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Lecture 11Surfaces

Science B44

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1. What problem do surfaces solve2. How are surfaces discovered, grouping3. Modal and amodal completion4. Border ownership and figure-ground5. Top-down help from object knowledge

Surfaces

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Almost everywhere you look, what you “see” is a surfaceWe don't represent the transparent air in frontWe don’t represent things behind the first opaque surfaceSurfaces are a level of description between low-level

receptive fields and objects in scenes

Surfaces

An object is a bounded 3D volume of connected materialCan you think of exceptions?A surface is the 2D boundary between the object and

whatever surrounds it4

1. What problem do surfaces solve?

These are the contoursof the image

A picture of a HenryMoore sculpture

How are the regions of the image integrated toform the object?

Start with grouping elements into surfaces

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Feature levelencoding:

Contours, regions,fragments, bits,

pieces

Grouprelated bits

intosurfaces

Combinesurfaces into

objects

1. What problem do surfaces solve?

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2. How are surfaces discovered

Gestalt Laws of groupingReflect the probability that bits belong

together as parts of the same objectPrinciples work even in the sparsest imagesGrouping is the first step in discovering

surfaces (and then objects)When the bits and pieces that group do not

form a bounded surface, we still perceivea grouping (like birds in a flock)

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Aside: What is grouping??

Absolutely basic visual phenomenon that isalmost impossible to define

First stage in collecting regions togetherthat are part of an object

Grouped elements seem to belong together(subjective report)

Processed as a unit (visual search)Grouping can be seen on isolated elements

as an attempt to find an object

NN NN N

NN NF

YN LS B

RQ GF

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Because an object is a bounded volume, two nearbybits are more likely part of the same object thanare two distant bits

a) Proximity: the discs appear to group in pairsdue to proximity

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On average, there is higher similarity of materialwithin an object than between a part of the objectand its surround

b) Similarity: the similar discs appear to group

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Objects are bounded volumes, musthave closed outer contours

c) Closure: the brackets that enclose space groupmore strongly than those that do not

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Objects are bounded volumes, must haveenclosing contours. On average these aresmooth. Can you think of exceptions?

d) Good continuation: the dots forming thesmoothest contour group together

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The best shapes are the most familiar onesor the most regular or the simplest

e) Good form: the contour bits forming the “best”shape group together

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f) Bad form: certain cues indicate that two surfacesdo not belong together. T, and X junctions

This is a T junctionThe stem is a contour on a surface

lying behindThe top bar is a contour on a surface

in frontThe two contours usually do not belong

together

Opaque objects interruptthe contours of otherobjects behind them

behind

in front

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If the front surface is transparentThe T becomes an XThis is always a good cue of

independent overlapping contoursfrom different surfaces

Contours of transparent objects makeXs with background contours

f) Bad form: certain cues indicate that two surfacesdo not belong together. T, and X junctions

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3. Modal and amodal completionNot limited to what is present in imageVisual system may add regions and contours to surfaces

beyond what is present in the imageAmodal completion

Surfaces completing behind an occluding surfacesSurface is not seen but is “registered”

Modal completionSurfaces completing in front of another even though

the border is not visibleCompleted surface is seenIn real life, much less common than amodal

completion16

Amodal completion

Black surface completes amodallyThe two black rectangles appear

to belong togetherBecause of similarity and

continuityCompletion of contours that are

not seen, hidden behind frontsurface

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Evidence that amodal completion occurs

1. Face recognition is better for faces behind, when depth is defined by stereopsis

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2 Amodal completion allows fragments to be groupedand recognized even when partially hidden

spot the 5letter Bs

From Bregman, 1990

samefragments

letter B

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3. The 3D shape is much more evident when occludingstrips permit the lines to complete amodally

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Modal completionContour that is absent in the

image but “visible” to usModal completion only happens

when two surfaces have samecolor and brightness

No contour in the image wherethe front surface covers theback one

But we may see a faint hint of acontour, a “subjective”contour

In this example, it is not clearwhich lies in front

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When it is unclear which surface is in front,the visual system seems to choose the one that

requires the shortest modal contoursEven when that is illogical

Modal completion

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Kanizsa, 1979

Subjective Contours show Modal andAmodal Completion

You would only be seeing a square (modal completionof contour over white background)

if you were assuming complete discs (amodal completionof green contours behind the “white” square)

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Here the interrupted contours give evidence ofcolor or transparency

The color and transparency spreads over thesubjective surface

This is a 3D example for those who can free-fuse.

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The cube in this figure can be seen as in front of theblack discs and the white bars are also “seen” in frontof the white background (modal completion).

Or behind, seen through the Swiss cheese holes against ablack black background. The white bars are now behindthe white surface (amodal completion).

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small modifications greatly weaken subjective surfacesby making alternate explanations more likely

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These subjective surfaces show how much thevisual system gambles in inventing surfaces

We see what ought to the most likely cause ofthese image patterns

The bet is often wrong in these sparse imagesbut seldom wrong in natural scenes wherethere are many more cues to support eachchoice

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4. Border ownership and figure-ground

Unlike borders betweencountries, borders betweenoverlapping surfaces belongto one of the surfaces andonly one of them.

Compare inter-nose spaceto width of vase

Surface extends to edgeof its borders 28

A border is always owned by the surface that lies infront.

The green-black border belongs to black not greenIt does not mark the end of the green surfaceGreen can continue behind

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Black is in frontWhy?T junctionsLow probability of an L shape fitting snugly against the

black squareAmodal completion allows green to extend into a square

itself --> good form and continuity

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A spherical pearl in a 4 point setting or an angel.The 4 point setting is in front of the pearl and owns

the borders.The pearl completes amodally under the setting.The angel is in front of the black background

Ambiguous figures: border ownership disputes

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Some objects expected or guessed based on a fewdistinctive features

This object knowledge helps us identify and joinremaining parts together

5. Top-down help from object knowledge

Group relatedbits intosurfaces

Feature level encoding:Contours, regions,

fragments, bits, pieces

Completeobject

description

Guides

Guessed orexpected

object

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We see a face behind bars, not simplyimage fragments.

Grouping, amodal completion, based onobject knowledge

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Object knowledge is not all powerful, howeverCompletion of surfaces (continuity) often

overrides object knowledgeAnd a regular pattern of figures, completely

predictable, is seen as different under anoccluder.

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Surfaces intermediate level of analysisPrinciples of groupingModal and amodal completionBorder ownership and figure-groundTop-down help from object knowledge

1 Minute QuizNo readings for next MondayObservation Paper Proposal, this week’s sectionLab Report 2 for Monday

Summary

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Modal vs amodal, Figure vs ground, In frontvs in back, Border ownership

Ground is in backAmodal completion in back and not visibleBorder is not owned by the surface in back

Figure is in frontModal completion in front when front and

back surfaces same color and brightnessFigure in front owns its borders

Glossary

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Terms figure-ground are also used for importance orinterest of object

Do not always correspond to front and back surfaces

Glossary