Chapter 5: Marks and Channels Flashcards
What are marks and channels?
Marks are basic geometric element that depict items or links. Channel control the appearance of marks.
What is a 2D mark?
A zero-dimensional mark is a point, a one-dimensional mark is a line and a two-dimensional mark is an area. A three dimensional mark is a volume.
Name 5 different visual channels and give an example of how it can be used.
Position: horizontal, vertical or a combination of both can be used as a channel.
Color: color is often used as an identifying channel or for grouping
Shape: shape can be used to alter the appearance of 0D or 1D marks as an identifying channel.
Tilt:
Size: length, area and volume can be used as magnitude channels for ordered data.
What kind of channels do bar charts consist out of?
- Size: length. The length of a bar encodes quantative information
- Position: Bars are spaced out to encode categorical information.
- Color can be used as an identifying channel to categorize or group bars.
How many dimensions can a bar chart encode?
2, one on the X-axis and one on the Y-axis
how many dimensions can a scatterplot encode?
A scatterplot can encode two, three or four data dimensions. Two dimensions are used to plot the x and y positions of individual data points, while an optional third dimension can be represented by the size or color of the data points. Another dimension can be added by using the size of the marks.
Name one example where an area mark cannot be size coded.
When the area mark denotes a state or province. The area then already has an intrinsic constraint on size.
Can line marks be altered through the size channel?
Yes, but there are constraints to the width of the lines. If the line is not very long and the width is increased incrementally, it will approach a rectangle shape, which might be confusing.
Explain the terms magnitude and identity channels.
Identity channels tell us information about what something is or where it is. In contrast, the magnitude channels tell us how much of something there is.
Explain on the basis of what marks and channels can be ranked.
The expressiveness principle dictates that the visual encoding should express all of, and only, the information in the dataset attributes. The most fundamental expression of this principle is that ordered data should be shown in a way that perceptual system intrinsically senses as ordered. Conversely, unordered data should not be shown in a way that perceptually implies an ordering that does not exist. The identity channels are the correct match for the categorical attributes that have no intrinsic order. The magnitude channels are the correct match for the ordered attributes, both ordinal and quantitative.
The effectiveness principle dictates that the importance of the attribute should match the salience of the channel; that is, its noticeability. The most important attributes should be encoded with the most effective channels in order to be noticeable, and then decreasingly important attributes can be matched with less effective channels.
In general what are the most effective magnitude channels for ordered attributes?
Spatial position and then 1D size
Explain steven’s power law and explain how it is related to the accuracy of a visual encoding.
Stevens’ power law is a principle in psychophysics that states that the perceived magnitude of a stimulus is proportional to the logarithm of its physical intensity. In other words, the perceived magnitude of a stimulus is not linearly related to its physical intensity, but is instead related to the logarithm of its physical intensity.
In practice this means that by making an element twice as bright, it is not perceived as twice as bright. This makes a big difference when a human interprets a visualization, because we have to think about how a human perceives a visual element.
Explain discriminability when it comes to visually encoding information, and what rules we should follow based on this when designing a visualization.
The question of discriminability is: if you encode data using a particular visual channel, are the differences between items perceptible to the human as intended? The characterization of visual channels thus should quantify the number of bins that are available for use within a visual channel, where each bin is a distinguishable step or level from the other.
Explain the difference between separability and discriminability and give an example of separability in encodings.
You cannot treat all visual channels as completely independent from each other, because some have dependencies and interactions with others, this is meant with separability. Where separability is about the interactions between two or more encodings, discriminability is about how well humans can perceive the difference between items using a particular channel. An example where separability is clear is color and location. There is no interaction there. An example where there is interaction is using two different axes of hue, like red and green, to encode quantitive data, as these combine into new hues.
Which four grouping channels are there?
- Link marks to link different items within a group to eachother
- Containment mark to put marks within a same area
- Proximity, so placing items within the same spatial region
- Similarity, using color, motion or shape to make different marks more similar.
