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diff --git a/Documentation/userspace-api/media/v4l/colorspaces.rst b/Documentation/userspace-api/media/v4l/colorspaces.rst new file mode 100644 index 000000000000..0846df9066c5 --- /dev/null +++ b/Documentation/userspace-api/media/v4l/colorspaces.rst @@ -0,0 +1,170 @@ +.. Permission is granted to copy, distribute and/or modify this +.. document under the terms of the GNU Free Documentation License, +.. Version 1.1 or any later version published by the Free Software +.. Foundation, with no Invariant Sections, no Front-Cover Texts +.. and no Back-Cover Texts. A copy of the license is included at +.. Documentation/userspace-api/media/fdl-appendix.rst. +.. +.. TODO: replace it to GFDL-1.1-or-later WITH no-invariant-sections + +.. _colorspaces: + +*********** +Colorspaces +*********** + +'Color' is a very complex concept and depends on physics, chemistry and +biology. Just because you have three numbers that describe the 'red', +'green' and 'blue' components of the color of a pixel does not mean that +you can accurately display that color. A colorspace defines what it +actually *means* to have an RGB value of e.g. (255, 0, 0). That is, +which color should be reproduced on the screen in a perfectly calibrated +environment. + +In order to do that we first need to have a good definition of color, +i.e. some way to uniquely and unambiguously define a color so that +someone else can reproduce it. Human color vision is trichromatic since +the human eye has color receptors that are sensitive to three different +wavelengths of light. Hence the need to use three numbers to describe +color. Be glad you are not a mantis shrimp as those are sensitive to 12 +different wavelengths, so instead of RGB we would be using the +ABCDEFGHIJKL colorspace... + +Color exists only in the eye and brain and is the result of how strongly +color receptors are stimulated. This is based on the Spectral Power +Distribution (SPD) which is a graph showing the intensity (radiant +power) of the light at wavelengths covering the visible spectrum as it +enters the eye. The science of colorimetry is about the relationship +between the SPD and color as perceived by the human brain. + +Since the human eye has only three color receptors it is perfectly +possible that different SPDs will result in the same stimulation of +those receptors and are perceived as the same color, even though the SPD +of the light is different. + +In the 1920s experiments were devised to determine the relationship +between SPDs and the perceived color and that resulted in the CIE 1931 +standard that defines spectral weighting functions that model the +perception of color. Specifically that standard defines functions that +can take an SPD and calculate the stimulus for each color receptor. +After some further mathematical transforms these stimuli are known as +the *CIE XYZ tristimulus* values and these X, Y and Z values describe a +color as perceived by a human unambiguously. These X, Y and Z values are +all in the range [0…1]. + +The Y value in the CIE XYZ colorspace corresponds to luminance. Often +the CIE XYZ colorspace is transformed to the normalized CIE xyY +colorspace: + + x = X / (X + Y + Z) + + y = Y / (X + Y + Z) + +The x and y values are the chromaticity coordinates and can be used to +define a color without the luminance component Y. It is very confusing +to have such similar names for these colorspaces. Just be aware that if +colors are specified with lower case 'x' and 'y', then the CIE xyY +colorspace is used. Upper case 'X' and 'Y' refer to the CIE XYZ +colorspace. Also, y has nothing to do with luminance. Together x and y +specify a color, and Y the luminance. That is really all you need to +remember from a practical point of view. At the end of this section you +will find reading resources that go into much more detail if you are +interested. + +A monitor or TV will reproduce colors by emitting light at three +different wavelengths, the combination of which will stimulate the color +receptors in the eye and thus cause the perception of color. +Historically these wavelengths were defined by the red, green and blue +phosphors used in the displays. These *color primaries* are part of what +defines a colorspace. + +Different display devices will have different primaries and some +primaries are more suitable for some display technologies than others. +This has resulted in a variety of colorspaces that are used for +different display technologies or uses. To define a colorspace you need +to define the three color primaries (these are typically defined as x, y +chromaticity coordinates from the CIE xyY colorspace) but also the white +reference: that is the color obtained when all three primaries are at +maximum power. This determines the relative power or energy of the +primaries. This is usually chosen to be close to daylight which has been +defined as the CIE D65 Illuminant. + +To recapitulate: the CIE XYZ colorspace uniquely identifies colors. +Other colorspaces are defined by three chromaticity coordinates defined +in the CIE xyY colorspace. Based on those a 3x3 matrix can be +constructed that transforms CIE XYZ colors to colors in the new +colorspace. + +Both the CIE XYZ and the RGB colorspace that are derived from the +specific chromaticity primaries are linear colorspaces. But neither the +eye, nor display technology is linear. Doubling the values of all +components in the linear colorspace will not be perceived as twice the +intensity of the color. So each colorspace also defines a transfer +function that takes a linear color component value and transforms it to +the non-linear component value, which is a closer match to the +non-linear performance of both the eye and displays. Linear component +values are denoted RGB, non-linear are denoted as R'G'B'. In general +colors used in graphics are all R'G'B', except in openGL which uses +linear RGB. Special care should be taken when dealing with openGL to +provide linear RGB colors or to use the built-in openGL support to apply +the inverse transfer function. + +The final piece that defines a colorspace is a function that transforms +non-linear R'G'B' to non-linear Y'CbCr. This function is determined by +the so-called luma coefficients. There may be multiple possible Y'CbCr +encodings allowed for the same colorspace. Many encodings of color +prefer to use luma (Y') and chroma (CbCr) instead of R'G'B'. Since the +human eye is more sensitive to differences in luminance than in color +this encoding allows one to reduce the amount of color information +compared to the luma data. Note that the luma (Y') is unrelated to the Y +in the CIE XYZ colorspace. Also note that Y'CbCr is often called YCbCr +or YUV even though these are strictly speaking wrong. + +Sometimes people confuse Y'CbCr as being a colorspace. This is not +correct, it is just an encoding of an R'G'B' color into luma and chroma +values. The underlying colorspace that is associated with the R'G'B' +color is also associated with the Y'CbCr color. + +The final step is how the RGB, R'G'B' or Y'CbCr values are quantized. +The CIE XYZ colorspace where X, Y and Z are in the range [0…1] describes +all colors that humans can perceive, but the transform to another +colorspace will produce colors that are outside the [0…1] range. Once +clamped to the [0…1] range those colors can no longer be reproduced in +that colorspace. This clamping is what reduces the extent or gamut of +the colorspace. How the range of [0…1] is translated to integer values +in the range of [0…255] (or higher, depending on the color depth) is +called the quantization. This is *not* part of the colorspace +definition. In practice RGB or R'G'B' values are full range, i.e. they +use the full [0…255] range. Y'CbCr values on the other hand are limited +range with Y' using [16…235] and Cb and Cr using [16…240]. + +Unfortunately, in some cases limited range RGB is also used where the +components use the range [16…235]. And full range Y'CbCr also exists +using the [0…255] range. + +In order to correctly interpret a color you need to know the +quantization range, whether it is R'G'B' or Y'CbCr, the used Y'CbCr +encoding and the colorspace. From that information you can calculate the +corresponding CIE XYZ color and map that again to whatever colorspace +your display device uses. + +The colorspace definition itself consists of the three chromaticity +primaries, the white reference chromaticity, a transfer function and the +luma coefficients needed to transform R'G'B' to Y'CbCr. While some +colorspace standards correctly define all four, quite often the +colorspace standard only defines some, and you have to rely on other +standards for the missing pieces. The fact that colorspaces are often a +mix of different standards also led to very confusing naming conventions +where the name of a standard was used to name a colorspace when in fact +that standard was part of various other colorspaces as well. + +If you want to read more about colors and colorspaces, then the +following resources are useful: :ref:`poynton` is a good practical +book for video engineers, :ref:`colimg` has a much broader scope and +describes many more aspects of color (physics, chemistry, biology, +etc.). The +`http://www.brucelindbloom.com <http://www.brucelindbloom.com>`__ +website is an excellent resource, especially with respect to the +mathematics behind colorspace conversions. The wikipedia +`CIE 1931 colorspace <http://en.wikipedia.org/wiki/CIE_1931_color_space#CIE_xy_chromaticity_diagram_and_the_CIE_xyY_color_space>`__ +article is also very useful. |