Visual Processing (contd.) Pattern recognition. Proximity the tendency to group pieces that are close together into one object.

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1 Objectives of today s lecture From your prior reading and the lecture, be able to: explain the gestalt laws of perceptual organization list the visual variables and explain how they relate to perceptual tasks Perception & Representation Visual Processing (contd.) Pattern recognition Human pattern recognition is not a simple matter of matching a pattern in the signal to a template in memory; it makes heavy use of context and knowledge. The organizing principles that enable us to see patterns of stimuli as meaningful wholes are defined as: Proximity the tendency to group pieces that are close together into one object. Similarity the tendency for elements of the same shape or colour to be seen as belonging together. Closure missing parts of a figure are filled in to complete it. Continuity where possible we will see continuity, rather than discreteness. Symmetry regions bounded by symmetrical borders tend to be perceived as coherent figures. Together these are known as the gestalt laws of perceptual organization. If we do not pay proper attention these aspects of human perception we can end up with an interface which exhibits visual noise; where we see false groupings and perceptual artefacts, with the intended meaning becoming obscured. Week 8, Lecture 1, Page 1

2 Text & Reading During reading, the eye makes jerky movements known as saccades followed by fixations (accounting for 94% of time elapsed). Perception occurs during the fixation periods. The eye moves backwards over text as well as forwards, in what are known as regressions. More complex text will result in more regressions. The average adult reads at about 250 words per minute. To read at this rate, words are not scanned serially character by character, but recognized as whole entities. This means that unfamiliar shapes for words will slow reading. THIS IS ONE REASON TO DISRECOMMEND PRESENTING TEXT IN ALL-CAPS (ALSO KNOWN AS SHOUTING). Similarly, very short or long lines of text are difficult to read, because in the former case they eye has to find the start of the line more often than normal, and in the latter case, a long distance to move from the end of one line to the start of the next makes finding the next line more difficult. Visual variables and perceptual tasks Visual variables are those things we can vary in the visual field, and are: Shape Size Intensity (aka Value or Luminosity) Week 8, Lecture 1, Page 2

3 Hue (obviously this can only be seen if this document is rendered in colour) Saturation (obviously this can only be seen if this document is rendered in colour) Texture Position Orientation In design, it is important that we use visual variables correctly so as to maximally support the users perceptual tasks. Users perceptual tasks fall into the following categories: associative and selective (together known as nominal), and ordered and quantitative. In nominal perception the user is concerned only with categorizing or differentiating the things being observed. In associative perception the viewer ignores variation on one visual dimension in reading the remainder of the display. A visual variable is considered associative if it does not affect the visibility of other dimensions in the elements to which it is applied. Week 8, Lecture 1, Page 3

4 Thus, we can recognize the hue of an object, regardless of its orientation. A visual variable is considered dissociative if visibility is significantly reduced for some values along that coding dimension. All values except size and value are associative, they can be overlooked when necessary. Note that value and saturation can associate to an extent, but as value decreases, it becomes harder to tell how much saturation there is; for example: (obviously this can only be seen if this document is rendered in colour) In selective perception the viewer attempts to isolate all instances of a given category and perceptually groups them into a single image. The task is to ignore everything but the target value on the dimension of interest to see at a glance where all targets are on the display. A visual variable is selective only if the grouping is immediate and effortless. All visual variables except shape. are selective. In ordered perception the viewer must determine the relative ordering of values along a perceptual dimension. There must be a natural ordering so that the element representing more of the corresponding quality is immediately obvious. Position, size, saturation and value are ordered importantly, hue is not ordered. In quantitative perception the viewer must determine the amount of difference between two ordered values, with the relative magnitudes being immediately apparent. Only position and size are quantitative. Week 8, Lecture 1, Page 4

5 Although position and size are both visual variables that allow us to make quantitative judgements, some kinds of size are easier to make perceptual judgements about than others. We are very good when it comes to perceiving differences in length, therefore: Line length provides us with quite accurate numerical comparisons. Area and volume are much less accurate, especially given that they are often misused. For example: Consider the following two diagrams, using mailboxes to compare the amount of mail sent in Washington and Oregon. Oregon Washington Oregon Washington Does either make you feel that Washington sends twice as much mail as Oregon? Week 8, Lecture 1, Page 5