The painter's color wheel

• Primary colors - the ``pure'' component colors from which are mixed other colors in a given color system
  
  
• Secondary colors - equal mixtures of two primary colors
  
  
• Tertiary colors - unequal mixtures of two or more primary colors
  
  
• A painters primaries can be somewhat aribitrarily chosen, but redish, yellowish, and blueish color primaries plus black and white often form the core of a painter's pallette
  
  

Metamerism

• Combinations of different frequencies striking the cone cells of the retina cause the sensation of different colors.
  
  
  • e.g. the sensation of purple requires power in both the red and blue spectral regions

  
  

• Different spectral distributions can result in the same (or very similar) encoded signals being sent to the brain and hence cause the perception of the "same" color. This phenomenon is called metamerism.
  
  
• The principal of metamerism underlies color reproduction systems.
  
  
• Various spectral stimuli can be effected in a controlled manner by proportional mixing of primary colors - typically three, and typically chosen to take advantage of the "long", "medium" and "short" wavelength sensitivities of the cones of the retina.

CIE (Commission Internationale de L'Eclairage) XYZ tristimulus colors

• Spectral sampling / tristimulus color
  
  
• Human visual response is relatively consistent - "CIE standard observer" - standardized in 1931
  
  
• CIE XYZ primaries are "imaginary" primaries, abstracted from statistical results from experiments with human observers, able to represent any perceivable color
  
  
• CIE color gamut for representative human perception
  
  

R,G,B additive primaries

• R,G,B flashlight demo
• R,G,B color wheel
• R,G,B swatch demo

Raster graphics displays

• Phosphor triads
  
  
  
  
• Proportionally excited by voltages
• Emissive display
• Colors are "added" at source

RGB Color Space

• RGB - Red, Green, Blue
  
  • R,G,B color cube
  • R, G, B sliders
  • Hardware display oriented
  • Not terribly intuitive for mixing tertiary colors

  
  

Subtractive primaries

• C,M,Y color wheel
• C,M,Y sliders
  
  

• C,M,Y reflective surface
  
  

CMYK Color Space

• CMY(K) - Cyan, Magenta, Yellow, (Black)
  
  • C,M,Y color cube
  • Process printing oriented
  • Also not terribly intuitive beyond secondary colors

• C,M,Y/R,G,B slider demo, ideally:
  
  
  • C = RGB White - R
  • M = RGB White - G
  • Y = RGB White - B

  
  But this is not true in practice.

Printers and inks

• Cyan(C), Magenta(M), Yellow(Y), Black(K) inks
• Proportionally mixed through screen densities
• Reflective display
• A combination of incident illumination and reflective substrate (paper) provides the "white point"
• Colors are "subtracted" by the inks at the reflected source
• CMY are not ideal filters in that there is some overlap of spectral absorbtion among the three primaries
  
  
• Note that Pantone Matching System (PMS) is fundamentally different from CMYK process color. PMS colors are recipes for mixing standard printing inks of various colors (much like painter's colors). CMYK colors are proportional, screened percentages of coverage by the CMYK primary inks.

Perceived Attributes of Color

• Hue - the primary wavelength(s) of a color
  
• Lightness - the perceived luminance of a color
  
• Saturation - the purity or vividness of a color

``Perceptual'' Color Spaces

HLS - Hue, Lightness, Saturation

• Developed by Gerald Murch at Tektronix, also late 1970s
• Perhaps a ``truer'' representation than HSV (see below)
• Compare to RGB cube rotated

HSV - Hue, Saturation, Value

• Developed by Alvy Ray Smith, around 1978, after artist's nomenclature
• User oriented
• Compare to RGB = 1 planes from RGB cube
• Sometimes also referred to as HSB

Color difference / component

• YUV; Y,R-Y,B-Y
  
  
• Separates luma or luminance component from chroma
  
  
• Avoids ``crosstalk'' (cross-luma, cross-chroma) in video systems
  
  
• Human eye is more sensitive to luminance change than to chroma change
  
  
• Chroma channels may be subsampled for transmission or storage efficiencies
  
  
• Color difference or component schemes often are used in conjunction with digital video graphics
  devices or JPEG and MPEG compression schemes

Application and/or device specific

• Named color schemes, pen indices, etc.
• Simple for user to specify color
• Suitable for business graphics, for example, where precise color specification is unimportant
• Often difficult for user to predict or control color

Key Points

• Tristimulus color primary sytems have been derived to take advantage of the unique spectral sensitivities of the human visual processing system
  
  
• RGB and CMY(K) primaries are designed for use with specific and very different techologies (monitors and process-printing)
  
  
• User-oriented color spaces (HLS, HSV, named indices), while conceptually convenient for the user, ultimately will be transformed to RGB or CMYK during the display or printing process
  
  
• Ultimately, your monitor displays RGB/voltages/phosphors; raster printers use CMYK/inks; at some level(s) in your system colors are transformed from one color space and/or device profile to another

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