Color

1
Color

• CSC361/661 -- Digital Media
• Burg/Wong

2
Color

• Light is electromagnetic energy in the 400- to
  700 nanometer wavelength part of the spectrum,
  perceived as the colors ranging through violet,
  indigo, blue, green, yellow, orange, and red.
• The colors that we see in the world around us are
  generally not pure colors consisting of a single
  wavelength. Rather, color sensation results from
  the dominant wavelength of the light reflecting
  off or emanating from an object.

3
Color Terminology

• Hue color
• Monochromatic color a color that is created
  from only one wavelength. (Most colors that we
  experience are NOT monochromatic. They result
  from a combination of wavelengths. The dominant
  wavelength gives us our color sensation.)
• Chrominance color information
• Luminance lightness or brightness information
• Additive color systems based on adding colored
  light (as in computer monitors). A combination
  of all colors gives white.
• Subtractive color systems based on adding
  pigments (as in printing). A combination of all
  colors gives black.

4
Human Perception of Color

• To humans, color sensation is a matter of
  subjective perception resulting from the effect
  of light on the cones of the eyes.
• There are three types of cones, each one with a
  particular sensitivity to red, green, or blue
  light.
• This decomposition of light into three color
  components is called the tristimulus theory of
  color and is the basis for the RGB color model.

5
Response of Cones in the Eyes

• This graphs shows the results of experiments in
  which the fraction of light absorbed by each type
  of cone is measured for each color in the visible
  spectrum. The green cone absorbs the most light.
  (Note that this is essentially the same graph as
  Figure 2.1 of the handout given in class, except
  that the units are different, eliminating the
  negative values.)

6
The Eyes Sensitivity to Different Colors

• This graph shows the eyes overall response as
  the dominant wavelength (i.e., the hue) is varied
  across the visible spectrum. (Luminance is kept
  constant.) The eyes sensitivity peaks at around
  550 nm, the wavelength of yellow-green light.
  Note that this graph is the sum of the three
  graphs in the previous slide.

7
Testing Color Response for RGB Model

• To create the RGB color model, it is necessary to
  determine how much of each color component is
  needed to create all the dominant wavelengths in
  the visible spectrum.
• This is done by projecting pure colors onto one
  screen, mixing amounts of R, G, and B on a
  neighboring screen, and asking a large number of
  people to say when the colors match.
• Matching color C is expressed by
• C RR GG BB

8
RGB Color Matching

• This graph shows the amounts of red, green, and
  blue light needed by an average observer to match
  color samples as they vary across the spectrum.
  (Luminance is kept constant.) A negative value
  means it is not possible to match the original
  color with RGB as primaries, so some R, G, or B
  has to be added to the original color sample.

9
RGB Color on a Computer

• The RGB color model works well for computers
  because it matches the technology of monitors.
• On a color monitor, color is produced by exciting
  three adjacent dots made of red, green, and blue
  phosphors. Because the dots are so small, they
  are blended into one color by the eye. Note that
  the color is not blended by putting one color of
  light over another it is blended by the eye.

10
RGB Color on a Computer

• Not all colors perceivable by humans can be shown
  on a computer screen with the RGB model.
• The same RGB values will not necessarily result
  in the same colors on two different monitors
  because monitors are not calibrated to a single
  standard.
• RGB colors are not pure, saturated colors. This
  is because the kind of light emitted by an
  excited phosphor is not of a single wavelength,
  but has a spectral power distribution over a band
  of frequencies.

11
RGB Color on a Computer

• RGB is perceptually non-linear. This means that
  equal differences in the RGB values do not
  correspond to equal differences in the perceived
  color. Low RGB values produce small changes in
  color on the screen (as you move from one low
  value to the next). Large RGB values produce
  very perceivable differences as you move from one
  high RGB value to the next.

12
CIE Color

• The earlier graph showing RGB color matching
  demonstrates that not all visible colors can be
  represented with RGB as primaries. (You cant
  add positive amounts of RGB to get all the
  colors. In some cases, you have to take some
  color away from the original sample being
  matched.)
• The Commission Internationale de lEclairage
  (CIE) decided that we need a standard color model
  that is based on primaries which, when mixed
  together, produce all the visible colors.

13
CIE Standardization

• Another motivation for the CIE model is that the
  RGB and CMYK models are device dependent. That
  is different monitors use different R, G, and B
  colors of phosphors. Different printers use
  different CMYK colors of ink. CIE
  standardization gives us a way to map between
  systems.

14
CIE Color Primaries

• The CIE color primaries X, Y, and Z replace R, G,
  and B. X, Y, and Z are artificial primaries,
  not visible colors like R, G, and B.
• These primaries can be combined in various
  proportions to produce all the colors the human
  eye can see.

15
CIE Color Matching

• This graph shows the amounts of X, Y, and Z light
  needed by an average observer to match color
  samples as they vary across the spectrum. Notice
  that no negative values are needed.

16
CIE Color Space

• The graph on the previous slide shows how X, Y,
  and Z can be combined to create any visible
  color, where all the colors in the spectrum are
  considered at the same luminance.
• This graph shows the entire CIE color space,
  where not only the color but the luminance
  varies.
• In the CIE color model,a color C is given by
• C XX YY ZZ

17
CIE Color Model on the XYZ 1 Plane

• If we want to consider each component as a
  percentage of the total amount of light, we can
  normalize the values

Note X Y Z is the total amount of light
energy. Also note that x y z 1
18
CIE Color Space

• This graph shows the amounts of X, Y, and Z
  needed for all colors in the visible spectrum.
  The X Y Z 1 plane is shown as the triangle
  embedded in the graph.
• The x, y, and z computed on the last slide lie on
  the XYZ1 plane. It is convenient to consider
  this plane only, which effectively reduces our
  consideration to constant luminance.

19
CIE Chromaticity Diagram

• Picture the portion of the CIE color space that
  intersects the XYZ1 plane.
• Now picture projecting that part of the XYZ1
  plane down onto the X, Y plane.
• This is how the CIE Chromaticity Diagram is
  created (next slide).

20
CIE Chromaticity Diagram

• This graph represents the hue and saturation of
  all colors in the visible spectrum. This is
  chromaticity information.
• All perceivable colors with the same chromaticity
  but differet luminances map into the same point
  in this graph.
• The 100 percent spectrally pure colors are on the
  curved perimeter of the graph. The dot in the
  center represents the chromaticity of daylight
  (white light).

21
Dominant Wavelength on CIE Color Diagram

• To determine what the dominant wavelength of a
  color A is from the diagram, draw a line between
  C (white) and the closest perimeter. The dominant
  wavelength is at B.
• The degree of saturation is given by the
  proportion of segment AB to segment CB. The
  closer A is to the perimeter, the more saturated
  the color.

22
Color Gamuts Represented on CIE Diagram

• All colors on the line IJ can be created by
  additively mixing colors I and J all colors in
  the triangle IJK can be created by mixing colors
  I, J, and K.

23
RGB Color Gamut in Terms of CIE Diagram

• We can see how much of the visible spectrum is
  displayable on a computer monitor by looking at
  the gamut within the RGB diagram, represented by
  the triangle.
• Phosphors on a computer monitor have these
  approximate values
• red green blue
• x .61 .25 .15
• y .34 .62 .063

24
CMYK model

• CMYK is primarily a printing color model.
• Cyan, magenta, and yellow are called the
  subtractive primaries.
• In practice, cyan, magenta, and yellow dont
  produce all the colors needed for printing.
  Blacks come out muddy. So a pure black is added
  in. Thats the K.

25
CMYK Model

• Cyan, magenta, yellow, and black
• Cyan is white light with red taken out. C G
  B W - R
• Magenta is white light with green taken out. M
  R B W - G
• Yellow is white light with blue taken out. Y
  R G W - B

26
CMYK vs. RGB

• The colors printable with the CMYK model do not
  overlap exactly with the colors displayable on an
  RGB monitor, as represented by their respective
  gamuts within the CIE diagram.

27
Hue, Saturation, and Lightness

• One way to represent color is by dividing it into
  its hue, saturation, and lightness components.
• Hue (or color) is determined by the dominant
  wavelength.
• Saturation is a matter of how much white light is
  added in. The less white light, the more
  saturated the color.
• Lightness is how much black is in the color.
• Hue and saturation are elements of chrominance.
  Lightness is a matter of luminance.

28
HSV

• HSV stands for hue, saturation, and value (where
  value represents lightness or brightness).
• Some people find the HSV model more intuitive
  than RGB. It is easier to think of colors in
  terms of their hue, tint, and shade rather than
  as combinations of red, green, and blue.

29
YUV Color Model

• YUV is a general term that refers to any color
  model that has one luminance component (Y) and
  two chrominance (i.e., color) components (U and
  V). (Youll also see references to the YIQ
  model, which is the same thing.)
• YCBCR is a specific instance of a YUV model.

30
YUV Color Model

• YUV is a color model appropriate to color TV
  because it makes it possible to send the color
  information separate from the luminance
  information, so that signals for black and white
  vs. color TV are easily separated.
• YUV is also a good representation for
  compression, because some of the chrominance
  information can be thrown out without loss of
  quality in the picture (since the human eye is
  less sensitive to chrominance than luminance).