PERCEPTUAL STRATEGIES FOR MATERIAL IDENTIFICATION

Qasim Zaidi Rocco Robilotto

Byung-Geun Khang

SUNY College of Optometry

1 2 3 4

•Three of the objects are made of identical gray paper while one is made of a different shade.•Illumination on the right is half the illumination on the left.

What is the number of the odd object?

3 1 2 4

“3” is the number of the odd object!

What strategy did you follow?

•Brightness discrimination threshold from % side-correct•Lightness identification threshold from % object-correct

Method of constant-stimuli

PREDICTIONS FOR TWO MODELS

Results of Lightness Identification

Hypothesis: P(I|D)=1.0Rejected at p=0.01Chi-squared > 16.8

Note systematic asymmetry in lightness identification compared to brightness discrimination.

1 2 3 4

•Choose the object that is most different in brightness.•Disregard the material and the illumination difference.

Brightness is different from photometer luminance because of adaptation and lateral interactions.

Results of Brightness Dissimilarity

Note systematic asymmetry in curves is similar to lightness identification results.

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Lightness Identification (Exp 1)

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Lightness Identification (Exp 1)

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Brightness Discrimination Thresholds (Reflectance units)

Lightness Identification and Brightness Dissimilarity Thresholds (Reflectance units)

Half Illum.

Full Illum.

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Adding adaptation to the photometer modelGain= k/(k+l)

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Lightness identification based on brightness dissimilarity responses of adaptation model

PERCEPTUAL STRATEGIES FOR LIGHTNESS IDENTIFICATION

•Under everyday conditions, observers consistently judge and remember surfaces as having a certain lightness or grayness.

•This phenomenology is not sufficient evidence that the visual system generically has access to the lightness of materials.

•For 3-D objects, we show conditions where lightness identification is limited solely by the limen of brightness discrimination, and other conditions where lightness identification is considerably worse.

•More importantly, we show that the same relative brightness based strategy reproduces both sets of results.

Differences between brightness and color information for material identification

Systematic changes in cone-coordinates

Algorithms for simultaneous identification for Lambertian and non-Lambertian 3-D objects

•Three of the filters are identical while one is different.•Illumination on the right is skylight, on the left is sunlight.

Which is the odd filter?

FILTER IDENTIFICATION ACROSS ILLUMINANTS

Identical filters will cause systematically different spectral changes in two spectrally different lights.

Given information about the filtered and unfiltered lights, can an observer identify two filters with identical spectral transmittances?

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8NATURAL LIGHTS (Taylor & Kerr,1941)

WAVELENGTH(nm)

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Y-- sunlight

-- skylight

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1COLOR FILTERS (Kodak CC50)

WAVELENGTH(nm)

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--- red --- green --- blue --- yellow --- magenta--- cyan

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8SUNLIGHT THROUGH SIX FILTERS

WAVELENGTH(nm)

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--- red --- green --- blue --- yellow --- magenta--- cyan

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8SKYLIGHT THROUGH SIX FILTERS

WAVELENGTH(nm)

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--- red --- green --- blue --- yellow --- magenta--- cyan

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Means of Material Chromaticities

SDs of Material Chromaticities

36 EVERYDAY MATERIALS (VRHEL et al 1994)

•Changes in means are systematic across illuminants.

•Filters can increase or decrease the color variation.

•Three of the filters are identical while one is different.•Illumination on the right is skylight, on the left is sunlight.

Which is the odd filter?

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1PSYCHOMETRIC CURVES

DELTA (CYAN VERSUS RED)

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SIDE CORRECT & FILTER CORRECT CURVES

•Method of constant stimuli•DELTA is distance between standard and distractor.•6 standards x 5 tests x 2 daylights x 6 deltas x 10 reps

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IDENTIFICATION VS DISCRIMINATION

FILTERS

RESULTS

Identification thresholds were generally similar to discrimination thresholds and were significantly greater only in 5%, 10%, 13%, and 23% of 60 pairs for 4 observers.

Observers can identify filters across different illuminants almost as well as they can discriminate filters within the same illuminant.

MATERIAL IDENTIFICATION ACROSS ILLUMINANTS

X-junctions at the edges of the circular filters, in conjunction with the systematic changes in chromaticities of partially overlaid objects, promote a percept of transparency.

Are these results influenced by transparency. By rotating the image of the circular filtered regions by 180, we abolished the edge-based cues to transparency while leaving unchanged the figural and color contents inside and outside the filters.

•Three patches are from the same material while one is different.•Illumination on the right is skylight, on the left is sunlight.

Which is the odd material?

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IDENTIFICATION VS DISCRIMINATION

MATERIALS

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IDENTIFICATION THRESHOLDS

DISCRIMINATION THRESHOLDS

FILTERS VS MATERIALS

RESULTS

Disrupting the X-junctions and color relations at the edges of the filters did not appreciably affect discrimination or identification.

Systematic color changes at the edges of the filter and X-junction cues to filter transparency were not necessary for good discrimination or identification performance.

COLOR IDENTIFICATION ACROSS ILLUMINANTS

•Both experiments can be conceived as identification of a circular ensemble of materials across the two natural lights.

•Both experiments indicate that identification of ensembles of materials is almost as good as discrimination between ensembles of materials.

•The significant cues are the chromaticity shifts across illuminants.

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CORRELATED CHROMATICITY CHANGES

•L/(L+M) coordinates translate

•S/(L+M) coordinates scale

•Means can be used to estimate translation and scaling constants

•Simple algorithms can exploit the affine transforms for material identification

# Observers Standard Test # Observer Standard Test

X- T- (Sunlight) (Skylight) X- T- (Skylight) (Sunlight)

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2 1 RED BLUE 3 2 BLUE MAGENTA

2 2 BLUE MAGENTA 1 1 MAGENTA RED

2 3 MAGENTA BLUE 2 2 BLUE RED

2 0 RED MAGENTA 1 1 RED BLUE

0 1 YELLOW GREEN 0 3 RED MAGENTA

3 1 CYAN BLUE 1 2 GREEN YELLOW

1 1 BLUE YELLOW

PAIRS OF CONFUSED FILTERS

Are RED and BLUE confused because observers judge deviations from directions of color vectors better than lengths of color vectors?

Means of Material Chromaticities

•If the material statistics are constant, then color identification only requires comparing rank-orders of chromaticities.•If material statistics change, affine transforms will give correct identification, but rank orders will not.

Color identification across illuminants within the same scene

PERCEPTUAL STRATEGIES FOR COLOR IDENTIFICATION

Identification based on similarity of appearance is affected by adaptation.

Identification based on systematic shifts of

appearance is based on geometrical operations in perceptual color space.

Identification strategies rely on associative

invariants of the natural world.