MAE152
Computer Graphics for Scientists
and Engineers
Colors In Computer Graphics
How much red was she with anger ?!
•
•
•
•
•
glColor3d (1.0, 0.78, 0.78);
May be! It’s YOUR perception.
Can color be quantified ?
Can a color be uniquely defined ?
Is there a “common understanding” about colors ?
Why “color management”
Source, Object, Observer
Reflected light = color of object
The Eye
Rods and Cones
• Rods: sensitive to color intensity (black and
white sensitivity in dark)
• Cones: three types – S, M and L
From left to right, the curves above show
the sensitivity of the S, M, and L cones
to various wavelengths of visible light
CIE Color Matching Experiment
Basis for industrial color standards and “pointwise” color models.
Color Matching Experiment
Image courtesy Bill Freeman
CIE Experiment Result
• Three pure light
source: R = 700.0
nm, G = 546.1 nm,
B = 435.8 nm.
Color Matching Experiment
CIE Color Space
• 3 hypothetical light
sources, X, Y, and
Z, which yield
positive (why?)
matching curves
• Y: roughly
corresponds to
luminous efficiency
characteristic of
human eye
The CIE 1931 Standard Observer represents the
color perception of a "normal" person. The curves
show the intensity of X, Y, and Z values (akin to
cone response) for a given wavelength
Principle of Tri-chromaticity
In general :
m
T   wi Pi
i 1
Pi are primaries, wi are weights. Example :
P1  645.16 nm (R), P2  526.32 nm (G), P3  444.44 nm (B)
Color matching experiment s imply that 3 primaries are enough
3
T   wi Pi
i 1
Grassman’s Laws
Color Spaces
• Use color matching functions to define a
coordinate system for color.
• Each color can be assigned a triple of coordinates
with respect to some color space (e.g. RGB).
• Devices (monitors, printers, projectors) and
computers can communicate colors precisely.
U ( )  f1 ( ) P1  f 2 ( ) P2  f 3 ( ) P3
S ( )  w1 P1  w2 P2  w3 P3
wi   f i ( ) S ( )d
3D Tri-chromatic Space
CIE Chart
A qualitative rendering
of the CIE (x,y) space.
The blobby region
represents visible
colors. There are sets
of (x, y) coordinates
that don’t represent real
colors, because the
primaries are not real
lights (so that the color
matching functions
could be positive
everywhere).
Slide courtesy Forsyth and Ponce
A plot of the CIE (x,y)
space: The spectral
locus (the colors of
monochromatic lights)
and the black-body locus
(the colors of heated
black-bodies) is shown.
The range of typical
incandescent lighting is
also plotted.
Slide courtesy Forsyth and Ponce
Some Colour Gamuts
Undisplayable Colours
• Suppose XYZ colour computed, but not
displayable?
• Terminology
– Dominant wavelength
– Saturation
A Maxwell Triangle, with white in the centre
Maxwell Triangle, showing where the spectral cyan matches
The spectral locus and the resulting RGB colour matching functions
The CIE tristimulus values and there relation to the spectral locus.
Colour might not be displayable
• Falls outside of the triangle (its chromaticity
not displayable on this device)
– Might desaturate it, move it along line QW
until inside gamut (so dominant wavelength
invariant)
• Colour with luminance outside of
displayable range.
– Clip vector through the origin to the RGB cube
(chrominance invariant)
RGB Cube Mapped to XYZ Space
Market for Display Technologies
TOTAL WORLD MARKET ( $27.8 BILLION )
CRT (58.7%)
LCD (35.7%)
Vacuum Fluorescent (2.3%)
LED (1.7%)
Plasma (1.1%)
EL (0.5%)
Cathode Ray Tube
Electron beam
Deflection yoke
Funnel
Face Plate
Electron gun
Phosphor screen
S hadow mask
Base
Neck
Convergence
magnet
Color Shadow Mask CRT
In-Line
Electron
guns
Green
Three-beam
electron
gun
Blue
Aperature
Red
Dot
Triad
B
R
G
Metal
mask
G
B
R
G
R
G
B
R
B
R
G
B
G
B
R
G
R
G
B
R
R
G
G
GR
R
B
Metal
strips
Phosphors on
glass faceplate
B
RG
Phosphors
on glass
faceplate
B
Color CRT Phosphor Pattern
Versus Spot Size
R
G
Dot
Pitch
G
R
G
R
B
R
B
G
R
G
G
B
R
B
R
B
R
B
G
B
Spot Size
G
B
R
Raster Display
• TV boom made it
cheap
• Entire screen painted
30 times/ sec
• Screen is traversed 60
times/ sec
• Even/ Odd lines on
alternate scans,
‘interlace’.
Pro/Con for Raster CRT Display
• Advantages
– Allows solids to be
displayed
– Leverages low- cost
CRT H/W
– Whole Screen is
constantly updated
•Disadvantages
•Requires screen- sized memory array
(frame buffer)
•Discrete spatial sampling (pixels)
•Moire patterns: when shadow- mask
and dot- pitch frequencies mismatch
•Convergence (varying angles of
approach distance of e-beam across CRT
face)
•Limit on practical size (< 40 inches)
•Spurious X- ray radiation
•Occupies a large volume
Color CRT
• Requires precision
geometry
• Patterned phosphors on
CRT face
• Aligned metal shadow
mask
• Three electron guns
• Less bright than
monochrome CRTs
Combining Colors
Additive (RGB)
Shining colored lights
on a white ball
Subtractive (CMYK)
Mixing paint colors and
illuminating with white light
Additive and subtractive color system
(r,g,b) RGB = (1,1,1) – (c,m,y)CMY
Maping (r,g,b) = (x,y,z)
2.739
-1.145
-.424
-1.110
2.029
0.033
.138
-.333
1.105
Raster Displays
•
•
•
•
Display synchronized with CRT sweep
Special memory for screen update
Pixels are the discrete elements displayed
Generally, updates are visible
Double Buffer
•
•
•
•
Adds a second frame buffer
Swaps during vertical blanking
Updates are invisible
Costly
Color Depth = 1 is black and 0 is white
Color Index scheme
Color Index Example
True Color
True Color Example
The RGB Cube
The HSB or HSL scheme
Primary Colors
Secondary Colors
Tertiary colors
The SV Grid
HSV Picker
HSB Color Picker
CMYK Color Picker
Metamerism
The same samples
shown in Figure 6 as
they appear under
incandescent room
lighting.
The samples represented above
appear identical under a D65 light
source, which contains little light in
the longer-wavelength end of the
visible spectrum
Dithering
The process of approximating colors you
don't have by mixing colors you do have.
100% of the pixels are a mixture 50% of the pixels are 100% red,
of 50% red and 50% white
50% of the pixels are 100% white
Color Model in OpenGL