| .. -*- coding: utf-8; mode: rst -*- |
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| .. _colorspaces: |
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| *********** |
| 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. |
| |
| |
| .. ------------------------------------------------------------------------------ |
| .. This file was automatically converted from DocBook-XML with the dbxml |
| .. library (https://github.com/return42/sphkerneldoc). The origin XML comes |
| .. from the linux kernel, refer to: |
| .. |
| .. * https://github.com/torvalds/linux/tree/master/Documentation/DocBook |
| .. ------------------------------------------------------------------------------ |