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COLOR MODELS |
| RGB model
- R
- Red, Green and Blue (RGB)
Most of the visible spectrum can be represented by mixing the 3 primary
colors red, green and blue (RGB).
When these colors are mixed in varying amounts and intensities
(overlap), they create the secondary colors of cyan. magenta and yellow.
When all three primary colors are combined they create white and are
thus called additive colors because they depend on the projection of
light as in the 3 RGB phosphor guns in a computer monitor.
Lighting, video, monitors and film recorders all use the additive
color model to represent color.
The human eye is based upon RGB color; the eye has red, green and blue receptors whose
signals are interpreted as color by the brain.
Photoshop assigns an intensity value to
each pixel, ranging from 0 (black) to 255 (white) for each of the RGB components. For
example, a bright red color might have and R value of 246, a G value of 20, and a B value
of 50. When the values of all three components are equal, the result is a shade of gray.
When the value of each component is 255, the result is pure white; when all components
have values of 0, the result is pure black. |
| CMYK model
- Cyan Magenta Yellow blacK. While RGB color depends on light transmission for its additive color scheme,
the CMYK model is based upon the light absorbing qualities of ink and paper therefore it
is a subtractive color-building process.
In theory, cyan, magenta and yellow should combine to absorb all
color and produce black. Because of the small impurities found in all inks, they actually
combine to form a muddy brown. This is the reason for the black ink. It finishes the job
by producing true black. This process is the basis for four-color process printing.
The CMYK color model is used for color-separation printing, and
represents only colors that can be created by using process inks. |
| Lab model
Earlier this century, there was a need to develop a
device independent color description method. So the CIE (Commision Internationale
d'Eclairage - the International Commision of Illumination) established a standard for
describing the primary colors. LAB color is the result.
Lab color is designed to be device independent; that is, it creates
consistent color regardless of the specific device, such as the monitor, printer, or
computer.
Lab color consist of a luminance, or lightness component (L) and two
chromatic components: the a component, which ranges from green to red, and the b
component, which ranges from blue to yellow.
The Lab mode is used most often when you are working with Photo CD
images or when you want to edit the luminance and the color values in an image
independently. You also use the Lab mode to maintain color fidelity when you're moving
images between systems and to print to PostScript Level 2 printers.
In the Lab mode, the lightness component (L) can range from 0 to
100. The a component (green-red axis) and the b components )blue-yellow
axis) can range from +120 to -120.
Lab color is the internal color model that Adobe Photoshop uses when
converting from one color model to another. |
| HSL Color Model
The Hue, Saturation, and Lightness
(HSL) color model defines a color based on its hue (color), saturation (purity of the
color), and lightness (brightness).
Hue is what we usually define as the color of an
object. The hue of a color is assigned a number from 0° to 360°. Red is defined at the
0° point, blue is 120°, cyan is 180°, green is 240°, and so on. The hue of the sky is
blue, and the hue of a banana is yellow.
Saturation of a color describes the purity of a
color. The range of saturation is defined in value percentages from 100% (full color
intensity) to 0% (no color intensity). A black-and-white photograph has a color saturation
value of 0%. Refer to the Saturation Shift slider in the Hue Map dialog box to see how
changes in saturation affect an image.
Lightness refers to the amount of white or black in
a color and is defined in percentages from 100% (totally white) to 0% (totally black).
Fifty percent lightness is the pure hue. Refer to the Lightness Shift slider in the Hue
Map dialog box to see how changes in lightness affect an image. |
| Grayscale
mode uses up to 256 shades of gray to represent the image [brightness value
ranging from 0 (black) to 255 (white)]. |
| COLOUR COUNTS |
| Color model |
Channels |
Bits |
Number of
colours |
Common name |
| Bytes per pixel |
| CMYK (cyan,
magenta, yellow, and black) |
4 |
32 |
16.7 ml |
|
| RGB (red, green and
blue) |
3 |
24 |
16.7 ml |
TrueColor |
| 2 |
16 |
65,536 |
HiColor |
| Grayscale |
1 |
8 |
256 values |
|
| One channel of color
where the red, green, and blue components for each pixel are written to a color table as a
fixed value. |
1 |
8 |
256 |
SVGA, Indexed |
| 0.5 |
4 |
16 |
VGA, Indexed |
| B&W only |
0.125 |
1 |
2 values |
Line Art |
| Each channel works
with eight bits of color information - a gamut of brightness values from 0 and 255. |
When
changing color modes
You can simulate additional colors by using different colored pixels
arranged closely together (dithering). Using this technique, the eye is
tricked into seeing more colors than are actually found in the color table. |
When
resizing or changing resolution
Interpolation is the averaging of two neighboring
pixels colour-values, to come up with a pixel that appears to fit smoothly between the
two. Interpolation adds and deletes pixels then reassigns different
colors to existing pixels to execute a particular command. |
Some terms:
Saturation is the purity of a color.
Hue: 100-percent saturated color.
Brightness is how intensely you perceive the energy of light to be.
Gamma is a measurement of contrast - how much different pixels vary in
their brightness.
Gamut is the range of colors that can be reproduced by a color model
or device.
Chromacity, or chroma, refers to the combination of two components of
color: hue and saturation.
Tones are produced when a mixture of black and white is added to a pure
hue.
Shade: when black is added to a pure hue.
Tint: when white is added to a pure hue.
Anti-aliasing, the placing of semitransparent pixels around the jagged
edges of a design detail.
Halftones are the representation of continuous-tone images by dots of
different sizes. Whenever an image is printed, it must be screened to create a halftone.
Screen frequency, also known as screen ruling, refers to the
number of halftone cells per inch in the halftone screen.
Light is a form of electromagnetic energy. Electromagnetic energy is
radiated in waves along a straight-line path. Visible light is electromagnetic energy
between wavelengths about 400nm and 700nm. Within this visible spectrum individual
wavelength bands produce sensation of colors; mixed together they appear white.
Calibration is the process of setting a device to known color
conditions.
Characterization, or profiling, is the process of creating ICC
profile that describes the unique color characteristics of a particular
device. |
A
Color Management System
(CMS) uses three components to map colors across devices:
- Reference color space - CIE
(Commission Internationale d'Eclairage) LAB color model
(consider device-independent)
- ICC (International Color Consortium) profiles -
define the color characteristics of particular devices and document by
embedding the profile in the file.
- A color management engine - translates color from
the source device to the destination device by a process called color
mapping.
|
| COLOR
EQUATIONS |
| red
+ green + blue = white (additive colors) |
| cyan
+ magenta + yellow = black (subtractive colors) |
| cyan
= blue + green |
magenta
= red + blue |
red
is opposite to green (adding red will reduce green and
vice versa)
blue is opposite to yellow (increase
in blue you can achieve either by adding blue or by reducing yellow) |
Red =
Yellow + Magenta
Green = Cyan/Blue + Yellow
Blue = Cyan + Magenta
Yellow = Red + Green |
Red + Yellow =
Orange
Yellow + Blue = Green
Blue + Red = Purple |
Equal amounts of Red, Green & Blue gives you Gray
0 R+ 0 G+0 B=Black
255 R+255 G+255 B=White |
| Color |
Name |
Effect |
| |
Cyan |
Absorbs Red |
| |
Yellow |
Absorbs Blue |
| |
Magenta |
Absorbs Green |
| |
Red |
Absorbs Blue and Green |
| |
Green |
Absorbs Red and Blue |
| |
Blue |
Absorbs Red and Green |
Cyan absorbs Red, Magenta absorbs Green, Yellow absorbs Blue |
| R |
G |
B |
= |
| 1 |
0 |
0 |
BRIGHT RED |
| 1 |
0.75 |
0.75 |
PINK |
| 0.5 |
0.2 |
0.8 |
PURPLE |
| 1 |
1 |
1 |
WHITE |
| 0 |
0 |
0 |
BLACK |
The human eye contains two types of cells:
- 6 to 7 million cones - concentrated
in the center of the eye, called the fovea - sensitive to color
- 150 million rods - responsible for
peripheral vision - more sensitive in dim light and not capable of
detecting color.
|
| Temperature |
Typical Sources |
| 1000-1500 K |
Candles; oil lamps |
| 2000 K |
Very early sunrise; low effect tungsten lamps |
| 2500 K |
Household light bulbs |
| 3000 K |
Studio lights, photo floods |
| 4000 K |
Clear flashbulbs |
| 5000 K |
Typical daylight; electronic flash |
| 5500 K |
Sunny daylight around noon |
| 6000 K |
Bright sunshine with clear sky |
| 7000 K |
Slightly overcast sky |
| 8000 K |
Hazy sky |
| 9000 K |
Open shade on clear day |
| 10,000 K |
Heavily overcast sky |
| 11,000 K |
Sunless blue skies |
| 20,000+ K |
Open shade in mountains on a clear day |
Steps to follow to reproduce color
accurately:
- Calibrate your monitor
- use Color Setting dialog box to specify
which color space to use (e.g. RGB on-line or CMYK for printing)
- proof the image
- check for out-of-gamut colors
- adjust colors as needed
- create color separations for printed
images
|
| A color separation is created when an image
is converted to CMYK mode. |
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