is defined in term of:

  1. Spatial substrate
  2. Graphical marks
  3. Graphical properties of the marks

Spatial substrate

The spatial substrate define a position of the space where we collocate graphical marks. The space is defined in term of axes. Card et al. (1999) define four elementary types of exes:

  • Unstructured axis (no axis)
  • Nominal axis (a region is divided into sub regions)
  • Ordinal axis (the order of the axis correspond to the order of the data)
  • Quantitative axis (there is a metric associated with the region)

In order to increase the quantity of data that can be encoded with in a single representation, Card et al. (1999) identified some techniques:

  • Composition
  • Alignment
  • Folding
  • Recursion
  • Overloading

Graphical marks

Marks are graphical entities visible to the humans located somewhere in the space. According to Bertin (1983) and Card et al. (1999) there are four elementary types:

  • Points
  • Lines
  • Areas
  • Volumes

Graphical properties for marks

Graphical proprieties (also called retinal proprieties) are proprieties processed by retina of the eyes which is sensitive to them independent of position. Bertin (1983) identify 6 retinal proprieties:

  • Colour
  • Size
  • Orientation
  • Gray Scale
  • Texture
  • Shape

Card at al. (1999) also add two proprieties which are considered in Information Visualisation:

  • Connection
  • Enclosure

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