Shadows (P) Lavanya Sharan February 21st, 2011. Anomalously lit objects are easy to see Kleffner &...
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Transcript of Shadows (P) Lavanya Sharan February 21st, 2011. Anomalously lit objects are easy to see Kleffner &...
![Page 1: Shadows (P) Lavanya Sharan February 21st, 2011. Anomalously lit objects are easy to see Kleffner & Ramchandran 1992Enns & Rensink 1990.](https://reader035.fdocuments.net/reader035/viewer/2022062407/56649d4b5503460f94a296a6/html5/thumbnails/1.jpg)
Shadows (P)Lavanya Sharan
February 21st, 2011
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Anomalously lit objects are easy to see
Kleffner & Ramchandran 1992
Enns & Rensink 1990
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Anomalously lit objects are easy to see
Kleffner & Ramchandran 1992
Enns & Rensink 1990
![Page 4: Shadows (P) Lavanya Sharan February 21st, 2011. Anomalously lit objects are easy to see Kleffner & Ramchandran 1992Enns & Rensink 1990.](https://reader035.fdocuments.net/reader035/viewer/2022062407/56649d4b5503460f94a296a6/html5/thumbnails/4.jpg)
Anomalously lit objects are easy to see
Ostrovsky et al. (2005): Only in these conditions.
Kleffner & Ramchandran 1992
Enns & Rensink 1990
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Notion of distractor homogeneity
Enns & Rensink (1990)
Target is lit from belowDistractors are lit from above
Task-relevant dimension = Light direction
Here, distractors are identical to each other => Complete distractor homogeneity.
Search tasks are easier when distractors are homogeneous.
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Notion of distractor homogeneity
Ostrovsky et al. (2005)
Target is lit from the sideDistractors are lit from above
Task-relevant dimension = Light direction
Here, distractors differ from each other in orientation => Decreased distractor homogeneity.
Task-irrelevant dimension = Orientation
Search tasks can get harder when distractors are
inhomogeneous.
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Ostrovsky et al. experiment
Ostrovsky et al. (2005)
Target and distractor lighting differ by 90 degrees.
Lighting conditions vary in 45 deg steps from 0 to 360.
Distractor cubes vary in orientation.
Number of items = 4, 9 and 12.
Presented for 100, 500 and 1000 ms.
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Search for anomalous illumination is not efficient
Ostrovsky et al. results contradict previous work.
When reaction time increases and accuracy decreases with number of items = signature of serial search (opposite of ‘pop-out’).
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What is going on?
No. They ran a baseline condition where distractors were identical and reproduced ‘pop-out’-like performance.
90% accuracy at 120 ms display time (chance = 50%).Performance invariant to number of items (4-12).
Something weird about their setup?
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What is going on?
Of course, distractor inhomogeneity makes a task hard. This is a stress test.
Perhaps. But,
i) Real world has a lot of inhomogeneity.ii) They conducted an experiment where task-relevant dimension was cube shape, and task-irrelevant dimension (as before) was orientation.
At 1000 ms, number of items = 9Shape task 92%, Illumination task 56% (chance = 50%)
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What is going on?
Advantage for top and top-left conditions, but even in those no pop-out.
Wait, we know there is a bias for top-left. They tested all directions, perhaps pop-out is in the top-left conditions.
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What is going on?
Cube shapes are weird. Ran experiment with other shapes, same results. Ran a baseline to confirm participants can estimate illumination direction from these new shapes.
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Second study with real-world stimuliDigitally modified images to have inconsistent illlumination (average diff = 90 deg)
This is clearly not pop-out.
When not explicitly instructed to look for illumination inconsistency, performance was at chance.
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Support from other studies
When stimuli can be seen as object + shadow vs. object + a second attached object, search is slower in shadow case.
Rensink & Cavanagh (2004)
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Support from other studies
Farid & Bravo (2010)When cast shadows in opposite directions, near perfect
detection.When cast shadows in same direction, near chance
performance.
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Conclusions
✓ Global illumination errors (and therefore shadow consistencies) are hard to detect, especially in complex scenes.
✓ If we are estimating illumination really well (and therefore estimating shape & reflectance really well), why do we make these mistakes? Inverse optics theories have to account for these errors.
✓ Unclear whether these mistakes happen because we are bad at estimating illumination (imperfect inverse optics) vs. unable to report correctly (read out issue).