11-12. color (phys/percep factors)

Question 1

Chromatic aberration?

Answer 1

• in a photo, it creates a thin green line on one side of an object and red line on the other side
• light rays of diff. wavelengths focus on slightly diff. parts of the image sensor

Question 2

Spectral power distribution def?

Answer 2

• describes the INTENSITY (power) of a light at each wavelength emitted by a SOURCE (ex. florescent bulb)
• ALSO used to describe ILLUMINATION when talking about color constancy and the light (wavelengths) that’s actually being reflected by an object

Question 3

Spectral reflectance def?

Answer 3

• proportion of light at each wavelength that the object will reflect (instead of absorb)
• what KINDS of light does the object reflect if illuminated?

Question 4

What does the spectral reflectance curves look like for black, white, and grey? How do they differ?

Answer 4

• they are all approximately horizontal
  –> reflect same % of all wavelengths
• differ in OVERALL AMOUNT of light reflected
  –> black reflects only 20% of all light
  – curve is really low on graph
  –> white reflects 80% or more
  – curve is really high on graph

Question 5

Is color a property? What 2 things are properties?

Answer 5

• Color is NOT a property
• Spectral Power Distribution of a light source IS a property
• Spectral Reflectance IS a property

Question 6

Monochromatic light? Ex?

Answer 6

Light that consists of only one wavelength
ex. laser

Question 7

3 PHYSICAL parameters of light?

Answer 7

1. wavelength
2. intensity
3. spectral composition

Question 8

3 psychological correlates?

Answer 8

1. Hue (color)
2. Brightness
3. Saturation

Question 9

What does saturation measure? What colors are high/low sat?

Answer 9

• Measures richness
• rich reds/greens/etc. –> HIGH
• pastels –> LOW
• greys (blacks&whites)
  –> completely UNSAT

Question 10

What type of color mixture for light? For paint?

Answer 10

additive = light
subtractive = paints & filters

Question 11

How do additive mixtures work?

Answer 11

• most light we see is not monochromatic –> its a mixture
• a mixture contains ALL wavelengths present in individual COMPONENTS

Question 12

How can we measure additive mixtures?

Answer 12

For computers…
- amounts of R/G/B light are measured from 0-255
- if you know the amount of R, G, & B you want, it will give you the color

Question 13

On the color wheel… mixing 2 lights will fall…?

Answer 13

on the line segment connecting the 2

Question 14

On the color wheel… mixing 3 lights will fall…?

Answer 14

within the triangle defined by the 3

Question 15

Primary colors def?

Answer 15

Any three colors that can be combined in different proportions to produce a range of other colors.

Question 16

Complementary colors def?

Answer 16

Pairs of colors (opposite sides) that, when combined in equal proportion, are perceived as a shade of gray.

Ex. 50% yellow and 50% blue
–> 25% G, 25% R, and 50% blue

Question 17

How do subtractive mixtures work?

Answer 17

paints:
- the only wavelengths reflected by mixture are those that are reflected by ALL COMPONENTS
- components always absorb/reflect the same things
- mixture color is the wavelengths that both components reflect
EX:
blue & yellow paint –> only color they both reflect is green

Question 18

How do sub. mixtures work in filters?

Answer 18

• the wavelengths in “mixture” / result are those that pass through every filter in use

EX.
white –> yellow (G&R pass, not B)
–> magenta (R passes, not G)
result: red light on wall

(when white light passes through a filter, all wavelengths NOT in the color of the filter get absorbed)

Question 19

What physical and perceptual factors does color perception depend on?

Answer 19

PHYSICAL:
- SPD of light source illuminating the object
- Spectral reflectance of the object

PERCEPTUAL:
1. Adaptation after-effects
2. (Simultaneous) Color contrast
3. Color assimilation
4. Color constancy

Question 20

Color adaptation after-effects (after-images) explanation?

Answer 20

• you become adapted by staring at an image for a long time. When you switch to staring at a blank/white/grey box, you see a version of the image where pairs of colors have switched
  –> R switches with G
  –> Y switches with B
• suggests that the perceptual system inherently links pairs of colors (Y/B & R/G)

ex. 4 colors in boxes, american flag

Question 21

(Simultaneous) color contrast explanation?

Answer 21

• target color (usually grey) seems tinged with the COMPLEMENTARY color of the surround

EX. (12. pg 9)
- Y diamond in B background looks normal yellow
- Y diamond in Y background looks bluish

Question 22

Color assimilation explanation? AKA?

Answer 22

• AKA spreading effect
• perception of a color is shifted towards a surrounding color
  –> not necessarily a complementary color

EX. (12. pg 11, esp. 12)
- green surrounded by red looks reddish, or brighter green
- green surrounded by blue looks blueish, or muted green

Question 23

Color constancy explanation? Why?

Answer 23

• the color we perceive remains constant regardless of illumination
• if identifying color is important for survival (ex food), you need perception to be the same in diff lighting (ex. noon vs dusk)

Question 24

What is white balance?

Answer 24

• a camera’s way of figuring out what “white” is in the image, so it can decide what the illumination is
• if the camera knows what the illumination is, it can better represent the colors in the image

Question 25

Best explanation for why people argue ab “the dress”?

Answer 25

• the photo allows for a variety of perceptual interpretations about the illumination of the dress
• observers make different implicit assumptions about illumination –> we perceive color differently

Question 26

Chromatic adaptation as an explanation for color constancy means…?

Answer 26

• if you’re OVEREXPOSED to a certain wavelength, you adapt to it –> overcorrect –> NOT AS SENSITIVE
• you don’t perceive those wavelengths as well as others

ex.
- stare at screen with cyan | yellow
- image of plane below (tinged with cyan | yellow illumination) looks normal