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Computational Vision

CSCI 363, Fall 2012Lecture 33

Color

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Three types of cone photoreceptors

The retina has three types of cone photoreceptors. Each has a different pigment molecule that absorbs best at a given wavelength.

S (Short wavelength), "Blue"=>440 nm

M (Middle), "Green" => 530 nm

L (Long), "Red" => 560 nm

A single wavelength stimulus will stimulate each type of cone by some amount.A mixed color match is a set of stimuli that stimulate the cones by the same amounts as the single wavelength.

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1 cone cannot detect color

1 cone type: Cannot tell change in brightness from change in Hue.

2 cone types: Can distinguish some colors.

3 cone types: Can distinguish more colors.

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Color perception requires comparison of responses

A given wavelength of light will cause a set of responses in the three receptor types.

If the intensity changes, the ratio of responses between the photoreceptors will stay the same (but all will increase or decrease).

If the wavelength (or spectrum) changes, the ratio of responses between the photoreceptors will change.

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Color Opponency

Observation: Color appears to be organized in terms of 3 sets of complementary colors. (This was first established by Hering)

Red/Green Blue/Yellow White/Black

Evidence: 1) We don't see a Reddish-Green when mix Red and Green light. Instead, we see less red and green and more yellow.

2) Color adaptation: If you stare at a Green surface for a while, then look at a white background, it will appear red.

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Color Adaptation

Stare at the white dot

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Color Adaptation

Stare at the black dot

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Color Opponent Cells

Retinal Ganglion Cells and LGN cells have center-surround receptive fields that show color opponency.

Concentric single-opponent

(G+R)-

(G+R)+

(G+R)+

(G+R)-

Concentric Broadband

B+

(R+G)-

B-

(R+G)+

Co-extensivesingle-opponent

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Types of Single-opponent cells

Concentric single-opponent: Signal Red vs. Green. Also useful for edge detection.

Concentric broad-band: Useful for overall luminance changes (responsive to white light changes). Useful for edge detection.

Co-extensive single-opponent: Signal Blue vs. Yellow.

Blue cones are probably not involved in edge-detection because of chromatic aberration. The image of the blue wavelength light is more distorted on the retina than of red or green wavelength light.

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Responses of Single-opponent cells

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Color ContrastObservation: The appearance of a color depends on the surrounding colors.

There is only one shade of pink in this picture.

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Color ConstancySome surfaces appear the same color, even when the ambient lighting changes.

The wavelength of the blue squares on the left matches the wavelength of the yellow squares on the right!

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Color Constancy

This is the same picture as before, but the surrounding squares have been masked out to show the "true" color of the blue and yellow squares.

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Double-opponent cellsIn V1, many color sensitive cells are double-opponent (found in the blobs).

G+R-

G-R+

G-R+

G+R-

B+(G+R)-

B- (G+R)+

B-(G+R)+

B+ (G+R)-

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Responses of double-opponent cells

Small red or green spot

Red or green annulus

Large red or green spot

Spot inside annulus

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Retinex Theory

The perception of a color depends on the colors of all objects in the scene.

Edwin Land came up with Retinex Theory to predict what appearance colors would have for a given scene.

Retinex theory integrates colors across space by measuring the ratios of color changes across color borders.

Because we can only measure the relative luminance of surfaces, the retinex theory uses the brightest region in the scene as an anchor. It sets this region to white, and all other regions are computed relative to the anchor.

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Color BlindnessA small proportion of the population is missing one of the types of cones. Another small proportion has an anomalous pigment.

Trichromats: 3 pigments: Normal color visionDichromats: 2 pigments:

Red/Green color blind (1% of males and 0.01% of females)Missing red cones: protanopiaMissing green cones: deuteranopia

Blue/Yellow color blind (0.02% of males, 0.01% of females)

Color anomalies: One of the pigments has a different absorption spectrum.

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Colorblindness test

Can you see the numbers?