Color 1, Color 2, Depth&Size

Electromagnetic Spectrum for Humans

400nm-700nm

Why have color vision?

Object Discrimination

S-cone

Sensitive to short wavelengths “blue cone” (420nm)

M-cone

Sensitive to middle wavelengths “green cone” (535nm)

L-cone

Sensitive to long wavelengths “red cone” (565nm)

Spectral Sensitivity

Referring to the sensitivity of a cell or a device to different wavelengths on the electromagnetic spectrum

Photopic

Referring to light intensities that are bright enough to stimulate the cone receptors and bright enough to “saturate” the rod receptors (daytime)

Scotopic

Referring to light intensities that are bright enough to stimulate the rods but too dim to stimulate the cone receptors (nighttime)

Univariance

Sensitivity response is independent of wavelength. One photoreceptor type cannot make color discriminations based on wavelength.

Hue

Color

Saturation

Amount of white “mixed in” with the color

Brightness

Amount of light

Physics of Object ‘color’

Stimulus spectrum is product of illumination and reflectance: S(w)=E(w) x R(w)

Color Constancy

The tendency of a surface to appear the same color under a fairly large range of illuminations

Subtractive Color Mixture

Physics - A mixture of pigments. If pigments A and B mix, some of the light shining on the surface will be subtracted by A, and some by B only the remainder contribute to the perception of color.

Broadband Light

Large spectrum of light of the same wavelength

Additive Color Mixture

Perception - A mixture of lights. If light A and light B are both reflected from a surface to the eye, in the perception of color the effects of those two lights add together.

Red + Green + Blue

White

Red + Green

Yellow

Pointilism

PSF of the eye is averaging the colors so you don’t see the dots

Trichromacy

The theory that the color of any light is defined in our visual system by the relationships of the three cones

Color Matching - Additive Color MIxing

Adjusting the intensities of three lights at the same time (primary lights) (each primary light stimulates one cone type) can make it match the test light

Isomeric

Physically the same (color matching works)

Metameric

Physically not the same (color matching doesn’t work)

Color Matching Math

Response of a given photoreceptor type to a light of a given wavelength is the product of the sensitivity and the intensity of the light I(units) x S(w) = response (do for each cone type) (if match the lights would look the same for a given cone type)

Protanope

Absence of L cones, red-green color blind

Protanomaly

Weakened L cone response (curve shifts left closer to M cones)

Deuteranope

Absence of M cones, red-green color blind

Deuteranomoly

Weakened M cone response (curve shifts right to L cones)

Why are protan and deutran defects both “red green blind”?

‘red vs. green’ discrimination depends on a subtraction – if either L or M is missing, subtraction impossible

Tritanope

Absence of S cones, blue-yellow color blind

Tritanomoly

Weakened S cone response

Cone Monochromat

An individual with only one cone type. Truly color-blind.

Rod Monochromat

An individual with no cones of any type. In addition to being colorblind they are also visually impaired in bright light

Why are men more likely to be red/green colorblind?

Gene that controls colorblindness is on the x chromosome.

Why did the L and M cones diverge?

Need for red-green discrimination developed (need to be able to determine red fruit from green leaves)

Opponent Color Theory (Hering)

Perception of color is based on the output of three mechanisms, each of them resulting from an opponency between two colors: red-green, blue-yellow, and black-white (can see both but not at the same time)

Negative Afterimage

Polarity is the opposite of the original stimulus. Light stimuli produce dark negative image. Colors are complementary: red produces green, yellow produces blue

Color Contrast

A color perception effect in which the color of one region induces the opponent color in a neighboring region

Hue Cancellation Experiment

Starts with a color (bluish green) and attempts to determine how much of the opponent color (red) of one of the starting color’s components must be added to eliminate any hint of that component from the starting color (green)

Unique Hues

Any of four colors that can be described with only a single-color term: red, yellow, green, blue.

Unique Red

Need at least 2 wavelengths of light to create

Color Opponent Ganglion Cell

One region is excited by one color/cone type and inhibited by the opponent color/cones (R+/G-). Another adjacent region would be inhibited by the first input and excited by the second (R-/G+)

Chromatic Pathway

Antagonistic relationship where some retinal and LGN ganglion cells are excited by the L-cone onset in their center and inhibited by the M-cone onset in their surround (L-M) cells. There are also (M-L), and ([M+L]-S)

Luminance Pathway

Cells excited by light onset are thought of as (L+M) cells, fast intensity information

Color Constancy Exceptions

Film doesn’t account for illumination: only the visual system can adjust

Also fails in sodium light

Sodium Light

Narrow band of wavelengths

“The Dress”

Visual system interprets the illumination differently

Blue/Black: Shadow

White/Gold: Sunlight

Visual Angle

Tells you the size of an object 2 degreed of visual angle (“stand in” for retinal image size)

Monocular Cues

Information about depth that can be determined by one eye

Binocular Cues

Information about depth that relies on information from both eyes.

Occlusion

A cue to relative depth order in which one object obstructs the view of part of another object

Pictorial Cues

A cue to distance or depth used by artists to depict three-dimensional depth in two-dimensional pictures

Texture Gradient

A depth cue based on the fact that items of the same size form smaller images when they are farther away. An array of items that change in size smoothly across the image will appear to form a surface tilted in depth

Relative Height

As a depth cue, the observation that objects at different distances from the viewer on the ground plane will form images at different heights in the retinal image. Objects farther away will be seen as higher in the image.

Relative Size

A comparison of size between items without knowing the absolute size of either one.

Familiar Size

A depth cue based on knowledge of the typical size of objects like humans or pennies

Aerial/Atmospheric Perspective

A depth cue based on the understanding that light is scattered by the atmosphere. Thus, more distant objects are subject to more scatter and appear fainter, bluer, and less distinct.

Linear Perspective

A depth cue based on the fact that lines that are parallel in the three-dimensional world will appear to converge in a two-dimensional image.

Vanishing Point

The apparent point at which parallel lines receding in depth converge

Motion Parallax

The retinal image of objects closer to the observer move faster

Optic Flow

The pattern of apparent motion of objects in the visual scene produced by the relative motion between the observer and the scene

Deletion

Occurs when an object is occluded by another object, making it appear as if it is moving behind the occluder

Accretion

Occurs when an object reappears after being occluded, indicating it is moving in front of the occluder

Accomodation

The process by which the eye changes its focus (in which the lens gets fatter as gaze is directed toward nearer objects)

Convergence

The ability of the two eyes to turn inward, often used in order to place the two images of a feature in the world on corresponding location in the two retinal images. (visual angle increases)

Divergence

The ability of the two eyes to turn outward, often used in order to place the two images of a feature in the world on corresponding location in the two retinal images. (visual angle decreases)

Binocular Overlap

The combination of signals from each eye that makes performance on many tasks better with both eyes than with either eye alone

Two vantage points so the retinal images differ because the retinas are in slightly different places in your head

Illustrates how geometric regularities are exploited by the visual system to achieve stereopsis from binocular disparity.

Binocular Disparity

The differences between the two retinal images of the same scene. This is the basis of stereopsis, a vivid perception of the three-dimensionality of the world that is not available with monocular vision

Stereopsis

The ability to use binocular disparity as a cue to depth

Corresponding Retinal Points

Two monocular images of an object in the world are said to fall on corresponding points if those points are the same distance from the fovea in both eyes.

Horopter

Imaginary surface containing all objects that fall on corresponding positions

Retinal Disparity

Degree and direction of non-correspondence of the retinal images of an object

Light rays projecting from the brown (a) and purple (b) crayons onto Bob’s retinas as he continues to gaze at the red crayon

The red and blue crayons sit on the horopter and have zero disparity. They form retinal images in corresponding locations. The brown crayon forms images with a small binocular disparity. The purple crayon, farther from the horopter, has larger binocular disparity.

Visual system can detect whether the disparity is crossed or uncrossed, and also detect its magnitude

Crossed Disparity

The sign of disparity created by objects in front of the plane of fixation (the horopter). The images appear to be displaced to the left in the right eye and to the right in the left eye

Uncrossed Disparity

The sign of disparity created by objects behind the place of fixation (the horopter). The images appear to be displaced to the right in the right eye and to the left in the left eye

Stereoscope

A device for simultaneously presenting one image to one eye and another image to the other eye.

Free Fusion

The technique of converging or diverging the eyes in order to view a stereogram without a stereoscope

Random Dot Stereogram (RDS)

Made of large number of randomly placed dots. This contains no monocular cues to depth, but you can still see depth. (Julesz, 1950’s)

Correspondence Problem

In binocular vision the problem of figuring out which bit of the image in the left eye should be matched with which bit in the right eye. The problem occurs when the image consists of thousands of similar features like RDS and dots.

Uniqueness Constraint

The observation that a feature in the world is represented exactly once in each retinal image. (Simplifies the correspondence problem)

Continuity Constraint

The observation that, except at the edges of objects neighboring points in the world lie at similar distances from the viewer. (Simplifies the correspondence problem)

Bayes Rule

Formalizes the idea that perception is a combination of the current stimulus and our knowledge about the conditions of the world (the theorem allows us to calculate the probability that the world is in a particular state given a particular observation)

Size-Distance Relationships

1. For a given object, the visual angle/retinal image size is inversely proportional to the distance

2. visual angle (or retinal image size), distance, object size Knowing any two of these is sufficient to determine the third.

Size Constancy

In judging size, visual system “corrects for” distance or depth (size stays the same)

Increasing distance → smaller retinal image (the two effects cancel)