Color

Contents

• Light and color

• The visible light spectrum

• Primary and secondary colors

• Color spaces– RGB, CMY, YIQ, HLS, CIE– CIE XYZ, CIE xyY and CIE diagram

properties

What do we see?

• Visible electromagnetic radiation• The type of electromagnetic wave that is

visible to the human eye.• Electromagnetic radiation that has a

wavelength in the range from about 4,000 (violet) to about 7,700 (red) angstroms and may be perceived by the normal unaided human eye.

http://www.answers.com/color

Electromagnetic radiation

• Electromagnetic radiation is a kind of energy radiated in the form of a wave

• The frequency define the energy– E = h – And the hue: = c

Electromagnetic spectrum and the visible light spectrum

What is color?

• It is a sensation• “Color is actually light waves that hit our eyes,

translated into nerve impulses, and interpreted by our brains as all the various colors around us.”

• Three components: RGB• Cones

– Reds (low frequency)– Greens (medium frequency)– Blues (high frequency)

http://www.stlukeseye.com/images/illustrations/anatomy_globe.jpg

Mixing colors

Primary and secondary colors

Additive Colors

Subtractive Colors

Color spaces

• Systems to define colors numerically• Device oriented

– RGB– CMY– YIQ

• Human oriented– HSV– CIE

(1, 1, 0) Yellow

(0, 1, 0) Green

(1, 0, 0) Red

(1, 0, 1)Magenta

(0, 0, 1) Blue (0, 1, 1) Cyan

(1, 1, 1)White

(0, 0, 0)Black

RGB

• Additive model– Intensity of the three

components

RGB

• Features– It is used by hardware devices

• True color: 2563 = 16.777.216

– It is a standard on computing• bgcolor="#FFFFFF”

• Problems– Their variations are not lineal– It isn’t intuitive

(1, 1, 0) Blue

(0, 1, 0) Magenta

(1, 0, 0) Cyan

(0, 0, 1) Yellow (0, 1, 1) Red

(1, 1, 1)Black

(0, 0, 0)White

(1, 0, 1)Green

CMY

• Subtractive model

CMY

• Complementary of RGB[CMY] = [1,1,1] - [RGB]

• Used in printing

• Use of CMYK– Black (K) is added

HSV

• Projection of RGB diagram about the biggest diagonal

• Polar coordinates– Hue

– Saturation

– Value or brightness

• Transformation to RGB (no lineal)

0 Red

S

H

V60 Yellow120 Green

180Cyan

240 Blue 300 Magenta

0.0 Black

1.0 White

HLS

• Two cones joined, with white on the upper vertex

• Variation of HSV– L: lightness

SH

L

60 Yellow120 Green180Cyan

240 Blue 300 Magenta

0.0 Black

1.0 White

0 Red

YIQ

• Lineal transformation of RGB

• Used by the NTSC (National Television Standards Committee)

• Y is brightness (used in black-and-white television monitors)

B

G

R

Q

I

Y

311.0523.0212.0

321.0275.0596.0

144.0587.0299.0

CIE Diagram

http://www.lumitroncorp.com/rsrcs/CIE.gif

CIE

• 1931: measurement of color by the CIE (Commission International de L'Eclairage)

• Based on Tristimulus Vision Theory

• Represents any color detected by the human being

Definition of CIE diagram

• Experimentally– Red (700 nm)– Green (546,1 nm)– Blue (435,8 nm)

• There is negative values (added the primary to the sample)– They can’t be used– It is made adding the primary to the

sample

http://www.imel1.kuis.kyoto-u.ac.jp/education/dip-arch/pre/images/graph10.gif

The three standard primaries of CIE

• They are not real colors

• The functions are defined in tabular form (at 1 nm interval)

• They are standards to define colors

http://www.imel1.kuis.kyoto-u.ac.jp/education/dip-arch/pre/images/xyz.gif

CIE XYZ

• One color is defined by the X, Y, Z components.

• The projection on the XY plane produces the CIE Chromatic Diagram.

X

Y

Z

CIE xyY

• Normalizing the values:x + y + z = 1

• Y is the luminance information

– function y() was chosen in this way

http://hypertextbook.com/physics/waves/color/chromaticity.jpg

Properties of CIE diagram

• Gamut of different devices

• The periphery is the spectrum colors

• The purple line are non-spectral colors

• The spectral component of a color is calculated with the line to the center

http://www.tu-harburg.de/rzt/tuinfo/periph/drucker/Color_Reproduktion/cie_spectrum.gif

Color spaces

http://www.gg.caltech.edu/~dhl/images.html

Cube RGB