NationMaster - Encyclopedia: Color balance

In photography and image processing, color balance (sometimes gray balance, neutral balance, or white balance) refers to the adjustment of the relative amounts of red, green, and blue primary colors in an image such that neutral colors are reproduced correctly. Color balance changes the overall mixture of colors in an image and is used for generalized color correction. Photography [fÓ™tÉ‘grÓ™fi:],[foÊŠtÉ‘grÓ™fi:] is the process of recording pictures by means of capturing light on a light-sensitive medium, such as a film or electronic sensor. ... UPIICSA IPN - Binary image Image processing is any form of information processing for which the input is an image, such as photographs or frames of video; the output is not necessarily an image, but can be for instance a set of features of the image. ... This article is about the book. ... Colour Correction by using Colour Gels, is a process used in Stage Lighting, Photography, Television, Cinematography to name a few disciplines, the intention of which is to alter the overall quality of the light, measured on a scale known as Colour Temperature. ...

Most electronic cameras have a means to select the type of illumination under which the photography is being done. Another option on some cameras is a button which one may press when the camera is looking at a gray card or other neutral object, to capture a "custom" color balance. A very common option is "automatic white balance" (AWB), which may be based on a scheme such as Retinex, an artificial neural network^[1] or a Bayesian method.^[2] Gray cards are used, together with reflective light meters, as a way to produce consistent images in film and photography. ... Color constancy is a feature of the human color-perception system which ensures that the perceived color of objects remains (almost) constant under varying light conditions. ... An artificial neural network (ANN), often just called a neural network (NN), is a mathematical model or computational model based on biological neural networks. ... Bayesian inference is statistical inference in which evidence or observations are used to update or to newly infer the probability that a hypothesis may be true. ...

Color balance and chromatic colors

Color balancing an image affects not only the neutrals, but other colors as well. An image that is not color balanced is said to have a color cast, as everything in the image appears to have been shifted towards one color or another.^[3] Color balancing may be thought in terms of removing this color cast.

Color balance is also related to color constancy. Algorithms and techniques used to attain color constancy are frequently used for color balancing, as well. Color constancy is, in turn, related to chromatic adaptation. Conceptually, color balancing consists of two steps: first, determining the illuminant under which an image was captured; and second, scaling the components (e.g., R, G, and B) of the image or otherwise transforming the components so they conform to the viewing illuminant. Color constancy is an example of subjective constancy and a feature of the human color-perception system which ensures that the perceived color of objects remains relatively constant under varying illumination conditions. ... Color vision is a psychophysical phenomenon that exists only in our minds. ... A standard illuminant is a profile or spectrum of visible light which is published in order to allow images or colours recorded under different lighting to be compared. ...

Viggiano^[4] found that white balancing in the camera's native RGB tended to produce less color inconstency (i.e., less distortion of the colors) than in monitor RGB for over 4000 hypothetical sets of camera sensitivities. This difference typically amounted to a factor of more than two in favor of camera RGB. This means that it is advantageous to get color balance right at the time an image is captured, rather than edit later on a monitor. If one must color balance later, balancing the raw image data will tend to produce less distortion of chromatic colors than balancing in monitor RGB. REDIRECT RGB color model ... A raw image file (sometimes written RAW image file ) contains minimally processed data from the image sensor of a digital camera or image scanner. ...

Mathematics of color balance

Color balancing is sometimes performed on a three-component image (e.g., RGB) using a 3x3 matrix. This type of transformation is appropriate if the image were captured using the wrong white balance setting on a digital camera, or through a color filter. RGB redirects here. ... In mathematics, a matrix (plural matrices) is a rectangular table of elements (or entries), which may be numbers or, more generally, any abstract quantities that can be added and multiplied. ...

Scaling monitor R, G, and B

In principle, one wants to scale all relative luminances in an image so that objects which are believed to be neutral appear so. If, say, a surface with R=240 was believed to be a white object, and if 255 is the count which corresponds to white, one could multiply all red values by 255/240. Doing analogously for green and blue would result, at least in theory, in a color balanced image. In this type of transformation the 3x3 matrix is a diagonal matrix. Achromatic redirects here. ... For other uses, see Red (disambiguation). ... For other uses, see Green (disambiguation). ... For other uses, see Blue (disambiguation). ... In linear algebra, a diagonal matrix is a square matrix in which the entries outside the main diagonal are all zero. ...

where

R

G

, and

B

are the color balanced red, green, and blue components of a pixel in the image;

R'

G'

, and

B'

are the red, green, and blue components of the image before color balancing, and

R' w

G' w

, and

B' w

are the red, green, and blue components of a pixel which is believed to be a white surface in the image before color balancing. This is a simple scaling of the red, green, and blue channels, and is why color balance tools in Photoshop and the GIMP have a white eyedropper tool. It has been demonstrated that performing the white balancing in the phosphor set assumed by sRGB tends to produce large errors in chromatic colors, even though it can render the neutral surfaces perfectly neutral.^[4] This example shows an image with a portion greatly enlarged, in which the individual pixels are rendered as little squares and can easily be seen. ... Adobe Photoshop is a bitmap graphics editor (with some text and vector graphics capabilities) published by Adobe Systems. ... For other uses, see Gimp (disambiguation). ... The title given to this article is incorrect due to technical limitations. ...

Scaling X, Y, Z

If the image may be transformed into CIE XYZ tristimulus values, the color balancing may be performed there. This has been termed a “wrong von Kries” transformation.^[5] Although it has been demonstrated to offer usually poorer results than balancing in monitor RGB, it is mentioned here as a bridge to other things. Mathematically, one computes: In the study of the perception of color, one of the first mathematically defined color spaces was the CIE XYZ color space (also known as CIE 1931 color space), created by the International Commission on Illumination (CIE) in 1931. ...

where

X

Y

, and

Z

are the color-balanced tristimulus values;

X w

Y w

, and

Z w

are the tristimulus values of the viewing illuminant (the white point to which the image is being transformed to conform to);

X' w

Y' w

, and

Z' w

are the tristimulus values of an object believed to be white in the un-color-balanced image, and

X'

Y'

, and

Z'

are the tristimulus values of a pixel in the un-color-balanced image. If the tristimulus values of the monitor primaries are in a matrix $mathbf{P}$ so that:

where

L R

L G

, and

L B

are the un-gamma corrected monitor RGB, one may use: Example of CRT gamma correction Plot of the sRGB standard gamma-expansion nonlinearity (red), and its local gamma value, slope in logâ€“log space (blue). ...

Von Kries's method

Johannes von Kries, whose theory of rods and three different color-sensitive cone types in the retina has survived as the dominant explanation of color sensation for over 100 years, motivated the method of converting color to the "LMS" tristimulus space, representing the effective stimuli for the Long-, Medium-, and Short-wavelength cone types that are modeled as adapting independently. A 3x3 matrix converts RGB or XYZ to LMS, and then the three LMS primary values are scaled to balance the neutral; the color can then be converted back to the desired final color space.^[6] Johannes von Kries (1853â€“1928) was a German physiological psychologist[1] who formulated the modern duplicity or duplexity theory of vision mediated by rods at low light levels and three types of cones at higher light levels. ... Human eye cross-sectional view. ... A comparison of different color spaces. ...

where

L

M

, and

S

are the color-balanced LMS cone tristimulus values;

L' w

M' w

, and

S' w

are the tristimulus values of an object believed to be white in the un-color-balanced image, and

L'

M'

, and

S'

are the tristimulus values of a pixel in the un-color-balanced image.

Matrices to convert to LMS space were not specified by von Kries, but can be derived from CIE color matching functions and LMS color matching functions when the latter are specified; matrices can also be found in reference books.^[6]

Scaling camera RGB

By Viggiano's measure, and using his model of gaussian camera spectral sensitivities, most camera RGB spaces performed better than either monitor RGB or XYZ.^[4] If the camera's raw RGB values are known, one may use the 3x3 diagonal matrix:

and then convert to a working RGB space such as sRGB or Adobe RGB after balancing. The title given to this article is incorrect due to technical limitations. ... The Adobe RGB color space is an RGB color space developed by Adobe Systems in 1998. ...

Contents

Color balance and chromatic colors

Mathematics of color balance

Scaling monitor R, G, and B

Scaling X, Y, Z

Von Kries's method

Scaling camera RGB

See also

References

External links

Contents

Color balance and chromatic colors GA_googleFillSlot("encyclopedia_square");

Mathematics of color balance

Scaling monitor R, G, and B

Scaling X, Y, Z

Von Kries's method

Scaling camera RGB

See also

References

External links

Color balance and chromatic colors