1Visual PerceptionCS 4460/7450 - Information VisualizationJanuary 21, 2010John StaskoSpring 2010 CS 4460/7450 2Agenda• Visual perception Pre-attentive processing Color Etc.2Spring 2010 CS 4460/7450 3Semiotics• The study of symbols and how they convey meaning• Classic book: J. Bertin, 1983, The Semiology of GraphicsSpring 2010 CS 4460/7450 4Related Disciplines• Psychophysics Applying methods of physics to measuring human perceptual systemsHow fast must light flicker until we perceive it as constant?What change in brightness can we perceive?• Cognitive psychology Understanding how people think, here, how it relates to perception3Spring 2010 CS 4460/7450 5Perceptual Processing• Seek to better understand visual perception and visual information processing Multiple theories or models exist Need to understand physiology and cognitive psychologySpring 2010 CS 4460/7450 6One (simple) Model• Two stage process Parallel extraction of low-level properties of scene Sequential goal-directed processingWare 2000Stage 1 Stage 2Early, paralleldetection ofcolor, texture,shape, spatialattributesSerial processing ofobject identification (usingmemory) and spatial layout,action4Spring 2010 CS 4460/7450 7Stage 1 - Low-level, Parallel• Neurons in eye & brain responsible for different kinds of information Orientation, color, texture, movement, etc.• Arrays of neurons work in parallel• Occurs “automatically”• Rapid• Information is transitory, briefly held in iconic store• Bottom-up data-driven model of processing• Often called “pre-attentive” processingSpring 2010 CS 4460/7450 8Stage 2 - Sequential, Goal-Directed• Splits into subsystems for object recognition and for interacting with environment• Increasing evidence supports independence of systems for symbolic object manipulation and for locomotion & action• First subsystem then interfaces to verbal linguistic portion of brain, second interfaces to motor systems that control muscle movements5Spring 2010 CS 4460/7450 9Stage 2 Attributes• Slow serial processing• Involves working and long-term memory• More emphasis on arbitrary aspects of symbols• Top-down processingSpring 2010 CS 4460/7450 10Preattentive Processing• How does human visual system analyze images? Some things seem to be done preattentively, without the need for focused attention Generally less than 200-250 msecs (eye movements take 200 msecs) Seems to be done in parallel by low-level vision systemHealey article6Spring 2010 CS 4460/7450 11How Many 3’s?1281768756138976546984506985604982826762980985845822450985645894509845098094358590910302099059595957725646750506789045678845789809821677654876364908560912949686Spring 2010 CS 4460/7450 12How Many 3’s?12817687561389765469845069856049828267629809858458224509856458945098450980943585909103020990595959577256467505067890456788457898098216776548763649085609129496867Spring 2010 CS 4460/7450 13What Kinds of Tasks?• Target detection Is something there?• Boundary detection Can the elements be grouped?• Counting How many elements of a certain type are present?Spring 2010 CS 4460/7450 14Example• Determine if a red circle is present• (2 sides of the room)8Spring 2010 CS 4460/7450 15HueCan be done rapidly (preattentively) by peopleSurrounding objects called “distractors”Spring 2010 CS 4460/7450 16Example• Determine if a red circle is present9Spring 2010 CS 4460/7450 17ShapeCan be done preattentively by peopleSpring 2010 CS 4460/7450 18Example• Determine if a red circle is present10Spring 2010 CS 4460/7450 19Hue and Shape• Cannot be done preattentively• Must perform a sequential search• Conjuction of features (shape and hue) causes itSpring 2010 CS 4460/7450 20Example• Is there a boundary in the display?11Spring 2010 CS 4460/7450 21Fill and Shape• Left can be done preattentively since each groupcontains one unique feature• Right cannot (there is a boundary!) since the twofeatures are mixed (fill and shape)Spring 2010 CS 4460/7450 22Example• Is there a boundary in the display?12Spring 2010 CS 4460/7450 23Hue versus ShapeLeft: Boundary detected preattentively basedon hue regardless of shapeRight: Cannot do mixed color shapes preattentivelySpring 2010 CS 4460/7450 24Example• Is there a boundary?13Spring 2010 CS 4460/7450 25Hue versus brightnessLeft: Varying brightness seems to interfereRight: Boundary based on brightness can bedone preattentivelySpring 2010 CS 4460/7450 26Example Applet• Nice on-line tutorial and example applet http://www.csc.ncsu.edu/faculty/healey/PP/index.html Chris Healey, NC State Prior pictures taken from site14Spring 2010 CS 4460/7450 27Preattentive Features• Certain visual forms lend themselves to preattentive processing• Variety of forms seem to workSpring 2010 CS 4460/7450 28Textons1. Elongated blobs2. Terminators3. Crossings of linesAll detected early15Spring 2010 CS 4460/7450 293-D Figures3-D visual reality has an influenceSpring 2010 CS 4460/7450 30Emergent Features16Spring 2010 CS 4460/7450 31Potential PA Featureslengthwidthsizecurvaturenumberterminatorsintersectionclosurehueintensityflickerdirection of motionbinocular lustrestereoscopic depth3-D depth cueslighting directionSpring 2010 CS 4460/7450 32Discussion• What role does/should preattentive processing play in information visualization?17Gestalt Laws• Background German psychologists, early 1900‟s Attempt to understand pattern perception Founded Gestalt school of psychology Provided clear descriptions of many basic perceptual phenomena Gestalt Laws of Pattern PerceptionSpring 2010 CS 4460/7450 33Gestalt Laws• Proximity Things close together are perceptually grouped together• Similarity Similar elements get grouped together• Connectedness Connecting different objects by lines unifies them• Continuity More likely to construct visual entities out of smooth, continuous visual elementsSpring 2010 CS 4460/7450 3418Gestalt Laws• Symmetry Symmetrical patterns are perceived more as a whole• Closure A closed contour is seen as an object• Relative Size Smaller components of a pattern as perceived as objects• Figure & Ground Figure is foreground, ground is behindSpring 2010 CS 4460/7450 35Spring 2010 CS 4460/7450 36Key Perceptual Properties• Brightness• Color• Texture• Shape19Spring 2010 CS 4460/7450 37Luminance/Brightness• Luminance Measured amount of light coming