chapter 5: perceiving objects and...
TRANSCRIPT
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Chapter 5: Perceiving Objects and Scenes
Figure 5-1 p96
Figure 5-2 p96
Why Is It So Difficult to Design a Perceiving Machine?
• The stimulus on the receptors is ambiguous. – Inverse projection problem: An image on
the retina can be caused by an infinite number of objects.
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Figure 5-4 p97 Figure 5-7 p98
Figure 5-5 p98 Figure 5-6 p98
A par&cular image on the re&na can be created by an infinite number of objects, among them the two rectangles and the <ed trapezoid shown here. This is why we say that the image on the re&na is ambiguous.
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Why Is It So Difficult to Design a Perceiving Machine?
• Objects can be hidden.
Figure 5-9 p99
Objects can be blurred.
People are also able to recognize objects that are not in sharp focus.
Why Is It So Difficult to Design a Perceiving Machine? - continued
• Objects look different from different viewpoints – Viewpoint invariance: the ability to
recognize an object regardless of the viewpoint
– This is a difficult task for computers to perform
Figure 5-10 p99
Although humans continue to perceive the chair as the same chair viewed from different angles, this isn’t so obvious to a computer
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Perceptual Organization
– How do humans overcome these complexities? • Answered by perceptual organization
Perceptual Organization
• Incoming stimuli in the environment become perceptually grouped to create our perception of objects. – Grouping: visual info are “put together” into
objects. – Segregation: separating object from another.
Figure 5-11 p100
Perceptual Organization
• Structuralism: Approach established by Wundt (late 1800s) – States that perceptions are created by combining elements called sensations.
According to structuralism, a number of sensations (represented by the dots) add up to create our perception of the face. Another principle of structuralism is that the combination of sensations to form perceptions is aided by the observer’s past experience.
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Perceptual Organization
• Structuralism could not explain apparent movement. Apparent movement can’t be explained by sensations, because there is nothing in the dark space between the flashing images.
Figure 5-14 p101
(a) One light flashes, followed by (b) a short period of darkness, followed by (c) another light flashing in a different position. The resulting perception, symbolized in (d), is a light moving from left to right. Movement is seen between the two lights even though there is only darkness in the space between them.
Figure 5-15 p101
The letters and numbers that appear to be moving smoothly across the screen are created by hundreds of small lights that are blinking on and off.
Other message is: the whole is different than the sum of its parts
Perceptual Organization
• Illusory contours- contours that appear real but have physical edge
The illusory contours clearly visible in (b) and (c) cannot be caused by sensations, because there is only white there. Sensations can’t explain illusory contours, because there aren’t any sensations along the contours.
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Gestalt Organizing Principles
• Gestalt psychologists proposed that perception depends on a number of organizing principles, which determine how elements in a scene become grouped together.
• Good continuation - connected points resulting in straight or smooth curves belong together
Figure 5-17 p102
Example of Good continuation
Figure 5-18 p102
Example of Good continuation Gestalt Organizing Principles
– Pragnanz - every stimulus is seen as simply as possible
This is usually perceived as five circles, not as the nine shapes in (b). So, also called the principle of simplicity
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Gestalt Organizing Principles
– Similarity - similar things are grouped together
This law causes circles of the same color to be grouped together.
Figure 5-21 p103
Waves, by Wilma Hurskainen
Gestalt Organizing Principles - continued
• Proximity - things that are near to each other are grouped together
Gestalt Organizing Principles - continued
• Common fate - things moving in same direction are grouped together. – E.g. when you see hundreds of birds all
flying together, you tend to see the flock as a unit, and if some birds start flying in another direction, this creates a new unit.
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Gestalt Organizing Principles - continued
• Common region - elements in the same region tend to be grouped together
This occurs because each oval is seen as a separate region of space.
Gestalt Organizing Principles - continued
• Uniform connectedness - connected region of visual properties are perceived as single unit
Figure 5-24 p104
Objects (the men’s legs) are partially hidden by another object (the gray boards). In this example, the men’s legs continue in a straight line and are the same color above and below the boards, so it is highly likely that they continue behind the boards.
Perceptual Segregation
• Figure-ground segregation - determining what part of environment is the figure so that it “stands out” from the background
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Figure 5-26 p105
(a) When the vase is perceived as figure, it is seen in front of a homogeneous dark background.
(b) When the faces are seen as figure, they are seen in front of a homogeneous light background.
Perceptual Segregation - continued
• What causes us to perceive one area as figure and the other as ground?
– Elements located in the lower part of displays are perceived as figure.
Figure 5-27 p105
They flashed stimuli like the ones on top for 150 milliseconds. And then they determined which area was seen as figure, the red area or the green one. Result: there is no left–right preference for determining figure, but there is a definite preference for seeing objects lower in the display as figure
Figure 5-28 p106
the lower part of the scene is figure and the sky is ground.
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Subjective Factors That Determine Which are is Figure
• Gestalt psychologists believed that experience and meaning play a role in perceptual organization.
• Experiments showed that figure-ground can affected by meaningfulness of a stimuli.
Figure 5-30 p107
(a) “W” on top of “M.” (b) When combined, a new
pattern emerges, overriding the meaningful letters.
Figure 5-31 p107
figure–ground formation can be affected by the meaningfulness of a stimulus
(a) The black area is more likely to be seen as figure because it is meaningful; (b) this effect doesn’t occur when meaningfulness is decreased by turning the picture upside down.
Figure 5-32 p108
At first glance, this scene appears to contain mainly trees, rocks, and water. But on closer inspection, you can see some faces in the trees
Once you perceive a particular grouping of rocks as a face, it is often difficult not to perceive them in this way.
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Perceiving Scenes and Objects in Scenes
• A scene contains: – background elements. – objects organized in meaningful ways with each
other and the background. • Difference between objects and scenes
– A scene is acted within – An object is acted upon
• E.g. if we are walking down the street and mail a letter, we would be acting upon the mailbox (an object) and acting within the street (the scene).
Perceiving Scenes and Objects in Scenes - continued
• You can identify most scenes after viewing them for only a fraction of a second. This general description of the type of scene is called the gist of a scene. (not be able to identify specific objects.)
• Research on perceiving gists of scenes – Potter showed that people can do this when a
picture is only presented for 1/4 second – Fei-Fei presented pictures for 27 ms to 500 ms
and asked observers to write a description of what they saw. He used masking to show that the overall gist is perceived first followed by details.
Figure 5-33 p109
Potter first presented either a description or picture, and then rapidly presented 16 pictures for 250 ms each. The observer’s task was to indicate whether the target picture/description had been presented.
When the target picture was only specified by a written description, such as “girl clapping,” observers achieved an accuracy of almost 90 percent. On the other hand, with pictures, her observers could do this with almost 100 percent accuracy even when the pictures were flashed for only 250 ms
Masking
– Fei-Fei – the perception of a visual stimulus continues for
about 250 ms (1/4 second) after the stimulus is extinguished.
– SO, when a picture is presented for 100 ms, it will be perceived as lasting about 350 ms. But the persistence of vision can be eliminated by presenting a visual masking stimulus, usually a random pattern that covers the original stimulus, so if a picture is flashed for 100 ms followed immediately by a masking stimulus, the picture is visible for just 100 ms.
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Masking
– Fei-Fei (results)
The overall gist of the scene is perceived first, followed by perception of details and smaller objects within the scene. See VL. (5.10)
Perceiving Scenes and Objects in Scenes - continued
• What enables observers to perceive the gist of a scene so rapidly?
• We used info called ‘Global image features of scenes’ – Degree of naturalness (Natural scenes, such
as the ocean and forest)
Perceiving Scenes and Objects in Scenes - continued
– Degree of openness (Open scenes, such as the ocean, often have a visible horizon line and contain few objects. The forest has a lo degree of openness)
– Degree of roughness (Smooth scenes (low roughness) like the ocean contain fewer small elements. Scenes with high roughness like the forest contain many small elements and are more complex.)
Perceiving Scenes and Objects in Scenes - continued
– Degree of expansion (The convergence of parallel lines, like what you see when you look down railroad tracks that appear to vanish in the distance. Street example)
– Color (Some scenes have characteristic colors, like the ocean scene (blue) and the forest (green and brown)
• Such features are holistic and perceived rapidly
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Regularities in the Environment: Information for Perceiving
• We easily use our knowledge of regularities in the environment to help us perceive, even though we may not be able to identify the specific information we are using. See VL.
• Physical regularities - regularly occurring physical properties – Oblique effect - people perceive horizontals
and vertical more easily than other orientations
– Uniform connectedness - objects are defined by areas of the same color or texture
Figure 5-36 p111
When we pick a point on the scene (such as A) and move slightly away from that point (to B), it is likely that the two points are on the same object if the color is the same. If, however, we move to C, the color changes, which means it is likely we are looking at a different object.
Regularities in the Environment: Information for Perceiving – continued
• Physical regularities - regularly occurring physical properties – Homogenous colors. and nearby objects
have different colors – Light-from-above heuristic - light in natural
environment comes from above us
Figure 5-37 p111
Light coming from above will illuminate the top of a shape that is jutting out.
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Figure 5-38 p112
Why does (a) look like indentations in the sand and (b) look like mounds of sand?
Regularities in the Environment: Information for Perceiving - continued
• Semantic regularities. Semantics refers to the meaning of a scene
• Palmer experiment – Observers saw a context scene flashed
briefly, followed by a target picture. – Results showed that:
• Targets congruent with the context were identified 80% of the time .
• Targets that were incongruent were only identified 40% of the time.
Figure 5-39 p113 Figure 5-40 p113
“the multiple personalities of a blob”
See rotating masks at VL.