Light
Light is the part of the
electromagnetic spectrum that causes
a reaction in our visual systems
Generally these are wavelengths in
the range of about 350-750 nm
(nanometers)
Long wavelengths appear as reds and
short wavelengths as blues
Luminance and Color Images
Luminance
Monochromatic
Values are gray levels
Analogous to working with black and
white film or television
Color
Has perceptional attributes of hue,
saturation, and lightness
Do we have to match every frequency in
visible spectrum? No!
Three-Color Theory
Human visual system has two types of
sensors
Rods: monochromatic, night vision
Cones
Color sensitive
Three types of cone
Only three values (the tristimulus
values) are sent to the brain
Need only match these three values
Need only three primary colors
Additive and Subtractive Color
Additive color
Form a color by adding amounts of three
primaries
CRTs, projection systems, positive film
Primaries are Red (R), Green (G), Blue (B)
Subtractive color
Form a color by filtering white light with Cyan
(C), Magenta (M), and Yellow (Y) filters
Light-material interactions
Printing
Negative film
The RGB Color Model – for CRT
Blue=(0,0,1) Cyan=(0,1,1)
Magenta=(1,0,1)
White=(1,1,1)
Green=(0,1,0)
Black=(0,0,0)
Red=(1,0,0) Yellow=(1,1,0)
Color Depth
Can choose number of bits for each of r, g
and b
More bits per component means more colors can
be distinguished, but image files will be larger
8 bits (1 byte) per component: 24-bit color,
millions of colors
If r = g = b, color is a shade of gray, so
grayscale can be represented by a single
value
8 bits permits 256 grays
The CMY Color Model – for hardcopy
Yellow=(1,1,0) Red=(1,0,0)
Green=(0,1,0)
Black=(0,0,0)
Magenta=(1,0,1)
White=(1,1,1)
Cyan=(0,1,1) Blue=(0,0,1)
Undercolor Removal: CMYK System
Real inks do not correspond to ideal
subtractive primaries
Combining three inks for black is
undesirable
Printers use four process colors, cyan,
magenta, yellow and black
CMYK gamut is not the same as RGB
Implications for using images prepared
for print (CMYK) on the Web (RGB)
The CMYK Color Model – for hardcopy
C = G+B = W-R
M = R+B = W-G
Y = R+G = W-B
K = min(C,M,Y)
C C-K
M M-K
Y Y-K
The HSV Color Model – for user-oriented
Alternative way of specifying color
Hue (roughly, dominant wavelength)
Saturation (purity)
Value (brightness)
Model HSV as a cylinder: H angle, S
distance from axis, V distance along
axis
Basis of popular style of color picker
The HSV Color Model – for user-oriented
H : hue Green V Yellow
120°
S : saturation
1.0
V : value Cyan White 0°Red
(or B for blight) 240°
Blue Magenta
0.0 H
S
Black
Color Models in Video
Largely derive from older analog methods of
coding color for TV. Luminance is separated
from color information.
YIQ is used to transmit TV signals in North
America and Japan. This coding also makes
its way into VHS video tape coding in these
countries since video tape technologies also
use YIQ.
In Europe, video tape uses the PAL or SECAM
codings, which are based on TV that uses a
matrix transform called YUV.
Digital video mostly uses a matrix transform
called YCbCr that is closely related to YUV.
The YUV Color Model – for PAL video
Can be useful to separate brightness and
color information, especially for video.
Y is for luminance and U and V are for
chrominance which are stored as two
color difference values B-Y and R-Y.
⎡ Y ⎤ ⎡ 0.299 0.587 0.114 ⎤ ⎡ R ⎤
⎢U ⎥ = ⎢ −0.299 −0.587 0.886 ⎥ ⎢G ⎥
⎢ ⎥ ⎢ ⎥⎢ ⎥
⎢⎣V ⎥⎦ ⎢⎣ 0.701 −0.587 −0.114 ⎥⎦ ⎢⎣ B ⎥⎦
The YUV Color Model – for PAL video
For dealing with composite video, it
turns out to be convenient to contain
U and V within the range −1/3 to
+4/3. So U and V are rescaled:
U = 0.492111( B − Y )
V = 0.877283( R − Y )
The chrominance signal =
the composite signal C:
C = U ⋅ cos(ωt ) + V ⋅ sin(ωt )
The YIQ Color Model – for NTSC color-TV
Y : luminance
I and Q : chromaticity
(rotated version of U and V)
I = 0.492111( R − Y ) ⋅ cos 33D − 0.877283( B − Y ) ⋅ sin 33D
Q = 0.492111( R − Y ) ⋅ sin 33D + 0.877283( B − Y ) ⋅ cos 33D
⎡Y ⎤ ⎡0.299 0.587 0.114 ⎤ ⎡ R ⎤
⎢ I ⎥ = ⎢0.596 −0.275 −0.321⎥ ⎢G ⎥
⎢ ⎥ ⎢ ⎥⎢ ⎥
⎣⎢Q ⎦⎥ ⎣⎢0.212 −0.528 0.311 ⎥⎦ ⎢⎣ B ⎥⎦
The YCbCr Color Model – for digital video
Cb = (( B − Y ) /1.772) + 0.5
Cr = (( R − Y ) /1.402) + 0.5
⎡ Y ⎤ ⎡ 0.299 0.587 0.114 ⎤ ⎡ R ⎤ ⎡ 0 ⎤
⎢C ⎥ = ⎢ −0.168 −0.332 0.5 ⎥ ⎢G ⎥ + ⎢0.5⎥
⎢ b⎥ ⎢ ⎥⎢ ⎥ ⎢ ⎥
scaled to [0-255]
⎢⎣Cr ⎥⎦ ⎢⎣ 0.5 −0.418 −0.082 ⎥⎦ ⎢⎣ B ⎥⎦ ⎢⎣ 0.5⎥⎦
⎡ Y ⎤ ⎡ 65.481 128.553 24.966 ⎤ ⎡ R ⎤ ⎡ 16 ⎤
⎢C ⎥ = ⎢ −37.797 −74.203 112 ⎥ ⎢G ⎥ + ⎢128⎥
⎢ b⎥ ⎢ ⎥⎢ ⎥ ⎢ ⎥
⎣⎢Cr ⎦⎥ ⎢⎣ 112 −93.786 −18.214 ⎥⎦ ⎢⎣ B ⎥⎦ ⎢⎣128⎥⎦
