Lightness

1


CPSC 533C
Information Visualization
Lightness
Brightness


Constancy
Contrast
Ming Huan Lee
Feb. 3, 2003
2
Quantity of Light

• Luminance
• the measured amount of light coming from some
  region of space
• Brightness
• the perceived amount of light coming from a
  source (self-luminous)
• Lightness
• the perceived reflectance of a surface

3
Merriam-Webster

• Luminance
• the luminous intensity of a surface in a given
  direction per unit of projected area
• Brightness
• the attribute of light-source colours by which
  emitted light is ordered continuously from light
  to dark in correlation with its intensity
• Lightness
• the attribute of object colours by which the
  object appears to reflect or transmit more or
  less of the incident light

4
Agenda

• Eyes Receptive Fields DOG Model
• Simultaneous Contrast Errors in Reading Maps
  in Computer Graphics
• Lightness Constancy Adaptation Contrast
• Computer Monitor
• Conclusion

5
Our Eyes

• light-sensing receptors
• no info about amount of light (light meter)
• signal of differences (contrast mechanism)
• relative amount of light (change meter)
• differ from neighbouring / change in the past
  instant
• nonlinear, no absolute values (but not
  inaccurate)
• reflectance of surfaces

6
Human Vision

• perceive objects from surface properties, not
  from the quality and quantity of illumination
• lightness is unaffected by illumination ?
  lightness constancy

7
Receptive Fields

• the receptive field of a cell is the visual area
  over which a cell responds to light
• stimulate receptive field
• on-centre emits pulses at a greater rate
• off-centre emits pulses at a lower rate
  (inhibited)

8
DOG Model

• Difference of Gaussians (DOG)
• the firing rate of the cell is the difference
  between two Gaussians (centre and surround)
• DOG receptive field explain a variety of
  brightness contrast effects
• Hermann Grid more inhibition between squares,
  hence brighter at the intersections

9
Simultaneous Brightness Contrast

• the general effect whereby a grey patch placed on
  a dark background looks lighter than the same
  grey patch on a light background

cross-cross snake
10
Simultaneous Contrast Errors

• contrast effects vs. visualization
• errors of judgment while reading quantitative
  (value) info encoded using gray scale
• ex. in the worst case, found substantial errors
  that averaged 20 of the entire scale

Gravity Map of North Atlantic large errors can
occur when values are read using the key
11
Gray Scale

• no perfect gray scale
• providing only rough approximations
• because the visual field is radically changed by
  many factors
• perceived illumination
• specular reflection from glossy surfaces
• local contrast effects
• contrast crispening
• differences are perceived as larger when samples
  are similar to the background color

12
Computer Graphics

• computer graphics images (not like real world)
  consist of simple luminous light patches
• much of the detailed (high level) texture and
  shadow info (like real world) is missing
• leads to errors and distortions in info displays
• consequences of contrast effects
• tend to show up the deficiencies in the common
  shading algorithms used in computer graphics
• lead to perception of illusory patterns
• smooth surfaces displayed using polygons (for
  simplicity and to speed up computer graphics
  rendering process)
• visual system enhances boundaries at edges of
  polygons (stand out)

13
Lightness Constancy

• lightness is unaffected by illumination ?
  lightness constancy
• two important mechanisms in lightness constancy,
  which help the visual system to factor out the
  effects of the amount and color of the
  illumination
• adaptation
• contrast

14
Adaptation

• the role
• the changing sensitivity of the receptors and
  neurons in the eye helps factor out the overall
  level of illumination
• allow visual system to adjust overall sensitivity
  to the ambient light level
• example
• briefly blinded when coming into a darkened room
  out of bright sunlight

15
Contrast
(a)
(b)

• the role
• help to achieve constancy by signalling
  differences in light levels, especially at the
  edge of objects
• Example
• (a) receives more light, however, it reflects
  about the same amount of light as (b)
• simultaneous brightness contrast
• (a) is lighter relative to its background than
  (b) is, relative to its background

medium gray
white
16
Additional Factors

• although both adaptation and contrast can be seen
  as mechanisms that act in service of lightness
  constancy, they are not sufficient
• brain must take the direction of illumination and
  surface orientation into account in lightness
  judgements
• brain seems to use the lightest object in the
  scene as a kind of reference white to determine
  the grey values of all other objects

17
Additional Factors (Contd)

• the ratio of specular and nonspecular reflection
• in the all-dark world, the ratio is much larger
  than in the all-white world

Every point on the black image is brighter than
the surroundings. How can we perceive something
to be black when it is a bright image?
18
Computer Monitor

• contrast illusions are much worse in CRT
• no texture (uniform pattern of pixels and
  phosphor dots)
• self-luminous (confound lightness constancy)
• the colour and the brightness of the surround of
  the monitor can be very important in determining
  how screen objects appear
• the adaptation effect produced by room light
• overall contrast is much reduced where the room
  light falls on the display
• LCD?

19
Conclusion

• nervous system computing difference signals
• visualization is not good for representing
  precise absolute numerical values, but rather for
  displaying patterns of differences of changes
  over time, to which the eye and brain are
  extremely sensitive
• contrast effects are severe in computer displays
  as a consequence of the impoverished nature of
  those displays, not of any inadequacy of the
  visual system

20
Color Use Guidelines for Data Representation
21
Color Schemes

• make the most of the information visualization by
  using perceptual dimensions of color in ways that
  parallel the logical structures in the data to
  allow its organization to be readily perceived
• sequential
• qualitative and binary
• diverging
• spectral
• two-variable

22
Sequential Schemes

• suited to ordered data that progress from low to
  high
• lightness to represent ordered data
• with light colors for low data values to dark
  colors for high data values
• ex. a graphic showing pollution level

23
Qualitative and Binary Schemes

• do not magnitude differences between legend
  classes, and hues are use to create the primary
  difference between classes
• best suited to representing nominal or
  categorical data
• ex. government spending for different departments
• binary schemes
• special case for the two categories

24
Diverging Schemes

• put equal emphasis on mid-range critical values
  and extremes at both ends of the data range
• the critical class or break in the middle of the
  legend is emphasized with light colors, and low
  high extremes are emphasized with dark colors
  that have contrasting hues

ex. increases and decreases in acid rain over a
given time period
25
Spectral and Two-Variable Schemes

• spectral (rainbow)
• often misused as a sequential scheme with poorly
  used lightness differences
• the most informative use of a spectral scheme is
  as a diverging scheme

• two-variable
• sequential-sequential
• sequential-qualitative
• binary-qualitative
• binary-diverging
• sequential-diverging
• diverging-diverging

26
Conclusion

• matching the organization of the perceptual
  dimensions of color (hue, lightness, saturation)
  to the organization of data being represented is
  one key to gaining insight from data visualization