Chapter 9 Flashcards
(24 cards)
Functions of colour perception
Color help us classify and identify objects.
Color facilitates perceptual organization of elements into objects.
Color vision may provide an evolutionary advantage in food foraging.
Relationship between predominant wavelengths reflected and colour percieved
WAVELENGTHS REFLECTED OR TRANSMITTED
Short blue
Medium Green
Long and medium Yellow
Long Red
Long, medium, and short White
, the color of things that are transparent, such as liquids, plastics, and glass, is created by selective transmission.
Selective transmission means that only some wavelengths pass through the object or substance (Figure 9.4c). For example, cranberry juice selectively transmits long-wavelength light and appears red
Selective reflection of long wavelengths = red
Equal reflection of all wavelength = white
Additive Colour Mixture
Mixing lights of different wavelengths
All wavelengths are available for the observer to see.
Superimposing blue and yellow lights leads to white.
Subtractive colour mixture
Mixing paints with different pigments
Additional pigments reflect fewer wavelengths.
Mixing blue and yellow leads to green
We can summarize the connection between wavelength and color as follows
Colors of light are associated with wavelengths in the visible spectrum.
■■ The colors of objects are associated with which wave- lengths are reflected (for opaque objects) or transmitted (for transparent objects).
■■ The colors that occur when we mix colors are also associated with which wavelengths are reflected into the eye. Mixing paints causes fewer wavelengths to be reflected (each paint subtracts wavelengths from the mixture); mixing lights causes more wavelengths to be reflected (each light adds wavelengths to the mixture).
Tri-chromatic theory
Normal human color vision
– 3 colors to match all samples
Cats, dogs and dichromats – 2 colors
HSV
We previously called colors like blue, green, and red chromatic colors. Another term for these colors is hues. Figure 9.8 shows a number of color patches, which we would describe as all having a red hue. What makes these colors appear different is their variation in the other two dimensions of color, saturation and value.
Saturation is determined by the amount of white that has been added to a particular hue. Moving from left to right in Figure 9.8, progressively more white has been added to each color patch and, as a result, saturation decreases. When hues become desaturated, they can take on a faded or washed-out appearance.
Value refers to the light-to-dark dimension of color. Moving down the columns in Figure 9.8, value decreases as the colors become darker.
color solid.
Another useful way to illustrate the relationship among hue, saturation, and value is to arrange colors systematically within a three-dimensional color space called a color solid. There are a number of different color solids, but we will focus on one example. Figure 9.9a depicts a cylindrical color solid called the HSV color solid, because its three dimensions are Hue, Saturation, and Value
the order of the hues around the cylinder matches the order of the colors in the visible spec- trum
Saturation is depicted by placing more saturated colors toward the outer edge of the cylinder and more desaturated colors toward the center. Value is rep- resented by the cylinder’s height, with lighter colors at the top and darker colors at the bottom. The color solid therefore cre- ates a coordinate system in which our perception of any color can be defined by hue, saturation, and value.
how differently colored lights will combine to produce new colors.
rranging colors geometrically not only shows how hue, saturation, and value are related, it also allows us to determine how differently colored lights will combine to produce new colors. For example, the color that would result from mixing
yellow and blue light can be determined by drawing a line that connects the yellow and blue hues
Any mixture of these two hues will fall on this line, with the exact location depending on the amount of each light added to the mixture. If we add some blue light to the yellow (75 percent yellow, 25 percent blue), the yellow light becomes less saturated. If we add even more blue light, so the blue and yellow lights are mixed equally (50 percent yellow, 50 percent blue) the result- ing color will fall on the midpoint of our line (aha, it’s white,
Researchers measured absorption spectra of visual pigments in receptors
They found pigments that responded maximally to:
Short wavelengths (419nm) dark blue
Medium wavelengths (531nm) green
Long wavelengths (558nm) yellow
Genetic differences for coding proteins for the three pigments (1980s).
Color perception is based on the response of the three different types of cones.
Responses vary depending on the wavelengths available.
Combinations of the responses across all three cone types lead to perception of all colors.
Color matching experiments show that colors that are perceptually similar (metamers) can be caused by different physical wavelengths.
, all visual pigments are made up of a large protein component called
called opsin and a small light-sensitive component called retinal. Differences in the structure of the long opsin part of the pigments are responsible for the three different absorp- tion spectra
The size of the cone symbolizes the size of the receptor’s response.
metamerism vs metamers.
which two physically different stimuli are perceptually iden- tical, is called metamerism, and the two identical fields in a color-matching experiment are called metamers
The reason metamers look alike is that they both result in the same pattern of response in the three cone receptors. For example, when the proportions of a 620-nm red light and a 530-nm green light are adjusted so the mixture matches the color of a 580-nm light, which looks yellow, the two mixed wave- lengths create the same pattern of activity in the cone receptors as the single 580-nm light
The 530-nm green light causes a large response in the M receptor, and the 620-nm red light causes a large response in the L receptor. Together, they result in a large response in the M and L receptors and a much smaller response in the S receptor. This is the pattern for yellow
Opponent-process theory
Proposed by Hering (1800s)
Color vision is caused by opposing responses generated by blue and yellow, and by green and red.
Color afterimages and simultaneous color contrast show the opposing pairings
Types of color blindness are red/green and blue/yellow.
These responses were initially believed to be the result of chemical reactions in the retina.
comparison of two theories
Each theory describes physiological mechanisms in the visual system
Trichromatic theory explains the responses of the cones in the retina.
Opponent-process theory explains neural response for cells connected to the cones further in the brain.
Color in the cortex
There is no single module for color perception.
Cortical cells in V1, and V4 respond to some wavelengths or have opponent responses.
These cells usually also respond to forms and orientations.
Cortical cells that respond to color may also respond to white.
Monochromats have
A very rare hereditary conditionwith only rods (no functioning cones)
only perceive in white, gray, and black tones
Poor visual acuity
Very sensitive eyes to bright light
Dichromats : 3 types, mostly males.
Color Constancy
Color constancy: perception of colors as relatively constant in spite of changing light sources
Sunlight has approximately equal amounts of energy at all visible wavelengths.
Tungsten lighting has more energy in the long-wavelengths.
Objects reflect different wavelengths from these two sources.
Experiment by Uchikawa et al.
Observers shown sheets of colored paper in three conditions:
Baseline: paper and observer in white light
Observer not adapted: paper illuminated by red light; observer by white
Observer adapted: paper and observer in red light
Color constancy
Effect of surroundings
Memory and color
Effect of surroundings
Color constancy works best when an object is surrounded by many colors.
Memory and color
Past knowledge of an object’s color can have an impact on color perception.
Adaptation changes color perception.
Memory and Color
Experiment by Hansen et al.
Observers saw photographs of fruits with characteristic colors against a gray background.
They adjusted the color of the fruit and a spot of light.
When the spot was adjusted to physically match the background, the spot appeared gray.
But when this was done for the fruits, they were still perceived as being slightly colored.
Lightness Constancy
Achromatic colors are perceived as remaining relatively constant.
Perception of lightness:
Is not related to the amount of light reflected by an object
Is related to the percentage of light reflected by an object
The ratio principle: two areas that reflect different amounts of light look the same if the ratios of their intensities are the same
This works when objects are evenly illuminated.
Lightness Perception Under Uneven Illumination
Lightness perception under uneven illumination
Perceptual system must distinguish between:
Reflectance edges: edges where the amount of light reflected changes between two surfaces
Illumination edges: edges where lighting of two surfaces changes
Information in shadows: system must determine that edge of a shadow is an illumination edge
System takes into account the meaningfulness of objects.
Penumbra of shadows signals an illumination edge.
