| References |
Definition |
| Additive mixing |
Method of color mixing for which two
or more primary colors are combined in specific amounts to create a
new color. The newly created color was not present in any of the primaries
but is created through the interaction of the primaries. Therefore
this method is said to be subjective. |
| Subtractive mixing |
Method of color mixing which filters
the incoming colored light, letting through only a particular color.
This method is objective: the desired color must be part of the incoming
light |
| Colorimetric system |
A (mathematical) way of rigorously classifying
and identifying colors by specific attributes |
| Color space |
The sum of all the colors that can be
represented starting from a set of primaries and a blending (color mixing)
rule.
In another definition, which is interchangeable
with 'colorimetric system', a color space is a mathematical description
of how colors can be uniquely identified by a suite of values called
color components. |
| Gamut |
The 'volume' of a color space, the
collection of all the colors that belong to a color space. A 'wider'
gamut implies that the space contains more colors |
| White point |
The reference white point in CIEXYZ
is the point for which x and y equal 1/3. In a color space, the white
point is the purest white that can be obtained using the primaries of
that color space. |
| Color temperature |
The associated temperature of a color
is the temperature at which heated, a black body would emit light of
that color. Colors become bluer as the color temperature increases and
more red as it decreases. |
| Planckian locus |
The geometric locus formed within the
chromaticity diagram by all the colors that the radiation (light) coming
from a black body can possibly have when heated to different temperatures.
Colors with equal color temperature but varying tint are situated on
the iso temperature lines drawn across the Planckian locus. |
| Tint |
The deviation of the white balance of
the image from the Plankian locus. This can be a green tint or a magenta
tint. |
| Color depth |
The number of discrete colors in a digital
representation of a particular color space. For obvious reasons this
is a power of 2 and the common representation is the actual number of
bits needed to describe the colors. A large bit depth is 48 bits. A
regular one is 24 bits. Because each color has three color coordinates,
an additional notation only takes into account the bits per color coordinate,
making 48 bits equivalent to 16 and 24 to 8. |
| Color management |
Ensemble of techniques employed for
maintaining consistent color representation across devices with different
input or output characteristics |
| CMM |
Color Matching Module (or Engine) -
routines in the image manipulation software responsible for the conversion
from the source color space to the destination color space according
to the desired rendering intent. CMM's from different manufacturers
have different names, for example:
Microsoft: ICM 2.0 - Image Color Management
Adobe: ACE - Adobe Color Engine
Apple: ColorSync
|
| Rendering intent |
Set of instructions for the Color Matching
Module describing the way translation between the source and the destination
color spaces needs to be made. The rendering intent is chosen by the
user and depends on the source image, source and destination color spaces
and the ... rendering intent.
Four rendering intents exist:
| |
Saturation or Graphic is for business graphics |
| |
Absolute colorimetric or Match clips hues outside the
destination space's gamut and does not preserve the white point graphics |
|
Relative colorimetric or Proof clips hues outside the
destination space's gamut and preserves the white point |
|
Perceptual or Picture stretches or squeezes the input
space's gamut to fit exactly in the output space's. Colors are usually
modified but no color information is lost in the process. |
Usually, Perceptual and Relative Colorimetric are used in the digital processing of photography.
|
| Chromaticity diagram,
Horseshoe diagram, CIEXYZ |
The three-dimensional color space of
the colors visible to the 'average human eye'. Also, the colorimetric
system used to describe colors by their parameters x, y and z. The z
axis is perpendicular to the chromaticity diagram. The chromaticity
diagram is a 2D representation of all the visible colors at maximum
brightness. It is bordered by the pure (saturated) colors - those found in the visible spectrum, and the purple line. Colors toward the
center are less saturated. |
| Hue |
Intrinsic property of a color which
differentiates it fundamentally from other colors. (A green is certainly
not red no matter what kind of green it is). Hue is measured on a circle,
in degrees, with red being at 0 deg. |
| Saturation, Purity |
A color a is the more saturated, the
closer is to the 'edge' of the chromaticity diagram, considering
the white point as the 'center'. Pure or completely saturated colors
are on the periphery of the chromaticity diagram. These are the colors
in the visible spectrum and in the purple line |
| Lightness, Value, Brightness,
Luminance |
Intrinsic property of a color which
allows classification by comparing it to a neutral gray scale.
|
| CIELAB, Lab, L*a*b* |
A geometrical transformation of the
CIEXYZ three-dimensional color space (other description of the same
entity). It allows a more uniform and intuitive representation of visible
colors, having as axes: L for luminance, a for green-magenta and b for
blue-yellow. Correct synonym for CIELAB is 'L*a*b*' but 'Lab'
liberally used. If white point is specified, CIELAB is an absolute (device
independent) color space |
| ICC profile |
For devices: file that denotes the capabilities
of an input or output device in terms of gamut and other rendering characteristics.
For color spaces: file that represents the actual color space by its
gamut. |
