EXP3202C Exam 5 Vision IV Perceiving Recognizing Objects Chapter 4 What is middle vision Middle vision is a loosely defined stage of visual processing It occurs after basic features have been extracted from the image early vision up to including striate cortex and before object recognition and scene understanding high level vision The goal of middle vision is to organize elements of a visual scene into groups that we can then recognize as objects The goal is to have a single answer emerge out of lots of opinions Middle vision is like a collection of specialists each with an area of expertise individual opinions about what the input might mean Why do you think the Gestalt school of thought was instrumental in identifying how we perceive images To answer this you need to know what Gestalt means Gestalt in german literally means form Pertaining to perception it is the name of a school of thought stressing that the perceptual whole could be greater than the apparent sum of the parts This grew out of failings of the structuralists who argued that perceptions are the sum of atoms of sensation like a crystal is built of atoms The most enduring contribution of this school was to begin the description of a set of organizing principles the Gestalt grouping rules that guide the visual system in its interpretation of the raw retinal image The distinction b t sensation and perception originates with the Gestalt psychologists ONE retinal image gives rise to MULTIPLE perceptions This is where we get the saying the whole is greater than the sum of the parts Another way to say that is the perception is greater than the sum of the sensation You want to think of sensation as a passive analytic process and perception as an active synthetic process Know the gestalt grouping principles good continuation occlusion texture segmentation similarity proximity connectedness common region how do those latter 2 relate to proximity what trumps what parallelism symmetry Also know dynamic occlusion Gestalt grouping rules is a set of rules describing which elements in an image will appear to group together The original list was assembled by members of the Gestalt school of thought Two visible bits of an edge make it easier to perceive a 3rd colinear line between them even if that middle segment is not actually visible If the set of lines forms a closed shape then the segments support each other even more strongly Good continuation two elements or lines will tend to group together if they seem to lie on the same contour A Looks like a pair of intersecting lines So does B But if change things C looks different BUT all other things being equal A and B it looks like a pair of intersecting lines So what s up with C Color trumps good continuation as an organizing principle Here all things are not equal EXP3202C Exam 5 Occlusion A cue to relative depth order in which one object obstructs the view of part of another object If an edge seems to stop the visual system guesses that it is stopping b c something else is in the way occluding it The illusory contour in a comes from two guesses The notches in the circles represent contours that can be extended and the vertical line stops at the top because something is occluding it BTW the notches themselves are perceived as a place where 1 object is occluding another In c each black line generates illusory contour at right angles to endpoints d circle Dynamic occlusion when visual regions are discontinuous not only in space but over time Texture segmentation Carving an image into regions of common texture properties Edge finding isn t helpful here it can only get us so far anyway Region of larger polygons coarser texture is separated from rest of image Texture grouping can be based on similar Color Size Orientation Aspects of form But combinations of features don t work well b Shows green squares and red diamonds on left red squares and green diamonds on right Did you even notice EXP3202C Exam 5 Similarity chunks that are similar to each other are groped together Here the similarity is based on form Proximity items near each other are grouped together Figure looks striped Connectedness common region trump proximity Black dots in each row are spaced in the same way Common region if 2 features appear to be part of the same common region they will group together 2 Connectedness if 2 items are connected they probably belong together 3 Parallelism a rule for figure ground assignment stating that parallel contours are likely to belong to the same figure Symmetry a rule for figure ground assignment stating that symmetrical regions are more likely to be seen as a figure Which lines go together Usually people say 2 3 and 7 8 How does color fit in to the gestalt grouping principles EXP3202C Exam 5 What is an ambiguous figure Ambiguous figure is when 2 cognitive demons shout equally loudly This is the exception that proves the rule the rule being that usually there is consensus among the committees Necker cube You could interpret a as b or c but not likely as d or other possibilities You also can t entertain both possibilities b and c at the same time What is an accidental viewpoint Accidental viewpoint a viewing position that produces some regularity in the visual image that is not present in the world like the sides of 2 independent objects lining up perfectly Moving just a tiny bit destroys the illusion Your visual system is way too smart to fall for this one What is figure and ground and what principles do we use to decide which is which in a visual scene What is evidence that maybe object recognition precedes establishment of figure ground Figure and Ground figure ground assignment is the process of determining that some regions of an image belong to a foreground object figure others are part of the background ground Principles we use to determine figure and ground Surroundedness if 1 region is entirely surrounded by the other it is likely the figure green swirl in previous slide Size smaller region is likely the figure swirl in previous slide Symmetry symmetrical region more likely the figure vase in previous slide Parallelism regions w parallel contours are more likely seen as figure Relative motion how surface details move relative to an edge Synchrony elements that change at the same time group together Extremal edges shading of objects tell us about figure and ground Extremal edges the set of surface points whose sight lines are tangent to the surface We perceive the thing w
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