lecture3 color representation in computer graphics(Computer graphics tutorials)

CSC 406: Applied Computer
Graphics
Lecture 3:
Color Representation in computer
graphics.

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Color Representation:Color Representation:
 Scope:
 Introduction.
 Additive and subtractive models.
 The standard CIE chromaticity.

Color Representation:
 Color is visible light between 400nm and
700nm.
 Visible light frequencies range between ...
 Red = 4.3 x 1014 hertz (700nm)
 Violet = 7.5 x 1014 hertz (400nm)
 Humans have trichromatic color vision
 Three color values

Color Representation…
 In the eye we have just three distinct 'cone'
cells for detecting light energy.
 These respond to a band of wavelength centered
around red (600), green (560) and blue (440);
respectively.
 The bands overlap, so, for example green light
excites all three types.

 The consequence of this is that each type of
cell may be excited similarly from very
different energy distributions
 Any distribution of wavelengths will be
perceived by us as a single color,
 but two entirely different distributions of
intensities could be perceived as the same color.

 In order to match a color, we can adjust the
brightness of 3 overlapping primaries until the two
colors look the same.
 C = color to be matched
 RGB = laser sources (R=700nm, G=546nm, B=435nm)
 Selecting three pure light sources (R, G, B) and
mixing them together while varying their respective
intensities, one may be able to create a large
number of colors.

 Each color satisfies the following linear
combination:
 X = r*R + g*G + b*B
 where r, g, b are intensities of the red, green and blue light
sources.
 Not all colors can be matched in this way.
 However, by adding one of the pure colors to an
unknown unmatchable color, we can make a match.

 This is in effect subtracting a color from the
mix.
 X + r*R = g*G + b*B
 or X + g*G = r*R + b*B
 or X + b*B = r*R + g*G

 The representation of colors as a mixture of
three components is called the tri-stimulus
representation;
 very commonly used in monitors and other
active color devices.
 The pure colors used are red, green and blue,
and are referred to as the additive primary
colors.

lecture3 color representation in computer graphics(Computer graphics tutorials)

 The human eye can distinguish hundreds of thousands of
different colors.
 When two colors differ only in hue, the wavelength between
just noticeably different colors varies with the wavelength!
 More than 10 nm at the extremes of the spectrum
 Less than 2 nm around blue and yellow
 Most JND hues are within 4 nm.
 Altogether, the eye can distinguish about 128 fully saturated
hues
 Human eyes are less sensitive to hue changes in less
saturated light (not a surprise)

Complementary colors:
 White represents the presence of all three colors –
in equal quantities.
 Black represents the absence of the three colors.
 Two colors are said to be complementary if their
mix gives white.
 A color can be generated by either:
 Adding the primary colors – R, G, and B
 Removing their complimentary colors.

The additive color model:The additive color model:
 Red, green and blue are the additive
primaries.
 Colors can be generated by adding the
additive primaries to black.
 This is referred to as the additive color
model.

The subtractive color model:
 Primaries – cyan, magenta and yellow.
 Also called the CMY model.
 Colors are generated by subtracting the three
primaries from other colors.

The CIE Chromaticity:
 Represents colors in terms of three new
primaries: x, Y and Z
 Basically an additive scheme.

Practical Colour Displays:
 When a transformation is necessary from the CIE standard colour chart
to the colour produced on a specific display device, we must express the
primary colours of the display device by x and y quantities.
 Good quality monitors will be calibrated for the CIE colour chart.
 For example, a colour CRT monitor may have the following primary colour
sources:
x y z
Red 0.628 0.346 0.026
Green 0.268 0.588 0.144
Blue 0.150 0.07 0.780
 These three points define a triangle on the CIE chromaticity diagram, as
shown on diagram 10.5.
 Only the points inside this triangle may be reproduced by the display device

lecture3 color representation in computer graphics(Computer graphics tutorials)

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