Surfaces. Science B44. Lecture 11 Surfaces. Surfaces. 1. What problem do surfaces solve? 2. How are surfaces discovered

Transcription

1 Science B44 Lecture 11 Surfaces Surfaces 1. What problem do surfaces solve 2. How are surfaces discovered, grouping 3. Modal and amodal completion 4. Border ownership and figure-ground 5. Top-down help from object knowledge 1 2 Surfaces 1. What problem do surfaces solve? Almost everywhere you look, what you see is a surface We don't represent the transparent air in front We don t represent things behind the first opaque surface Surfaces are a level of description between low-level receptive fields and objects in scenes An object is a bounded 3D volume of connected material Can you think of exceptions? A surface is the 2D boundary between the object and whatever surrounds it 3 A picture of a Henry Moore sculpture These are the contours of the image How are the regions of the image integrated to form the object? Start with grouping elements into surfaces 4 1. What problem do surfaces solve? 2. How are surfaces discovered Feature level encoding: Contours, regions, fragments, bits, pieces Group related bits into surfaces Combine surfaces into objects Gestalt Laws of grouping Reflect the probability that bits belong together as parts of the same object Principles work even in the sparsest images Grouping 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 perceive a grouping (like birds in a flock) 5 6

2 a) Proximity: the discs appear to group in pairs due to proximity Aside: What is grouping?? N N N N N FN N N Q R G Y N FL S B Absolutely basic visual phenomenon that is almost impossible to define First stage in collecting regions together that 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 7 b) Similarity: the similar discs appear to group 8 c) Closure: the brackets that enclose space group more strongly than those that do not On average, there is higher similarity of material within an object than between a part of the object and its surround 9 d) Good continuation: the dots forming the smoothest contour group together Objects are bounded volumes, must have enclosing contours. On average these are smooth. Can you think of exceptions? Because an object is a bounded volume, two nearby bits are more likely part of the same object than are two distant bits Objects are bounded volumes, must have closed outer contours 10 e) Good form: the contour bits forming the best shape group together The best shapes are the most familiar ones or the most regular or the simplest 11 12

3 f) Bad form: certain cues indicate that two surfaces do not belong together. T, and X junctions f) Bad form: certain cues indicate that two surfaces do not belong together. T, and X junctions behind This is a T junction The stem is a contour on a surface lying behind The top bar is a contour on a surface in front The two contours usually do not belong together If the front surface is transparent The T becomes an X This is always a good cue of independent overlapping contours from different surfaces in front Opaque objects interrupt the contours of other objects behind them 13 Contours of transparent objects make Xs with background contours Modal and amodal completion Not limited to what is present in image Visual system may add regions and contours to surfaces beyond what is present in the image Amodal completion Surfaces completing behind an occluding surfaces Surface is not seen but is registered Modal completion Surfaces completing in front of another even though the border is not visible Completed surface is seen In real life, much less common than amodal completion 15 Amodal completion Black surface completes amodally The two black rectangles appear to belong together Because of similarity and continuity Completion of contours that are not seen, hidden behind front surface 16 Evidence that amodal completion occurs spot the 5 letter Bs same fragments letter B From Bregman, Face recognition is better for faces behind, when depth is defined by stereopsis 17 2 Amodal completion allows fragments to be grouped and recognized even when partially hidden 18

4 3. The 3D shape is much more evident when occluding strips permit the lines to complete amodally 19 Modal completion Contour that is absent in the image but visible to us Modal completion only happens when two surfaces have same color and brightness No contour in the image where the front surface covers the back one But we may see a faint hint of a contour, a subjective contour In this example, it is not clear which lies in front 20 Modal completion When it is unclear which surface is in front, the visual system seems to choose the one that requires the shortest modal contours Even when that is illogical Subjective Contours show Modal and Amodal Completion Kanizsa, 1979 You would only be seeing a square (modal completion of contour over white background) if you were assuming complete discs (amodal completion of green contours behind the white square) Here the interrupted contours give evidence of color or transparency The color and transparency spreads over the subjective surface This is a 3D example for those who can free-fuse. 23 The cube in this figure can be seen as in front of the black discs and the white bars are also seen in front of the white background (modal completion). Or behind, seen through the Swiss cheese holes against a black black background. The white bars are now behind the white surface (amodal completion). 24

5 small modifications greatly weaken subjective surfaces by making alternate explanations more likely These subjective surfaces show how much the visual system gambles in inventing surfaces We see what ought to the most likely cause of these image patterns The bet is often wrong in these sparse images but seldom wrong in natural scenes where there are many more cues to support each choice Border ownership and figure-ground Compare inter-nose space to width of vase Surface extends to edge Unlike borders between countries, borders between overlapping surfaces belong to one of the surfaces and only one of them. A border is always owned by the surface that lies in front. The green-black border belongs to black not green It does not mark the end of the green surface Green can continue behind of its borders Ambiguous figures: border ownership disputes Black is in front Why? T junctions Low probability of an L shape fitting snugly against the black square Amodal completion allows green to extend into a square itself --> good form and continuity 29 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 30

6 5. Top-down help from object knowledge Some objects expected or guessed based on a few distinctive features This object knowledge helps us identify and join remaining parts together Guessed or expected object Feature level encoding: Contours, regions, fragments, bits, pieces Group related bits into surfaces Guides Complete object description 31 We see a face behind bars, not simply image fragments. Grouping, amodal completion, based on object knowledge 32 Object knowledge is not all powerful, however Completion of surfaces (continuity) often overrides object knowledge And a regular pattern of figures, completely predictable, is seen as different under an occluder

7 Summary Surfaces intermediate level of analysis Principles of grouping Modal and amodal completion Border ownership and figure-ground Top-down help from object knowledge 1 Minute Quiz No readings for next Monday Observation Paper Proposal, this week s section Lab Report 2 for Monday Glossary Modal vs amodal, Figure vs ground, In front vs in back, Border ownership Ground is in back Amodal completion in back and not visible Border is not owned by the surface in back Figure is in front Modal completion in front when front and back surfaces same color and brightness Figure in front owns its borders Glossary Terms figure-ground are also used for importance or interest of object Do not always correspond to front and back surfaces 39