Chapter 6 Color

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Chapter 6Color

• Multimedia Systems

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Key Points

• Colour is a subjective sensation produced in the
  brain.
• The measurable physical quantity that best models
  colour is a spectral power distribution (SPD).
• In the RGB colour model, a colour is represented
  by three values, giving the proportions of red
  (R), green (G) and blue (B) light which make up
  light of the desired colour.

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Key Points

• The RGB colour gamut is a proper subset of the
  visible colours.
• The colour depth of an image is number of bits
  used to hold a colour value.
• Indexed colour The stored 8-bit value is used as
  an offset into a colour lookup table (CLUT) or
  palette containing up to 256 24-bit colour
  values.

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Key Points

• Dithering helps avoid the posterization that can
  occur when indexed colour is used, by using
  patterns of dots to simulate a wider range of
  colours using optical mixing.
• The CMYK model is important for printing, as it
  models the way colour is produced by mixing ink
  or paint.
• Other colour models include HSV, YUV and its
  variants, and the CIE device-independent spaces
  Lab and Luv.

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Key Points

• A 24-bit image can be thought of as being made up
  of three 8-bit channels, one each for red, green
  and blue in RGB colour.Colour correction etc.
  can be performed by using techniques from Chapter
  5 on individual channels.
• Colour management software uses device profiles
  to help maintain consistent colour.

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Color Science

• Color is a subjective sensation produced in the
  brain.

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Color Science

• Light is a form of electromagnetic radiation, we
  can measure its wavelength.
• http//micro.magnet.fsu.edu/primer/lightandcolor/i
  ndex.html

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Microwave Ovens

• The wavelength is tuned to frequencies that are
  readily absorbed by water molecules in food
  causing them to absorb energy and release heat as
  they vibrate.
• Containers holding the food in microwave ovens
  usually contain very little water and tend to
  remain cool.

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Color Science

• Visible light (???) 400nm700nm

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Color Science

• Spectral power distribution (SPD)

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Color Science

• Receptor Cells
• Rods night-vision and cannot distinguish color
• Cones in three sorts which respond to different
  wavelengths

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Cone Response
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Tristimulus Theory

• Any colors can be specified by just three values,
  giving weights of three components.
• Each type of cone responds to R, G, B

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Primary Colors

• Additive mixing
• TV, monitors
• Subtractive mixing
• Paint absorbs light

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Color Temperature

• The color temperature model is based on the
  relationship between the temperature of a
  theoretical standardized material, called a black
  body radiator, and the energy distribution of its
  emitted light as the radiator is brought to
  increasingly higher temperatures, measured in
  Kelvin (K).
• Although this radiator does not exist, many
  metals behave very similar to a black body so we
  can take a metal pot as an example for this
  discussion.

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Color Temperature
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RGB Colors

• RGB color model
• Not possible to represent all visible color as a
  combination of fixed R, G, B.

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Other Color Spaces
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Color Depth

• Color depth
• 24, 30, 36, 48 bits
• Grey scale image RGB
• PNG format up to 48 bits
• Scanner 48 bits
• Accurate approximations when reduced to a lower
  resolution for display
• Millions of colors, true color 24 bitsThousands
  of colors, hi-color 16 bits256 colors 8 bits
• Grey-level 256 8 bits

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Indexed Color

• Direct color
• Indexed color
• Palette of 256 colors
• Color lookup table (CLUT)
• Logical colors gt physical colors
• 24 bits color gt indexed color
• 1/3 of data
• PNG, BMP, TGA, TIFF paletteGIF only 256 colors
  JPEG 24 bit color
• Missing color the nearest color
• Visible artifacts
• Posterization gradations replace by sharp
  boundaries
• Web-safe Palette
• 216 colors, Displayed on any platform

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Posterization

• Area of a single color are replaced by a pattern
  of dots of several colors
• Pink red white
• Dithering high resolution
• Halftoning greyscale images
• N by N pixels N21 grey levels

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Other Color Models

• CMYK
• C GB W-RM RB W-GY RG W-B
• Complementary color
• Art and design
• The dyes do not supply color to light reflected
  off surface, but to light that penetrates through
  them and gets reflected or scattered back from
  beneath it.

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Mixing Colored Inks
White light
White light
Cyan light
Cyan light
Absorption of green light
Absorption of red light
Cyan ink
Magenta
Absorption of red light
Cyan ink
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• Not possible to manufacture inks which absorb
  only light of precisely the complementary
  color.Some unwanted colors are absorbed.
• RGB gamut gt CMY gamut
• CMM ! perfect black
• With black CMYK
• Hue, saturation, brightness
• As white is mixed in, saturation decreases.

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Color wheel
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HSV
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Color Pickers
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Color Differences

• Brightness(RGB)/3
• LuminanceY0.2125R 0.7154G 0.0721BGreen
  contributes far more
• Color differences
• UB-Y and VR-Y
• YUV
• YCBCR

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Device-independent Color Spaces

• RGB, CMYK device-dependent
• CIE XYZ device-independent
• Not perceptually uniformthe same change in one
  of vales produced the same change in
  appearanceR (1 to 11) ! (101 to 111)
• Perceptually uniform
• CIE Lab as in CMYK for prepress industry
• CIE Luv as in RGB for monitors and scanners

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Channels and Color Correction

• Color images R B C channels
• Each channel can be manipulated separately.
• Color correction
• Compensate for the deficiencies of scanners and
  other input devices
• Time-consuming
• Wizards, assistants

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Color Correction

• Color balance hue saturation adjustment
• Color balance
• 3 sliders CR, MG, YB

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• Replace a specified color with a different color
• CMYK printing YCBCR video
• Alpha channeldifferent from color channels
• Blue screen alpha blue channel
• Mask out background
• JPEG process each channel separately
• YCBCR downsample color difference

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Consistent Color

• Color profiles
• White(255,255,255)specified in a
  device-independent color space
• Gamma

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Gamma Correction
Sample Input
Graph of Input
Gamma Corrected Input
Graph of Correction L' L (1/2.5)
Monitor Output
Graph of Output
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Color Profile

• EPS, TIFF JFIF, PNG color profile
• At least R, G, B, white point, gamma
• Fig. 6.13

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ICC

• International Color Consortium
• Apples ColorSync
• Kodak Precision Color Management System
• TIFF, EPS complete ICC profiles
• sRGB standard RGB
• ITU-R BT.709 standard
• Gamma2.2 and white point 6500 K