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Color Constancy: Perception of Object Colors under Different Lighting Conditions - Prof. R, Study notes of Biology

Northern Illinois University (NIU)Biology
Prof. Richard Hahin

The concept of color constancy, which refers to the ability of the visual system to recognize the same color of an object under different lighting conditions. Experiments conducted by land and mondrian, the role of reflectance and lightness, and the biological basis of color constancy. It also covers the types of cells found in v1 and v4, and their roles in generating lightness records and perceiving color.

Typology: Study notes

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Uploaded on 08/18/2009

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Tovée, Chapter 7
Color Constancy
The color constancy problem
The visual system must recognize objects under different conditions
Object constancy-
Objects do not change size
If objects change size then--
Interpretation--the object moved closer to or farther from us
Lightness constancy-
Lightness of an object does not change as conditions change
Lightness is measured relative to background
Color constancy-
The color of objects do not change with lightning conditions
Color must not be determined soley by reflected light from object
The Land Mondrian experiments
Land experiments (Land, 1964)
Subjects view a multicolored display (Color Mondrian)
Display characteristics--
-Display had patches of different size, shape & color
Controls for learning and memory of object shapes & sizes
-No patch was surrounded by another of a single color
-Patches surrounding another patch differed in color
Controls for induced colors and color contrast
-Patches were matte
Reflects light so that it can be seen at different angles
Lighting
-Illuminated by 3 projectors with filters for Red, Green & Blue
-Intensity was measured using a photometer
Experiment 1
Light reflectance from a green patch when other patches are visible
60 units red
30 units green
10 units blue
Subjects observation ⇒ GREEN
Experiment 2
Light reflectance from a green patch when viewed in isolation
60 units red
30 units green
10 units blue
Subjects observation ⇒ RED
pf3
pf4

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Tovée, Chapter 7

Color Constancy

The color constancy problem The visual system must recognize objects under different conditions Object constancy- Objects do not change size If objects change size then-- Interpretation--the object moved closer to or farther from us Lightness constancy- Lightness of an object does not change as conditions change Lightness is measured relative to background Color constancy- The color of objects do not change with lightning conditions Color must not be determined soley by reflected light from object

The Land Mondrian experiments Land experiments (Land, 1964) Subjects view a multicolored display (Color Mondrian) Display characteristics-- -Display had patches of different size, shape & color Controls for learning and memory of object shapes & sizes -No patch was surrounded by another of a single color -Patches surrounding another patch differed in color Controls for induced colors and color contrast -Patches were matte Reflects light so that it can be seen at different angles Lighting -Illuminated by 3 projectors with filters for Red, Green & Blue -Intensity was measured using a photometer Experiment 1 Light reflectance from a green patch when other patches are visible 60 units red 30 units green 10 units blue Subjects observation ⇒ GREEN Experiment 2 Light reflectance from a green patch when viewed in isolation 60 units red 30 units green 10 units blue Subjects observation ⇒ RED

CONCLUSION--

Perceived color is not determined by dominant reflected wavelength Perceived color depends upon the colors of other nearby objects

Reflectance and lightness: the search for constancy in a changing world Reflectance of objects doesn’t change with illumination Red objects reflect red best Green objects reflect green best Relative reflectance for a particular wavelength is a measure of lightness Each scene will present a lightness record to the visual system (Zeki, 1993) Land’s retinex theory proposes-- ‹relative lightness records for three wavelengths = color ‹ d (^) i i mean i

( ) log

( ) ( )

λ

λ λ

=

Surface reflectance background reflectance where λi represents either λ 1 =green, λ 2 =red, or λ 3 =blue and d is the designator or lightness record

‹Color = a( d (λ 1 )) + b( d (λ 2 )) + c( d (λ 3 ))

The biological basis of color constancy Type of cells found in V1 blobs ‹Cells responsive to primary reflected wavelength (red, green, or blue) ‹Single opponent receptive field cells ‹Double opponent receptive field cells Double opponent cells are presumed to generate lightness records Red +^ Grn -^ center Grn +^ Red -^ surround Double opponent cells possess small receptive fields Thus- They can only act as lightness detectors for a small area of visual field Type of cells found in V Wavelength selective cells that correlate with perceived color V4 lesioned monkeys can not perceive color constancy V4 lesioned monkey can discriminate between wavelengths Spit-brain patients can not perceive color correctly in certain experiments Conditions-- -Present colored patch to one hemifield -Present background colored patches to opposite hemifield -Colored patch & background are separated by 3.5 o Normal subjects perceive color of patch -Split brain subjects can not perceive color