Color

Sources: http://www.rgbworld.com/color.html

http://www.semmix.pl/color/

Light and Color

• Visible Light – The wavelengths our eyes can detect – a small portion of the electromagnetic energy spectrum • Primary Colors – Red, Green, Blue • Secondary Colors – Cyan, Magenta, Yellow • Color Systems – Additive (RGB) – Subtractive (CYMK)

Additive Color System (RGB)

• • • • • The additive color system involves light emitted directly from a source, before an object reflects the light. The additive reproduction process mixes various amounts of red, green and blue light to produce other colors. Combining one of these additive primary colors with another produces the additive secondary colors cyan, magenta, yellow. Combining all three primary colors produces white.

Television and computer monitors create color using the primary colors of light. Each pixel on a monitor screen starts out as black. When the red, green and blue phosphors of a pixel are illuminated simultaneously, that pixel becomes white. This phenomenon is called additive color.

All image capture devices utilize the additive color system to gather the information needed to reproduce a color image. These devices include digital cameras, flatbed scanners, drum scanners, and video cameras.

To summarize: Additive color involves the use of colored lights. It starts with darkness and mixes red, green and blue light together to produce other colors. When combined, the additive primary colors produce the appearance of white.

Subtractive Color System (CMY)

• • • • • The subtractive color system produces color by subtracting or absorbing certain wavelengths of color while reflecting other wavelengths back to the viewer. A red apple, for example, really has no color; it has no light energy of its own, it merely reflects the wavelengths of white light that cause us to see red and absorbs most of the other wavelengths which evokes the sensation of red. The viewer (or detector) can be the human eye, film in a camera or a light-sensing instrument.

Printing presses use color inks that act as filters and subtract portions of the white light striking the image on paper to produce other colors. Printing inks are transparent, which allows light to pass through to and reflect off of the paper base. It is the paper that reflects any unabsorbed light back to the viewer. The offset printing process uses cyan, magenta and yellow (CMY) process color inks and a fourth ink, black. The black printing ink is designated K to avoid confusion with B for blue. Overprinting one transparent printing ink with another produces the subtractive secondary colors, red, green, blue.

The illustrations show process inks printed on white paper. Each process printing ink (cyan, magenta, yellow) absorbs or subtracts certain portions of white light and reflects other portions back to the viewer. Process printing inks are transparent. It is the paper that reflects unabsorbed light back to the viewer.

To summarize: Subtractive color involves colorants and reflected light. It uses cyan, magenta and yellow pigments or dyes to subtract portions of white light illuminating an object to produce other colors. When combined in equal amounts, pure subtractive primary colors produce the appearance of black.

Color Space

• RGB Color = (R, G, B) • CMY Color = (C, M, Y) • (C, M, Y) = (1, 1, 1) – (R, G, B) • CMYK Color = (C-K, M-K, Y-K, K) K=min(C, M, Y)

Computer Display

• • • Additive Color System Grey Level (GL): 8bits/pixel (8bit Unsigned Integer) – Look-Up-Table (LUT) GL=LUT DN GL range =[0, 255] = [00, FF] • RGB color: 24bits/pixel (3 byte Unsigned Integer) RGB =[GL R , GL G , GL B ] T

Red Green Blue R

255

G

0

B

0 0 255 0 0 0 255

Decimal Red Green Blue

0 17 34 51 68 85 102 119 136 153 170 187 204 221 238 255

Windows

True color High color Indexed color

Bit depth

24-bit 16-bit 8-bit

Number of colors

16,777,216 65,536 256

Macintosh

Millions of colors Thousands of colors 256 colors

Internet Stuffs

• • •

http://www.decemb

er.com/html/spec/c olorcodes.html

http://www.visibone.

com/colorlab/ http://www.geocities

.com/SoHo/3505/hel p/colorcube.html

silver C0C0C 0 Gray 808080 Maroon 800000 green 008000 Navy 000080 Purple 00080 Olive 808000 Teal 008080 White FFFFFF black 000000 Red FF0000 Lime 00FF00 Blue 0000FF Magenta FF00FF Yellow FFFF00 Cyan 00FFFF