UNIT 50 - COLOR

• What is color?
• What gives an object its color?

• CIE
• Uniform color spaces

• Munsell color system

• RGB system
• HLS system
• HVC (hue, value, chroma) system

The slide set contains images to illustrate this unit (#31 to 40).

UNIT 50 - COLOR

Compiled with assistance from Jon Kimerling, Oregon State University

A. INTRODUCTION

What is color?

• a complex eye-brain response to electromagnetic radiation in the visible portion of the electromagnetic spectrum, commonly called "light"

• the average person perceives solar radiation from approximately 400 nm to 700 nm (1nm = 10-9m) in wavelength

• this range can be visualized as a series of six "spectral" colors grading from violet through red

• colors such as red cover a greater proportion of the spectrum than others such as yellow

• other colors are mixtures of these in varying proportions and a few colors, like fluorescent pink, are "non- spectral" since they cannot be created as a spectral mixture

• colors of most objects we see are a function of:
  • the spectral properties of the illumination source, i.e., the amount of light at each wavelength coming from the source
  • the ability of the object to reflect light at each wavelength, often graphically portrayed as a spectral reflectance curve
  • the sensitivity of the cones in our eyes to each wavelength

• a CRT generates color by selectively exciting dots of three different phosphors - red, green and blue
  • the spectral emittance characteristics of the phosphors and our sensitivity to light emitted by them determine the colors we see

• the gamut of a device is the range of colors which it is capable of generating
  • generally, it is difficult to match the gamuts of different devices or media (e.g. CRT and paper), so colors tend to change when an image is displayed on a different device or medium

• differences in spectral sensitivities of receptors in the eye's retina give us color vision
  • Maxwell trichromatic theory of color vision is based on the fact that cone cells in our retinas, termed b, c and q, are primarily sensitive to blue, green, and red light, respectively
  • color seen is a function of the relative amount of blue, green and red light striking the closely packed mixture of cone cells that, along with rod cells sensitive only to light intensity, form the retina

• visual signal transmission from the rod and cone cells appears not to be carried out by four different types of nerve fibre, but rather by nerve cells connected so as to

  produce only three different signals in the fibre

  • the Opponent Process theory of color vision postulates that these signals interact to produce four perceptually unique "pole" colors: blue, green, yellow, and red
  • all other colors will be seen as mixtures of these maximally discriminable "poles"

• color constancy refers to the ability of our visual system to adapt to light sources of different intensities and colors so that object colors remain the same
  • e.g. skier's experience of again seeing snow and trees as white and green after wearing goggles of a different color for a few seconds
  • e.g. colors on CRT monitors appear the same as the screen brightness is lowered
  • the Retinex theory, proposed by Edwin Land of Polaroid fame, explains color constancy by saying that our eyes do not function as cameras, since we do not perceive color by wavelength alone
    • our mind does not determine the color of an object in isolation, but by comparing the object with its surround and continually adjusting to light source differences so that the object color appears the same

• perceptual dimensions of color describe the three basic ways in which we see variation in color, that is, color varies by: 1. hue - the attribute of color whereby an area appears similar to an opponent process "pole" color (red, yellow, green, or blue) or a mixture of any two "pole" colors 2. lightness - the brightness of an area relative to the brightness of a similar area that appears white in color 3. chroma - the colorfulness of an area relative to the brightness of a similar area that appears white in color - the strength or weakness of a color

• light measuring devices called spectrophotometers are employed to measure the light reflected from or emitted by an object, giving data needed for physical color specification

  slide 31 - spectrophotometer for CRT screen

  • a spectrophotometer is a device that detects visible light either reflected from a surface lit by a "standard illuminant" or emitted by a CRT screen with a known "white point", disperses the light into a spectrum, and measures the amount of light at

    small wavelength intervals along the spectrum relative to the standard light source

  • amount of light per wavelength interval is recorded in graphical or digital form suitable for subsequent use in color specification systems

• methods of specifying color used in optics

• Commission International de l'Eclairage color system

• widely used

• allows precise numerical specification of color, based on spectrophotometric measurements
  • a numerical way to match colors to a standard and to determine color differences

• colors are defined by (x,y,Y) coordinates that give a location on a chromaticity diagram

  slide 32 - CIE chromaticity diagram

  • plotting the (x,y) coordinates of the color and the illumination source, one finds that a straight line drawn from the light source to the color and continued to the edge of the diagram gives the color's dominant wavelength, a numerical description of hue
  • the straight line distance on the chromaticity diagram between the light source and color, divided by the distance from the light source, through the color, and to the diagram edge gives the color's purity, which is similar in concept to chroma
  • the Y coordinate gives the color's luminosity, this being a mathematical counterpart to lightness

• the true nature of the CIE system is best illustrated by a three dimensional figure, that for process color printing and CRT monitors resembles a six sided crystal with black and white tips

  slide 33 - 3-D perspective diagram of CIE color gamut for process color printing

  • all colors that can be created by a display device, such as a plotter or CRT monitor, will fall within the boundaries of this type of solid figure, which normally encompasses only part of the entire CIE color space.

  slide 34 - the vertical dimension of the CIE color space

• equal differences in coordinates signify equal perceptual differences
  • desirable when color progressions are to be determined based on physical color measurements
  • the CIE (x,y,Y) system is not a uniform color space, but the related CIE (L*,u*,v*) color space is
  • (L*,u*,v*) is a non-linear transformation of (x,y,Y) coordinates

Munsell color system

• differs from CIE by being based on perceptual experiments to determine equal appearing steps of hue, value (perceived lightness), and chroma
  • color of a surface is determined by comparing it visually to a set of painted color chips
  • colors specified by 0-100 hue range, 0-10 value range, and 0-20+ chroma range
  • complex mathematical procedures exist to convert CIE to Munsell colors, based on a table look-up approach

  slide 35 - the Munsell color system

• color progressions for quantitative areal data displayed using the choropleth or dasymetric mapping method are often based on Munsell value and/or chroma steps, whereas qualitative data often are portrayed with a series of Munsell hues

• color CRT displays are fundamentally different from color printers and plotters
  • electrons in red, green, and blue (RGB) phosphor atoms are excited to higher energy levels by a moving electron beam, only to give off photons of the corresponding wavelengths upon return to their normal state after the beam has passed
  • the monitor screen is made up of hundreds of thousands of tiny red, green, and blue phosphors arranged as rows and columns of triads

• the RGB color system is closest to the physical design of monitors, since colors are specified by amounts of red, green, and blue which can be directly translated into the electron beam strengths to be delivered to each phosphor in a triad
  • system can be viewed as a cube with red, green, and blue axes

  slide 36 - RGB cube
  • cube corners are white, black, red, yellow, green, cyan, blue, and magenta
  • all possible RGB combinations are within the cube

• number of colors within the cube which are actually displayable depends upon the number of bit planes in the display driver or color monitor adaptor card
  • e.g. many adaptors (EGA, VGA) have 4 bit planes in normal modes, use 3 for colors, 1 for lightness - 3 colors give the eight corners of the RGB cube
  • 24 bit plane driver gives 224, or over 16 million different colors per pixel, organized so that there are 28 or 256 levels of red, green, and blue, plus all combinations thereof

  slide 37 - RGB cube diagram
  • (0,0,0) gives black, (255,255,255) gives white, and the 254 intermediate triplets form a progression of grey tones running diagonally through the cube
  • colors along the white-yellow [(255,255,255)- (255,255,0)], white-magenta and white-cyan cube edges, as well as diagonal rows from white to red, green, and blue form "tint" progressions
  • opponent process "pole" colors and mixtures thereof can be easily specified, since RGB components change smoothly
    • e.g. 254 gradations between blue (0,0,255) and green (0,255,0) can be created by holding red at 0, incrementing green by 1, and decrementing blue by 1

• Tektronix developed the HLS system to simplify the selection of color progressions similar to tints and shades

  slide 38 - HLS color solid

  • a double cone with the central axis forming a lightness progression identical to the black to white diagonal line through the RGB cube
  • hues are specified by angle, starting with blue at 0o and progressing around the perimeter in the same order as found in the CIE chromaticity diagram when its boundary is traversed counterclockwise
  • lightness and saturation vary from 0 to 1

• the triangular slice for each hue can also be viewed as a plane cut from the RGB cube and deformed into the HLS triangle

  slide 39 - RGB - HLS deformation

  • the transformation is linear, and hence simple equations can be used to transform HLS specifications into RGB values, and vice-versa.

slide 40 - HVC system
• Tektronix has worked for several years to develop a color specification system essentially identical to the Munsell, the HVC system being the end product
  • created by making spectrophotometric measurements of thousands of RGB combinations, determining the CIE chromaticity coordinates for each, transforming all (x,y,Y) coordinates to their (L*,u*,v*) counterparts, and determining equal increments of hue, value, and chroma in this uniform color space
  • closely resembles the Munsell system - an irregular solid with vertical axis forming the value scale
  • hues progress from 00 to 3600 around the axis, with red at 00
  • each vertical slice into the solid exposes a page of value-chroma combinations for a particular hue.

• the HVC-RGB transformation is far more difficult than the HLS-RGB, requiring a computer program of several hundred statements

Dent, B.D., 1985. Principles of Thematic Map Design, Addison- Wesley, Reading, MA, pp. 353-357.

Eastman, J.R. 1986. "Opponent Process Theory and Syntax for Qualitative Relationships in Quantitative Series," The American Cartographer. 13(4):324-333.

Hunt, R.W.G., 1987. Measuring Color, John Wiley & Sons, New York, pp. 1-102.

Murch, G.M. and J.M. Taylor, 1988. "Sensible Color," Computer Graphics World, July 1988:69-72.

Niblack, Wayne, 1986. An Introduction to Digital Image Processing. Prentice-Hall, Englewood Cliffs, NJ.

Robinson, A.H., R.D. Sale, J.L. Morrison, and P.C. Muehrcke, 1984. Elements of Cartography, 5th edition, John Wiley & Sons, New York, pp. 170-177.

EXAM AND DISCUSSION QUESTIONS

1. How has the Munsell color system been adapted for display screen color specification?

2. What is the relationship between bit planes and the number of colors possible on a CRT monitor?

3. How is it that we see objects as the same color under different sources of illumination?

4. Explain the relationship between physical, perceptual and CRT color specification schemes, and give examples of each.

5. Explain the meaning of the term "gamut", and the problems which occur because of differences in gamuts between different display devices and media.

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