RGB color space

An RGB color space is an additive color space replicates, the color perception by the additive mixing three primary colors (red, green and blue ). The color vision of humans is characterized by three types of cones. This color space is based in principle on the three-color theory.

  • 3.1 The CIE - XYZ color space,
  • 3.2 CIE - RGB color space
  • 3.3 The color space of the early NTSC
  • 3.4 color space of PAL and SECAM and NTSC later ( 3213 EBU, ITU -R BT.470 -2, SMPTE -C)
  • 3.5 The sRGB color space
  • 3.6 Adobe RGB (1998 ) color space
  • 3.7 The Adobe Wide Gamut RGB color space
  • 3.8 European Color Initiative: The eciRGB color space
  • 3.9 The ProPhoto RGB color space
  • 3:10 Recent Developments
  • 3:11 RGBY
  • 6.1 limits
  • 6.2 Color Correction

Basics

After initial study and reflection on the phenomenon of color vision in the 18th century resulted mainly scientific investigations in the 19th century to the first quantitative theories. One is the three-color theory. One can simulate almost all any color stimuli by mixing three primary colors. The light can be composed with the spectrometer completely different between the original stimulus and the simulated stimulus. The human eye but that can not distinguish. The two colors are metameric colors. Can not distinguish between the two color stimuli, so it is not necessary to store the exact spectral distribution for a reconstruction of the hues. In order to replicate this color stimulus, it is sufficient to store a number triple that describes the amount of red, green and blue light.

Just as a color in the RGB space is described. Is a red, a green and a blue in maximum intensity defined as the red component R, the amount of green G and blue component B can describe the color.

The ranges of values ​​for the color stimuli (R, G, B) can be defined differently. The classical representation allows for values ​​between 0 and 1 ( ergo 0 percent and 100 percent). This is based on the practical implementation using classical damping of available light. Computer -based applications often use the ajar to the classical form of saving notation, it will be integers between 0 and a maximum number stored. Such conventional maximum numbers are 7, 31, 255, 1023, 4095, 16383, 65535.

Since the intensity of human perception is not linear according to the Weber -Fechner rule, a non-linear coding for the intensities is usually performed. This often is referred to as the gamma function, since the first implementations of the function ~ used as an approach. The coefficient gamma described here, the curvature of the curve. The inverse function was mathematically accessible simply by.

The coordinate system, in addition to this non-linear encoding a total of nine degrees of freedom that are set for a particular RGB space. This can indicate different, which can lead to confusion for the user. For all three primaries, there are various possibilities

  • Using the standard color chart (x, y ) with the addition of the white point as the reference brightness
  • By means of the matrix (Y, x, y) with the chromaticity coordinates x and y and the tristimulus value Y, which is used here as a measure of the brightness of
  • By means of the matrix (X, Y, Z ), and thus all three tristimulus values ​​X, Y, Z based on the 1931 CIE color matching functions defined by the.

Modern computer -based applications and interfaces use at least internally, more and more floating-point numbers that break both from the interval [ 0,1] as well as larger ranges of values ​​with the same relative accuracy can represent innately (16 bit ≈ 12 orders of magnitude, 32 bit ≈ 83 orders of magnitude). Setting a maximum brightness is also avoided, one stores the absolute magnitudes. The number of degrees of freedom is reduced to 6, the color cube becomes a vector space.

The colored portion of the XYZ space is the set of all visible colors. The CIE standard colorimetric system is graphically represented by the color bodies by Rösch. Are ICC profiles for color input and output color devices such as monitors, scanners, printers, any necessary color spaces (RGB, CMYK) transformed. However, this transformation is not clearly possible. Of the material in each case viable RGB color space is located on the Farbarttafel, more specifically in the CIE color space within a triangle. Such a triangle is outlined in black in the adjacent illustration. Different transformations (usually 3 × 3 matrix ) of numbers and now better technical availability, there are differently defined and standardized variants (s- RGB, Adobe RGB, Bruce RGB).

Application

The RGB color space is used for self-luminous ( farbdarstellende ) systems that are subject to the principle of additive color mixing, also called light mixture. After Grassmann's laws to colors can be defined by three pieces of information, in the RGB color space, these are the red, the green and the blue. The concrete form of the color space depends on each concrete technical system for which the respective color space was determined.

SRGB ( standard RGB ) has been developed for monitors, whose coloring base are three phosphors ( phosphors ). Such a substance is when struck by electrons from a spectrum of light, thereby suitable phosphors are those with narrow-band emission at wavelengths in the range of perception qualities blue, green, red, the viewer gets defined in the RGB color space color impression ( at a sufficient distance from the screen go the pixels additively into each other ). The intensity of the excitation beam corresponds to the triples in the RGB color space and, for example, as a decimal fraction (0 to 1 or 0 to 100%) or discrete with 8 bits per channel (0 ... 255) are indicated (8 -bit TIFF). Depending on the application specific value representations are preferred.

With larger storage media pitches were possible from 16 bits per channel. So are three times from 0 to 65535 () possible, so a total of 281 trillion colors, such as when 16 -bit TIFF, 16 -bit PNG. Good technical output devices can reproduce more colors than humans can distinguish even the trained person comes to only about 500 000 colors. For special applications, 16 -bit values ​​, however, are quite reasonable. In evaluations in diagnostic radiology as accurate observations are possible.

The color reproduction in cases such as color images from a PC printer, color photos based on silver halide, the pressure of a magazine, color pictures in books done by remission on the presenting surface. Here, then, the laws of subtractive color mixing, for the CMY color space was developed because of the depth of color usually black with deep color as CMYK color space.

The representation of the RGB color space is done (less clearly than other color spaces ) in the Cartesian coordinate system as a cube. The figure on the left shows the view of the back wall, in the middle of the upward glance, right an insight into the interior. Red, green and blue components follow the axes; in the corners of which are yellow, magenta, cyan to be found. At the origin of coordinates with R = G = B = 0 is black, along the body diagonal gray. Corner until in White

Use of RGB color spaces for image reproduction

RGB color spaces as additive color spaces serve as a basis for representing color images by means of image reproduction devices, the colors of three or more colors together additively. In addition to CRT and TFT displays, these are also video projectors.

It is immaterial how the individual color channels are driven, if. By an analog or a digital signal of 5, 8, 10 or 16 bits per color channel

Usually, the three primary colors red, green and blue are used for display. To enlarge the gamut or the maximum brightness but can also be used more colors. So may be better represented by polygon colors covered, at least at lower luminosities. The restriction to the RGB triangle enclosed by the horseshoe omitted. To increase the maximum brightness and white can be used as an additional primary color. For larger magnitudes are represented, however, with further loss of gamut. Both methods are used in DLP projectors.

However, in these cases, further processing of the RGB data of the graphics card by the output device is necessary. In the case of multi-color projection, a suitable working space of the graphics card is required to take advantage of the benefits.

The vertices of the RGB Farbartdreiecks can be chosen arbitrarily, they are not limited by the availability of phosphor crystals. There is no inseparable connection to the three (basic ) light colors that can produce the phosphors of the output device. Color values ​​outside the specified by the key points of the triangle can not be displayed. Thus, in a picture tube lacks many of the powerful, rich greens and blues that occur in nature, and also the spectrally red and violet missing in the RGB space.

, The phosphors of a screen used by LEDs or similar elements for red, green, blue, changes in the color effect with respect to this description nothing, provided they can cover the RGB space used. For example, have flat screens no picture tube and produce colors by electrical field stimulation. Other phosphors require a different location of the RGB triangle (shown on the xy chromaticity sole). Technical requirement is to adjust the position of the graph vertices for LCD screen as possible to the situation in picture tubes. If this fails, must be a mathematical conversion, which, however, colors can be omitted, since the coordinates can not have negative values ​​. If it fails, the conversion, the colors are distorted. So maybe shades of color between red and ( yellow-orange ) are displayed noticeably different on different devices.

Applying the RGB color space for image recording

Although it looks at first sight, as if the image acquisition is subject to the same laws as the image rendering, so there are fundamental differences for the image recording for image reproduction:

  • Unfavorable spectra for the primaries result in image reproduction, only a small gamut, within this small gamut but is a perfect reproduction of colors possible (the triangle is small).
  • Not suitable spectral sensitivities of the primary colors of an image recording device usually lead to uncorrectable color errors ( you bend the shoe ).
  • It is not possible to build a monitor that can display all colors perceivable by humans.
  • The Dead and hot pixels a camera can be ausmappen, for a display, however, this is not easily possible.

Standard RGB color spaces,

In principle, there are an infinite number of color spaces that by defining the primaries, white point and Gradationkurve (gamma ) can be specified ( exactly occurs in matrix - ICC profiles ). The primaries set the color triangle of displayable colors at low luminosities determine the white point of the intensity ratio for color triplets with three identical components, thus indirectly the ratio of maximum to maximum red green and blue.

The following table provides an overview of the history of the usual RGB color spaces.

After opening the tables all information are available, which are needed for converting color spaces. All values ​​are normalized to the interval [ 0,1], with fixed-point number arithmetic other operations that are necessary. Here also associated individual documents are called.

Which can represent colors in a color space of the color values ​​restriction on the interval [0,1 ?] These are the CIE xy diagram the points ( xR, yR ), ( xG, yG ) and ( xB, yB ) draw in and connect. The enclosed triangle describes the gamut of the color space. Colors outside of the triangle to cause at least one component that is less than 0, and can not be represented. Values ​​within the triangle may be obtained at higher intensity, if necessary, also not shown, as one or more components to be greater than 1. Illustrating this phenomenon, it is necessary to calculate the resultant four-, five -or six- corner for higher brightness, and display the graph. For higher brightness levels of this polygon degenerate back to a triangle, to a point. We view the sectional planes of constant brightness by the RGB cube at the beginning of the article.

CIE XYZ color space

This XYZ color space from 1931 is the first standardization attempt to find a unified representation system in the world. The starting point for this was the experimentally determined Zapf sensitivities. The measurement technique used and the experimental evaluation corresponds to the state of the art of the 1920s. Nevertheless, the color space is still frequently used in practice. Color measurement at that time the used thereby "trick" that can be generated in color light by mixing light in the " Istseite " so to speak on the negative color stimuli " reference side." The XYZ color space should include all colors perceivable by humans. Although the XYZ color space is primarily a measurement color space, but can also be used for displaying colors.

Then he encloses the entire "horseshoe " of all kinds of colors, all existing colors are captured by him. The main problem resides in its uniformity. The Green as the same perceived color distances are larger than in red and blue. The primaries of this presentation are not real existing colors, but chosen so that the color coordinates are easy to demonstrate. So there is no real body color in RGB, which could reflect this color space.

Notes:

  • The transformation matrix is obtained from the definition of the XYZ color space as a reference.
  • For the same reason, E is obtained as a white point (X = Y = Z).
  • An indication of the Referenzluminanz is not necessary due to the nonlinearity of the space.
  • The Referenzluminanz is therefore useful to be determined by the number format used ( floating-point number: 1 cd / m², fixed-point number: Lmax = maximum occurring meaningful luminance).

CIE - RGB color space

The real CIE RGB color space created by the conversion of the virtual CIE XYZ color space ( which is based on non-displayable color stimuli ) to the calibration stimuli of well reproducible spectral lines:

  • Red: 700 nm ( for the human eye are practically all wavelengths above 650 nm in the same color, so all spectral lines above 650 nm are of practical use, such as the deep red 690.7 -nm Hg line )
  • Green: 546.1 nm ( green Hg - line)
  • Blue: 435.8 nm (blue Hg line )

This ensured an almost perfect coverage of red, orange, yellow and blue and violet range. Significant weaknesses are, however, in turquoise and green area by the unfortunate choice of the green stimulus.

In particular, not all CMYK colors are displayed, again especially in the green to turquoise range (480 nm to 510 nm).

Notes:

  • The transformation matrix was removed from the CIE Normvalenzreizen for 700 nm, 546 nm and 436 nm.
  • See also: The CIE XYZ color space ( here in the article)
  • The spectral lines of the elements in the visible range of the spectrum

The color space of the early NTSC

With the introduction of the NTSC color television in 1953, the color phosphors (then) used were used as primaries:

  • Green: silver doped zinc cadmium sulfide (Ag ZnS / CdS)
  • Blue: zinc sulfide ( ZnS)

The primaries are derived from the emission spectra of the phosphors used. The traditional NTSC color space is replaced by a the EBU color space, more similar SMPTE-C color space 1979 by the ATC ( predecessor of the ATSC).