M2 Lecture 10: Feb 14 Flashcards
(41 cards)
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
