M2 Lecture 10: Feb 14 Flashcards
what is Opponent color theory
The theory that perception of color depends on the output of three mechanisms, each of them based on an opponency between two colors: red–green, blue–yellow, and black–white
Ewald Hering (1834–1918) noticed that some color combinations are “legal” while others are “illegal.” explain
We can have bluish green (cyan), reddish yellow (orange), or bluish red (purple).
We cannot have reddish green or bluish yellow
explain Hue cancellation experiments
Start with a color, such as bluish green.
The goal is to end up with pure blue.
Shine some red light to cancel out the green light.
Adjust the intensity of the red light until there is no sign of either green or red in the blue patch
If you do hue cancellation across the entire spectrum, there are certain colors that …
can’t be “cancelled” by adding red/green or yellow/blue
what is Unique hue:
Any of four colors that can be described with only a single color term: red, yellow, green, blue.
For instance, unique blue is a blue that has no red or green tint.
Note: “unique red” has a longer spectral locus
Cones in general are very sensitive to the intensity of the light, for a wide range of wavelengths.
-> The absolute level of activity of a particular cone type is therefore relatively what
uninformative (i.e. “there is a lot of light”).
The difference of activity between types of cones is what
much more informative
what are the equations to determine what wavelength you are actually seeing
[L – M] or [M – L] compute red vs. green.
- [L + M] – S or S – [L + M] compute blue vs. yellow. - [L – S] or [S – L] and [M – S] or [S – M] are not that informative because L and M are so similar and are not used - [L + M + S] encodes general brightness
what are Afterimages
A visual image seen after a stimulus has been removed.
afterimages are caused by what
habituation (chromatic adaptation) of activated cones
what is Negative afterimage
An afterimage whose polarity is the opposite of the original stimulus.
Light stimuli produce dark negative afterimages.
Colors are complementary. Red produces green afterimages and blue produces yellow afterimages (and vice versa).
This is a way to see opponent colors in action
what is Color constancy
The tendency of a surface to appear the same color under a fairly wide range of illuminants.
To achieve color constancy, we must what
discount the illuminant and determine what the true color of a surface is regardless of how it appears.
what is Illuminant
The light that illuminates a surface.
For the same surface, the pattern of activation of S, M and L cones can vary greatly depending on the what.
illuminant
How is constancy achieved?
- Discounting the illuminant
2. Deduction
what is Discounting the illuminant
- Calculating the mean spectrum across the visual field and subtracting it from the pattern of cone activity.
- Habituation (chromatic adapation): the cones activated by the illuminant spectrum get “tired”
what is Deduction
- Intelligent guesses about the illuminant
- Assumptions about light sources
- Assumptions about surfaces
what is Euclidian geometry
Parallel lines remain parallel as they are extended in space.
Objects maintain the same size and shape as they move around in space.
Internal angles of a triangle always add up to 180 degrees, etc.
are the images projected onto the retina are Euclidean!
no
Therefore, our brains work with non-Euclidean geometry all the time, even though we are not aware of it.
(Euclidean ) line that are parrallel in the external world, converge towards the ____ in our retinal image of the world.
horizon
(Euclidean ) Objects that are the same size in the external world,_____ with distance in our retinal image of the world.
shrink
How do we perceive space?
Monocular cues
Binocular Vision and Stereopsis
what are the types of monocular cues
Optic flow Metrical depth cue Nonmetrical depth cue: Relative size Familiar size Relative height texture gradient Relative metrical depth cue: linear perspective Vanishing point motion parlax
what is Occlusion:
A cue to relative depth order in which, for example, one object partially obstructs the view of another object.
most reliable of all of the depth cues (only other possibility is what
accidental viewpoint
what are the types of Occlusion
Metrical depth cue
Nonmetrical depth cue
what is Metrical depth cue
A depth cue that provides quantitative information about distance in the third dimension.
what is Nonmetrical depth cue:
A depth cue that provides information about the depth order (relative depth) but not depth magnitude.
what is Relative size
A comparison of size between items without knowing the absolute size of either one.
All things being equal, we assume that smaller objects are farther away from us than larger objects.
what is Familiar size
A comparison of size between items when knowing the absolute size one of the items.
In this case, we can infer the absolute distance because we know the exact size of one of the objects.
what is Relative height:
For objects touching the ground, those higher in the visual field appear to be farther away. In the sky above the horizon, objects lower in the visual field appear to be farther away.
what is Texture gradient:
A depth cue based on the geometric fact that items of the same size form smaller, closer spaced images the farther away they get.
Texture gradients result from a combination of the cues of relative size and relative height.
The metrical cues of relative size and height can give the visual system more or less information than a nonmetrical cue like occlusion can.
more
what is Relative metrical depth cue:
A depth cue that could specify, for example, that object A is twice as far away as object B without providing information about the absolute distance to either A or B.
what is Linear perspective
Lines that are parallel in the three-dimensional world will appear to converge in a two-dimensional image as they extend into the distance
what is Vanishing point:
The apparent point at which parallel lines receding in depth converge.
Linear perspective also result from what
a combination of the cues of relative size and relative height.
what is Aerial perspective:
A depth cue based on the implicit understanding that light is scattered by the atmosphere.
More light is scattered when we look through more atmosphere.
Thus, more distant objects appear fainter, bluer, and less distinct.
what is Motion parallax
Images closer to the observer move faster across the visual field than images farther away.
The brain uses this information to calculate the distances of objects in the environment.
Head movements and any other relative movements between observers and objects reveal motion parallax cues.
what is Optic flow
the apparent motion of objects in a visual scene produced by the relative motion between the observer and the scene.
- Objects that are closer to the observer will move more rapidly.
- Objects in the focus of expansion won’t move
