diff options
Diffstat (limited to 'Documentation/media/uapi/v4l/colorspaces.rst')
| -rw-r--r-- | Documentation/media/uapi/v4l/colorspaces.rst | 170 |
1 files changed, 0 insertions, 170 deletions
diff --git a/Documentation/media/uapi/v4l/colorspaces.rst b/Documentation/media/uapi/v4l/colorspaces.rst deleted file mode 100644 index 4f6c82fa057f..000000000000 --- a/Documentation/media/uapi/v4l/colorspaces.rst +++ /dev/null @@ -1,170 +0,0 @@ -.. 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/media/uapi/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. |