CIS736-Lecture-20-20040310

1
Constructive Solid Geometry (CSG), Volume
Graphics, and Color Basics, concluded
2
Interactive Specification of Color
(1/2)

• English-language names
• ambiguous and subjective
• Numeric coordinates in color space with slide
  dials

Left Lab color picker from Adobe Photoshop for
Windows Below RGB color picker from Adobe
Photoshop for Mac OS
3
Interactive Specification of Color
(2/2)

• Interact with visual representation of the color
  space
• Important for user to see actual display with new
  color

HSB color picker from Adobe Photoshop
HSV color picker from Mac OS Xs Finder
4
Interpolating in Color Space (1/2)

• Interpolation needed for
• Gouraud shading
• antialiasing
• blending images together in a fade-in, fade-out
  sequence
• Results depend on the color model used
• RGB, CMY, YIQ, CIE are related by affine
  transformations, hence straight line (i.e.,
  interpolation paths) are maintained during
  mapping
• not so for HSV, HLS for example, interpolation
  between red and green in RGB
• interpolating in HSV
  
• midpoint values in RGB differ by 0.5 from same
  interpolation in HSV
• (60º, 1, 0.5) (60º, 1, 1)

red (1, 0, 0), green (0, 1, 0) midpoint
(0.5, 0.5, 0)
red (0º, 1, 1) green (120º, 1, 1) midpoint
(60º, 1, 1) RGB_to_HSV (60º, 1, 0.5)
5
Interpolating in Color Space (2/2)

• RGB, red is (1, 0, 0) and cyan is (0, 1, 1) which
  interpolate to (0.5, 0.5, 0.5), gray
• in HSV, that is (UNDEFINED, 0, 0.5)
• In HSV, red is (0º, 1, 1) and cyan is (180º, 1,
  1) which interpolates to (90º, 1, 1)
• new hue at maximum value and saturation, whereas
  the right result of combining equal amounts of
  complementary colors is a gray value
• Thus, interpolating, transforming
  transforming, interpolating
• For Gouraud shading (see Rendering unit), use any
  of the models because interpolants are generally
  so close together that interpolation paths are
  close together
• For blending two images, as in fade-in fade-out
  sequence or for antialiasing, colors may be quite
  distant
• use additive model, such as RGB
• If interpolating between two colors of fixed hue
  (or saturation), maintain fixed hue (saturation)
  for all interpolated colors by HSV or HLS
• note fixed-saturation interpolation in HSV or HLS
  is not seen as having exactly fixed saturation by
  viewer!

6
Using Color in
Computer Graphics

• Aesthetic uses
• establish a tone or mood
• promote realism
• Highlight
• Code numeric quantities
• temperature across the U.S.
• vegetation and mineral concentrations on Earth,
  moon, and planets (LandSat, Mars missions)
• speed of fluids in computational fluid dynamics
  (streamlines)

7
Using Color in Computer
Graphics Functionality

• Careless use of color is perilous
• in experiment, color reduced user performance by
  one-third
• Decorative use of color subservient to functional
  use
• test with real users
• provide best judgment defaults
• allow user to override defaults
• design first for a monochrome display (color use
  is redundant in monochrome displays and for
  color-blind users)

8
Using Color in Computer
Graphics Approach

• Color harmony
• select colors according to some method, typically
  by traversing a smooth path in a color model
• restrict colors to planes or hexcones in a color
  space
• space colors at equal perceptual distances
• light-dark contrast more important than hue
  contrast
• Specifics
• if a chart contains just a few colors, the
  complement of one of the colors should be used as
  the background
• use neutral (gray) background for an image with
  many colors
• separate non-harmonious colors by thin black
  border
• Coding
• be redundant (dual coding)
• show reference scale
• color can carry unintended meanings
• bright, saturated colors stand out (may give
  unintended emphasis)
• display elements of same color may incorrectly be
  associated by user

9
Using Color in Computer Graphics
Physiological Rules (1/2)

• Eye is more sensitive to spatial variation
• fine detail should vary from the background not
  just in chromaticity, but in brightness
• Blue and black, yellow and white are particularly
  bad combinations
• dont use blue for text
• For color-blind users, avoid reds and greens with
  low saturation and luminance
• Color of small objects less than 20-40 minutes of
  arc are not distinguishable

This is hard to read.
So is this.
And this, too! Yikes!
10
Using Color in Computer Graphics
Physiological Rules (2/2)

• Difficult to perceive color when used with small
  objects gt dont use color coding for them
• Perceived color of object is affected by color of
  surrounding area
• Strongly saturated colors produce after images
• Color affects perceived size
• red seen as larger than green
• colors refract differently through our lens
  appear at different depths, e.g., red (closer)
  vs. blue (farther) strongest effect
• Apply color conservatively, not gratuitously
• color-itis as bad as font-itis

11
Take Away Ideas

• To describe the character of light (ignoring
  phase) requires an infinity of amplitude
  wavelength data light spectra lie in an
  infinite-dimensional space
• Human perception, below certain intensity levels,
  is not at all linear
• At reasonable levels light perception is
  log-linear displays also not linear, influences
  color-table design
• Less is More in judicious use of color use
  some simple guidelines based on aesthetics and
  psychophysics

• If you are interested in learning more about
  color, check out Anne Morgan Spalters book, The
  Computer in the Visual Arts, available on
  Amazon.com

12
For More Information

• For more on color, check out the following web
  sites
• Color Matters
• http//www.colormatters.com
• Educational Color Applets
• http//www.cs.rit.edu/ncs/color/
• Gamma FAQ and Color FAQ
• http//www.inforamp.net/poynton/Poynton-colour.ht
  ml
• More Frequently Asked Questions about color
• http//www.ergogero.com/FAQ/cfaqhome.html
• info on color blindness and the way we interpret
  color
• Comprehensive List of Color Models
• http//www.colorsystem.com/
• Online book on color
• http//www.colorvoodoo.com/cvoodoo4.html
• Map Coloring
• http//www.personal.psu.edu/faculty/c/a/cab38/
• Get your own color cube!!!
• http//www.colorcube.com/intro.htm
• Excerpt from Thomas Youngs paper