Shadows (P)Lavanya Sharan

February 21st, 2011

Anomalously lit objects are easy to see

Kleffner & Ramchandran 1992

Enns & Rensink 1990

Anomalously lit objects are easy to see

Kleffner & Ramchandran 1992

Enns & Rensink 1990

Anomalously lit objects are easy to see

Ostrovsky et al. (2005): Only in these conditions.

Kleffner & Ramchandran 1992

Enns & Rensink 1990

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.

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.

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.

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’).

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?

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%)

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.

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.

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.

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)

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.

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).