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Laboratoire Psychologie de la Perception
- 1. Recognize shadows
- 2. Use their information for
relative position and surface shape
- 3. Throw them out
What does the visual system know about shadows Patrick Cavanagh - - PowerPoint PPT Presentation
What does the visual system know about shadows Patrick Cavanagh - - PowerPoint PPT Presentation
Laboratoire Psychologie de la Perception What does the visual system know about shadows Patrick Cavanagh Universit Paris Descartes Processing shadows 1. Recognize shadows 2. Use their information for relative position and surface shape
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Shadow terms
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Some simple rules Help from conjoint shape-shadow recovery
- 1. Recognizing shadows
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- No. 277, Jiro Takamatsu, 1968
Shadow-object inconsistency OK as long as shadow is darker Can’t always check the shape
Simple rules for what is a shadow
Cavanagh & Leclerc 1989
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Inconsistency goes unnoticed
Fra Carnevale, Birth of the Virgin, 1467
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When is an inconsistency noticed?
Pawan Sinha altering newpaper photo
- f photographer Cornell Capa
Shadows must be consistent within object
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How quickly do we find a BAD illuminant?
Ostrovsky, Cavanagh, Sinha, 2005
About 5000 ms
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A shadow should not have its own contour or texture
Simple rules for what is a shadow
Cavanagh & Leclerc 1989
Cavanagh & Kennedy, Science 2000
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But not darker, not good Can be wrong color
Maurice de Vlaminck, Still Life Georges Braque
Simple rules for what is a shadow
Cavanagh & Leclerc 1989
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Can be wrong color but must be darker
Simple rules for what is a shadow
Cavanagh & Leclerc 1989
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Simple rules for what is a shadow
Cavanagh & Leclerc 1989
Shadows should not have volume — must appear to lie flat on surface
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2-tone images: cannot tell black shadow from black pigment Unless you already know what the object is But cannot identify object until you identify shadow vs pigment
Recognizing object and shadow together
Tolstoy, Giorgio Kienerk, 1904
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How?
Two-tone image Contours Ignore cast shadow contours 2D match finds some characteristic contours of a face Retrieve prototypical head from memory, check against image
Cavanagh, 1991
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The contours alone are often impossible to recognize
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The recognition requires a familiar object Here same, parts, different arrangements
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Recognize a shadow
Simple shadow rules: darker, no border, no volume Can be checked LOCALLY Do not need to check if shadows consistent across the whole scene. Sometimes shadows come as part of object recognition
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- 2. Recover information
Relative position Object shape Surface shape
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Relative position
Pascal Mamassian
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Relative position
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Relative position
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Relative position
Offset or absent cast shadow can make you float
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Object shape
Shadows are silhouettes of the casting object Seen from the light’s viewpoint Silhouettes are sometimes interpretable
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Object shape
But often not And a different viewpoint is not readily integrated into object information
!
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Surface structure
Often too complicated
!
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- 3. Throw them out
Image All contours “Depth sketch”
Shadow contours are noise when it comes to identifying object shapes At some point they need to be removed
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Rensink & Cavanagh, 2004
Visual search for an odd tilt Much slower if the tilted element appears to be a shadow Than if not Slow Faster When are they thrown out?
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Search was slow
Target Distractor
Search was fast
Target Distractor
Darker, no contour Taken as shadow and removed before search can access them
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How quickly do we determine what is a shadow? Faster than the beginning of rapid visual search About 150 msec
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- 3. Throw them out
Where are they thrown out? Areas of visual system that no longer represent shadow borders.
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- 1. Recognize shadows
- 2. Use their information for
relative position and surface shape
- 3. Throw them out