Transcript of Copyright @ 2002 by Jim X. Chen: jchen@cs.gmu.edu 1 Understand brightness, intensity, eye...
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- Copyright @ 2002 by Jim X. Chen: jchen@cs.gmu.edu 1 Understand
brightness, intensity, eye characteristics, and gamma correction,
halftone technology, Understand general usage of color
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- Copyright @ 2002 by Jim X. Chen: jchen@cs.gmu.edu 2 Quantity of
light physics sense of energy -- intensity and luminance
psychological sense (perceived intensity) -- brightness Intensity
and Brightness They are related but are not the same. Checkout the
3- way switch, you will go from 50watt to 100, and 100 to 150, but
the brightness are levels are not even. ACHROMATIC LIGHT
(Grayscale)
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- Copyright @ 2002 by Jim X. Chen: jchen@cs.gmu.edu Gamma
correction Characteristic of the eye: it is sensitive to ratios of
intensity levels rather than to absolute values of intensity. On a
brightness scale, the differences between intensities of 0.1 and
0.11 and of 0.5 and 0.55 are equal. Brightness is called perceived
intensity. Sometimes, without confusion, simply intensity. 3
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- Copyright @ 2002 by Jim X. Chen: jchen@cs.gmu.edu The minimum
attainable intensity I 0 for a CRT is anywhere from about 1/200 to
1/40 of the maximum intensity of 1.0. 4 To find 256 perceived
intensities starting from lowest I 0 to a maximum of 1: And in
general for n+1 intensities:
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- Copyright @ 2002 by Jim X. Chen: jchen@cs.gmu.edu 5 Dynamic
range -- the ratio between the maximum and minimum intensities (1/I
0 ), the bigger the better. The intensity is related to the number
of electrons N in a CRT I = kN where k & are constants; is
between 2.2 to 2.5 N is proportional to V (the control-grid
voltage), so for another constant K: I = KV
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- Copyright @ 2002 by Jim X. Chen: jchen@cs.gmu.edu 6 Given a
desired intensity I, we can determine the voltage (intensity)
needed in the hardware: V j = ROUND((I j /K) 1/ ) And we know that
I = KV ; V = (I/K) Therefore, gamma correction means:
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- Copyright @ 2002 by Jim X. Chen: jchen@cs.gmu.edu 7 The values
of K, , and I 0 depend on the CRT in use, so in practice the
look-up table is loaded by a method based on actual measurement of
intensities. Use of the look-up table in this general manner is
also called gamma correction. If the display has hardware gamma
correction, then I j rather than V j is placed in entry j of the
look-up table or refresh buffer.
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- Copyright @ 2002 by Jim X. Chen: jchen@cs.gmu.edu 8 How many
intensities are enough? when r < 1.01, the eye cannot
distinguish between intensities I j,I j+1. Thus the appropriate
value for n, the number of intensity levels: r = 1.01 = (1/ I 0 )
1/n It depends on the lowest intensityvalue I 0. If I 0 = 1/200, n
= log 1.01 200 = 532
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- Copyright @ 2002 by Jim X. Chen: jchen@cs.gmu.edu 9 Halftone
Approximation Spatial integration -- if we view a very small area
from a sufficiently large viewing distance, our eyes average fine
detail within the area and record only the overall intensity. An
n*n group of bi-level pixels can provide n 2 +1 intensity levels
using halftoning technique. It is a trade-off between spatial
resolution and intensity resolution.
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- Copyright @ 2002 by Jim X. Chen: jchen@cs.gmu.edu10 The pixel
patterns to approximate the halftones must be designed not to
introduce visual artifacts in an area of identical intensity
values: a) form agrowth sequence so that any pixel intensified for
intensity level j is also intensified for all levels k>j. b) The
patterns must grow outward from the center. c) For certain hardware
system, all pixels that are on must be adjacent to other on
pixels.
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- Copyright @ 2002 by Jim X. Chen: jchen@cs.gmu.edu 11 Halftone
approximation is not limited to bi- level displays. For each point,
we can have Multiple level of intensities. Error diffusion: the
error is added to the values of the four image-array pixels to the
right of and below the pixel in question (7/16 of the error to the
pixel to the right, 3/16 to the pixel below and to the left, 5/16
to the pixel immediately below, and 1/16 to the pixel below and to
the right.) Dither matrix: to display an intensity I, we turn on
all pixels whose values are < I
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- Copyright @ 2002 by Jim X. Chen: jchen@cs.gmu.edu12 CHROMATIC
LIGHT Discussions of color perception: Hue -- distinguishes among
colors such as red, green, and yellow. Saturation -- refers to how
far color is from a gray of equal intensity. Red is highly
saturated; pink is relatively unsaturated; unsaturated colors
include more white light than do the vivid, saturated colors.
Brightness (Lightness) -- perceived intensity In graphic design
profession, colors are specified by matching to printed color
samples.
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- Copyright @ 2002 by Jim X. Chen: jchen@cs.gmu.edu13 The
percentage of pigments that must be mixed to match a color can be
used as a color specification. tints -- results from adding white
pigment to a pure pigment shade -- comes from adding a black
pigment to a pure pigment tone -- is the consequence of adding both
black and white pigments to a pure pigment Pure color Shades Black
White Grays tints Artists often specify color as different tints,
shades, and tones of saturated, or pure, pigments
(subjective).
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- Copyright @ 2002 by Jim X. Chen:
jchen@cs.gmu.edu14Psychophysics The above color specifications are
subjective: human observers judgements, the lighting, the size of
the sample, the surrounding color, etc. Light is electromagnetic
energy in the 400- to 700-nm wavelength part of the spectrum, which
is perceived as the colors from violet through indigo, blue, green,
yellow, and orange to red. The amount of energy present at each
wavelength is represented by a spectral energy distribution.
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- Copyright @ 2002 by Jim X. Chen: jchen@cs.gmu.edu15 A
quantitative way of specifying color: colorimetry The above
wavelength and energy distribution corresponds to a light. The
distribution represents an infinity of numbers, one for each
wavelength in the visible spectrum. A pure color is 100% saturated,
containing no white light. White light and grays are 0% saturated,
containing no color of any dominant wavelength.) Dominant
wavelength -- is the wavelength of the color we see; corresponds to
the perceptual notion of hue Excitation purity -- corresponds to
the saturation of the color Luminance -- corresponds to the
intensity (brightness, lightness)
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- Copyright @ 2002 by Jim X. Chen: jchen@cs.gmu.edu16 We can
describe the visual effect of any spectral distribution by dominant
wavelength, excitation purity, and luminance. e1=e2: excitation
purity=0; e1=0: excitation purity=100%. The dominant wavelength may
not be the one whose component in the spectral distribution is
largest. Two spectral energy distributions that look the same are
called metamers.
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- Copyright @ 2002 by Jim X. Chen: jchen@cs.gmu.edu17 Tristimulus
theory of color perception: the retina has 3 kinds of color sensors
(cones), with peak sensitivity to R, G, or B lights
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- Copyright @ 2002 by Jim X. Chen: jchen@cs.gmu.edu18 The
luminous-efficiency function, the eyes response to light of
constant luminance, as the dominant wavelength is varied from 400
to 700: our peak sensitivity is to yellow-green light of wavelength
around 550. Tristimulus theory of color perception: the retina has
3 kinds of color sensors (cones), with peak sensitivity to R, G, or
B lights
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- Copyright @ 2002 by Jim X. Chen: jchen@cs.gmu.edu19 A negative
value means if one of the primaries is added to the color sample,
the sample (after addition) can then be matched by a mixture of the
other two primaries. Colors can be specified by positively weighted
sums of red, green, and blue (the so-called primary colors). This
notion is almost true.
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- Copyright @ 2002 by Jim X. Chen: jchen@cs.gmu.edu20 Certain
colors cannot be produced by RGB mixes, and hence cannot be shown
on an CRT. Our eye can distinguish side-by-side colors. When colors
differ only in hue, the wavelength between just noticeably
different colors varies (mostly within 4 nm) 400 700 nm nm 2
Wavelength 4 10 8 6 Cant tell the difference Very
distinguishable
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- Copyright @ 2002 by Jim X. Chen: jchen@cs.gmu.edu Color
Measurement Any color can be matched using a combination of three
primaries. The primaries are not necessarily red, green, and blue.
Any three different colors can be used. The range of colors that
can be produced from a given set of primaries is the gamut.
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- Copyright @ 2002 by Jim X. Chen: jchen@cs.gmu.edu22 The CIE
Chromaticity Diagram In 1931, the Commission Internationale de
lEclairage (CIE) defined three matching primaries, called X, Y, Z,
to replace the RGB.
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- Copyright @ 2002 by Jim X. Chen: jchen@cs.gmu.edu Color
standard CIE (Commission Internationale dclairage) Primaries chosen
for mathematical properties: do not actually correspond to colors.
These virtual colors X, Y, and Z are called tristimulus values. Y
is the same as luminance
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- Copyright @ 2002 by Jim X. Chen: jchen@cs.gmu.edu24 The
primaries can be used to match, with only positive weights, all the
colors we can see. Y matches the luminous-efficiency function The
CIE chromaticity diagram, the projection onto the (X,Y) plane of
the X+Y+Z=1 plane Chromaticity values depend only on dominant
wavelength and saturation, and are independent of the amount of
luminous energy (luminance). The amounts of X, Y, and Z primaries
needed to match a color with a spectral energy distribution P( ),
are: k is a constant chosen according to the engery distribution
P
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- Copyright @ 2002 by Jim X. Chen: jchen@cs.gmu.edu For every
wavelength in spectrum, calculate (X,Y,Z) from CIE color matching
functions From (X,Y,Z), calculate (x, y) Plot (x,y) for all
wavelengths in spectrum Generates a horseshoe shaped diagram All
physical colors lie inside the horseshoe
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- Copyright @ 2002 by Jim X. Chen: jchen@cs.gmu.edu Chromaticity
Diagram
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- Copyright @ 2002 by Jim X. Chen: jchen@cs.gmu.edu Artist s
Rendition of Chromaticity Diagram All physical colors inside or on
boundary Monochromatic wavelengths on boundary White light near (x,
y) = (1/3, 1/3)
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- Copyright @ 2002 by Jim X. Chen: jchen@cs.gmu.edu Barycentric
Color System I.e., center of gravity 2 colors: P and Q Combine P
and Q in different amounts Can generate any color on straight line
connecting P and Q
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- Copyright @ 2002 by Jim X. Chen: jchen@cs.gmu.edu Dominant
Wavelength and Purity Dominant wavelength Draw line from white
point through the (x,y) point Extend line to boundary D Purity
Percentage of distance from white point to edge. Purity is 0% at
white point Purity is 100% at boundary
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- Copyright @ 2002 by Jim X. Chen: jchen@cs.gmu.edu Dominant
Wavelength Example White point at (0.33, 0.33) (x,y) = (0.2, 0.6)
Draw line from white point through point Extend it to boundary D =
515 nm Purity 55% 45% white light + 55% 515 nm light
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- Copyright @ 2002 by Jim X. Chen: jchen@cs.gmu.edu Complementary
Wavelength P and Q are complementary Line passes through white
point I.e., combination of light from P and Q can give white
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- Copyright @ 2002 by Jim X. Chen: jchen@cs.gmu.edu Color Gamuts
Any three colors form a triangle Combinations of 3 colors must lie
inside triangle. Why? 1 2 3 Physical Region
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- Copyright @ 2002 by Jim X. Chen: jchen@cs.gmu.edu Color Gamuts
and Color Reproduction Best color reproduction Use biggest color
gamut True for all media, print, monitor, film, slides No 3
primaries can reproduce human vision Human Vision
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- Copyright @ 2002 by Jim X. Chen: jchen@cs.gmu.edu34 The
interior and boundary of the horseshoe-shaped region represent all
visible chromaticity values. (All perceivable colors in 3D with the
same chromaticity but different luminances map into the same point
within this region in 2D.) The 100% spectrally pure colors of the
spectrum are on the curved part of the boundary. A standard white
light, meant to approximate sunlight, is formally defined by a
light source illuminant C, marked by the center dot.
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- Copyright @ 2002 by Jim X. Chen: jchen@cs.gmu.edu35 It allows
us to measure the dominant wavelength and excitation purity of any
color by matching the color with a mixture of the three CIE
primaries. A=B+C; AC/BC is the excitation purity of A; B is the
dominant wavelength Complementary colors are those that can be
mixed to produce white light (D and E). The CIE chromaticity
diagram is useful in many ways:
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- Copyright @ 2002 by Jim X. Chen: jchen@cs.gmu.edu36 Nonspectral
F, no dominant wavelength; B is the complement dominant wavelength.
CF/CG is the excitation purity. Take a flat spectral distribution
and delete some of the light at frequency B, the resulting color
will be perceived as F. The CIE chromaticity diagram is useful in
many ways:
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- Copyright @ 2002 by Jim X. Chen: jchen@cs.gmu.edu37 Color
gamuts (ranges), show the effect of adding colors together. I and J
can be added to produce color between I and J; A third color K can
be used with I and J to produce the gamut of all colors in triangle
IJK. The CIE chromaticity diagram is useful in many ways:
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- Copyright @ 2002 by Jim X. Chen: jchen@cs.gmu.edu Examples of
Gamuts