Color

2014, Fall

Colorimetry: Definition (1/2)

Colorimetry Light is perceived in the visible band

• from 380 to 780 nm

distribution of wavelengths (light's spectrum) Human distinguish 10 million different colors three different types of cone( 원추체 ) receptors in the retina Standard condition for measuring color

• Proposed by CIE(Commission Internationale d’Eclairage)

Retina ( 망막 )

Lens( 수정체 )

Human Eye

Retina( 망막 ) The photosensitive part of the eye composed of two types of cells, called rods and cones Only the cones are responsible for color perception. Cones are most densely packed within a region of the

eye called the fovea.

Wavelength vs. Frequency 사이클 (cycle) : 일정한 시간 간격마다 반복되는 동일한 모양 주기 (period) : 한 사이클이 걸리는 시간 ( 주파수와 역수관계 ) 주파수 (Frequency): 1 초에 사이클이 몇번 반복되는가 ? 파장 (Wavelength) : 한 사이클의 간격

Frequency

Wavelength = C / Frequency

C= 3*108 m/s ( 빛의속도 )the speed of light

( 300 million meters per second)

Colorimetry: Definition (2/2)

긴 파장( 저주파 )

짧은 파장( 고주파 )

Colorimetry: Visible Light

Visible light from 380 to 780 nm The lowest visible frequency : red the highest : violet. White light is an even distribution of all visible frequencies. Rainbows and prisms divide white light into the colors of the

spectrum. Black : the absence of light

Colorimetry: Color Model (RGB Color)

Color Matching (Color Models) RGB Color Model (Fig12-11)

• Primary colors: RED, GREEN, BLUE.

• Secondary colors: YELLOW = red + green, CYAN = green + blue, MAGENTA = blue + red.

• WHITE = red + green + blue.

• BLACK = no light.

• Disadv– cannot directly represent all

visible colors (negative value)

빛의 삼원색( 가산혼합 )

Colorimetry: Color Model (Grayscale)

Grayscale• BLACK = 0% brightness, 100% grey.

• WHITE = 100% brightness, 0% grey.

• NTSC phosphors (older)– Y=0.30R+0.59G+0.11B

• CRT and HDTV phosphors (modern)– Y=0.2125R+0.7154G+0.0721B

Colorimetry: Color Model (Paint Color)

Paint colors Primary colors:

• red, yellow, blue Secondary colors:

• orange = red + yellow,green = yellow + blue,purple = blue + red.

All colors:• red + yellow + blue = muddy brown.

Black and white are separate colors, which may be mixed with others to make lighter or darker shades or tones.

Colorimetry: Color Model (CIE XYZ Color)

CIE XYZ Color Model defined three new hypothetical light sources, x, y, and z,

which yield positive matching curves

Colorimetry: Color Model (CIE XYZ Color)

chromaticity diagram ( 색도도 )• curved line color of the

spectrum

• purple line line connecting the ends of the spectrum

• white point x=y=z=1/3

• Saturation( 채도 )– The relative distance of the

color point compared to the distance to the edge of the region

• Hue( 색상 )– the point on the region edge

Colorimetry: Color Model (CIE XYZ Color)

Gamut

• specify ranges of colors that can be produced by a particular model

Colorimetry: Color Model (CIE XYZ Color)

Disadvantage• the 2D diagram failed to give a uniformly-spaced visual

representation of what is actually a three-dimensional color space

Colorimetry: Color Model (CIE LUV)

CIE LUV

CIE LUV CIE LU’V’

Colorimetry: Color Model (CIE LAB)

CIE LAB retinal color stimuli are translated into distinctions

• between light and dark

• between red and green

• between blue and yellow.

CIE LAB indicates these values

with three axes: L*, a*, and b*.

Colorimetry: Color Model (HSV)

HSV (=HSB) Hue, Saturation, Value (=Brightness) HUE( 색상 ) : the actual color.

• measured in angular degrees around the cone

• Ex) red = 0 or 360 (so yellow = 60, green = 120, etc.).

SATURATION( 채도 ) : the purity of the color

• measured in percent from the center of the cone (0) to the surface (100).

• At 0% saturation, hue is meaningless. BRIGHTNESS( 명도 )

• measured in percent from black (0) to white (100).

• At 0% brightness, both hue and saturation are meaningless.

색의 삼속성( 삼요소 )

Colorimetry: Color Model (HLS)

HLS Hue, Lightness, Saturation (similar to the HSV cone)

• the primary colors located at L = 0.5

• the colors of black and white acting as ends of the cones.

Colorimetry: Color Model (CMYK)

CMYK Primary colors

• CYAN, MAGENTA, and YELLOW. Secondary colors

• BLUE = cyan + magenta• RED = magenta + yellow• GREEN = yellow + cyan.

BLACK• = cyan + magenta + yellow (in theory). • BLACK (K) INK is used in addition to C,M,Y to produce

solid black. WHITE

• no color (on white paper, of course). Standard Color Printer

색의 삼원색( 감산혼합 )

Colorimetry: Color Model (YIQ)

YIQ Used by US commercial color television broadcasting (Used

by NTSC standard) Y: encodes luminance I, Q: encode color (chromaticity) For black and white TV, only the Y channel is used People are more sensitive to the illuminance difference

• We can use more bits (bandwidth) to encode Y

• and less bits to encode I and Q

Color Model Applet: http://www.cs.rit.edu/~ncs/color/

Color Models for Computer Graphics