Marks and channels

Definitions

Marks are basic geometric elements that depict items or links. Channels control the appearance of the marks. This way you can describe the design space of visual encodings as an

orthogonal combination of these two aspects that allows to analyze and design the visual elements.

Channel synonyms: attribute, dimension, variable, feature, carrier. Visual synonyms: graphic, perceptual, retinal.

Marks can be classified according to the number of dimensions required for their representation:

Zero: points

One: lines

Two: surfaces

Three: volumes

Marks

Spatial position: Alignment: Depth (3D) Regions:

Color: Hue: Saturation: Luminance:

Size: Length: Area:

Volume: Orientation: Curvature: Shape: Stipple patterns:

Dots: Dashes:

Motion

Visual channels

Example combining channels and marks

Attributes: qualitative and quantitative Two quantiative attributes

Three quantitative attributes Three quantitative attributes and one qualitative

Channel types

Humans have two basic types of sensory modalities: Channels related to what and where channels: provide

information about identity and location. Examples: shape, hue, spatial patterns or motion.

Los canales relacionados con el qu y el dnde: proporcionaninformacin sobre la identidad y la ubicacin. Ejemplos: forma, tono, patrones espaciales o movimiento.

Channels related to how-much: provide quantitative information. Examples: length, area, volume, saturation, luminance, orientation.

Mark types

In a table, a mark represents an item. In a network, marks can be nodes or links. Links represent

relationships between items. There are two types of link marks:

Connection marks: define pairwise relationships between two items, using a line.

Containment marks (enclosure, nesting): define hierarchical relationships using nested areas at multiple levels.

Using mark and channels

The use of marks and channels should be guided by the principles of expressiveness and efficiency.

They allow the creation of a ranking of channels that are suitable for visual encoding of the data types to be used.

Expressiveness principle

The expressiveness principle specifies that visual encoding should express all, an only, the information in the dataset attributes.

It follows that the ordered data should be presented in a way that our perceptual system detects it as an intrinsic order. Conversely, not ordered data should never be displayed in a way that we perceive some order.

What/Where channels are adequate for categorical attributes that have no intrinsic order.

How-much channels are adequate for ordered attributes.

Effectiveness principle

The effectiveness principle indicates that the importance of the attribute should match the salience of the visual channel, ie with its noticeability.

The most important attributes should be encoded with the most effective channels.

Channels ranking: ordered attributes

Spatial position (2D) on common scale: Spatial position (2D) on unaligned scale: Length: Orientation: Area: Depth (3D position): Luminance: Saturation: Curvature: Volume:

Channels ranking: categorical attributes Spatial region: Hue: Motion Shape:

The attributes encoded with spatial positions will be the most predominant in the mental model of users.

Criteria for defining the effectiveness channels ranking Accuracy. Discriminability. Separability. Popout. Grouping.

Criteria: accuracy

It determines how close is human perception to some objective measure of stimulation.

Psychophysics is a subfield of psychology that studies the systematic measurement of human perception.

The apparent magnitude of all sensory channels follow a potential function based on the stimulus intensity: = where S is the perceived sensation, I is the magnitude of the physical stimulus. It is what is called the Stevens Law.

Criteria: discriminability

It determines it there are noticeable differences between different items encoded with a particular visual channel.

The characterization of a visual channel should quantify the number of bins that are available for being used, where each bin represents a new level of discrimination from the previous bin.

The key factor is that the number of values of a given attribute dont exceed the number of bins available in the visual channel.

If this restriction is not met, you will need to add values or choose another visual channel.

Criteria: separability

You can not consider that all visual channels are completely independent.

You can establish a continuous gradation between pairs of channels ranging from those channels that are orthogonal and independent separable to the channels whose combination is inherently integral (not separable).

The visual coding will be easy to do if separable channels are used, but it will fail if integral channels are used because users will not be able to access the information required for each attribute, but they will receive a combination of unwanted stimuli.

Fully separable: position and color

Significant interference: size (width and height)

Some interference: size and color

Major interference: colors red and green

Criteria: popout

Many visual channels provide a visual popout, by which an item stands out from the rest immediately.

The great value of the auto attendant is that the time required to identify the different object does not depend on the number of distracting objects.

This process is performed by the human visual system unconsciously.

20 items color encoded Many items color encoded

Shape-based encoding

[Heyley]

Visual encoding based on shape and color

Orientation Size Shape

Spatial proximity Shadow direction Paralelism

Criteria: popout

All visual channels previously presented support popout individually . As a general rule, there should only be one channel at a time used to

popout items, although some combinations of pairs of channels also respect this criterion: i.e. spatial position and color; movement and shape.

In addition, popout is not a binary phenomenon (all or nothing): it depends on the channel and the context in which the target item is located.

Criteria: grouping

The perception of groups mainly arises from the use of link marks or from what channels encoding categorical attributes.

Marks are based on using containment areas or lines connecting items.

Containment is the more powerful cue for grouping. Then, it comes connection.

Criteria: grouping

All items associated with a particular visual representation of a categorical attribute are perceived as a group if we can focus our attention exclusively on the categorical attribute selected.

This encoding is not as powerful as visual marks, but it doesnt overload the image with additional marks that need to be incorporated along with the data.

Another form of grouping is to use spatial proximity. The last possible option is the similarity with any of the categorical

channels, mainly hue and motion, tone color and movement, and to a lesser extent, shape.

Webers Law

Human perception is based on Weber's Law: the minimum detectable amount of a stimulus intensity I is a fixed value K proportional to its magnitude: = .

This Law holds for any sense perceived by human beings. Esta Ley se cumple para cualquiera de los sentidos percibidos por los

humanos. This must be taken into account when some criteria such as accuracy

or discriminability are analyzed.

Webers Law

Unalignedand unframedrectangles

Unalignedand framedrectangles

Alignedrectangles

For example: estimate the length with a common scale is much easier to do that doing it without the common scale.

Webers Law For example: the perception of color and luminance depends on

contextual information, based on the contrast with the colors around[Adelson].

References

Tamara Munzner. Visualization Analysis and Design. A K Peters Visualization Series. CRC Press. Oct 2014.

William S. Cleveland and Robert McGill. Graphical Perception: Theory, Experimentation, and Application to the Development of Graphical Methods." Journal of the American Statistical Association 79:387 (1984), 531-554.

S. S. Stevens. Psychophysics: Introduction to its Perceptual, Neural, and Social Prospects. Wiley, 1975.

[Healey]: www.csc.ncsu.edu/faculty/healey/PP/ [Adelson]:

http://web.mit.edu/persci/people/adelson/checkershadow_illusion.html

Marks and channelsDefinitionsMarksVisual channelsExample combining channels and marksChannel typesMark typesUsing mark and channelsExpressiveness principleEffectiveness principleChannels ranking: ordered attributesChannels ranking: categorical attributesCriteria for defining the effectiveness channels rankingCriteria: accuracyCriteria: discriminabilityCriteria: separabilitySlide Number 17Criteria: popoutSlide Number 19Slide Number 20Criteria: popoutCriteria: groupingCriteria: groupingWebers LawWebers LawWebers LawReferencesSlide Number 28