Red, Green and Blue separately. Colors we see are sensations these wavelengths in the eye. Primaries standardized by the CIE in In the above Chromaticity diagram, white is at the center. Edges are color at the maximum saturation. Chromaticity diagram are three dimensional.
Above diagram is the cross section of the below diagram. Figure 2 shows the used color matching functions. One can then take x and y as chromaticity coordinates, determining color tones for a certain luminance. That system is called CIE xyY, because a color value is defined by the chromaticity coordinates x and y in addition to the luminance coordinate Y.
Unfortunately, a computer monitor can display colors only in a limited part of the XYZ color space, e. Therefore, such CIE chromaticity diagrams, whether printed on paper or displayed on a screen, are usually showing colors only in quite approximate ways.
A rather accurate reproduction could in principle be done with a laser-based display employing carefully controlled mixtures of several primary colors, e. That would go substantially beyond the color limitations of displays with RGB sources using three primary colors , but would also be correspondingly more complex and expensive, and is therefore not common.
Colors for monochromatic light are located on the spectral locus of the chromaticity diagram, which is the curved boundary of the shown gamut; wavelength values in nanometers are indicated in Figure 3. The lower boundary of the gamut, called the line of purples , basically represent some mixtures of red and violet, which cannot be reached with monochromatic light. In real life, one may encounter different white color tones like warm white somewhat shifted towards yellow-orange , cold white with higher blue content , etc.
Figure 3 also shows the color values of blackbody radiation thermal radiation with spectrally uniform emissivity for a range of temperatures. Relatively common e. On computer monitors and projection displays with RGB sources , colors are usually obtained by additive mixing of red, green and blue light. For example, these may be emitted by different phosphors of a cathode ray tube CRT or by light-emitting diodes , or obtained with a suitable optical filters from an LED background light or a high intensity discharge lamp.
For such applications, it is obviously convenient to have colors defined with RGB red—green—blue coordinates, i.
One may then directly control the red, green and blue intensities according to the color coordinates, or sometimes first needs to apply a transformation. In principle, one may also work with a different set of colors, but with a combination of red, green and blue one can best produce a reasonably large color gamut.
The RGB color model defines only the basic principle; one also needs to define the exact primary colors and some other details e. There is a substantial number of different RGB color spaces:. Note that mathematically one can cover the complete gamut of human vision with RGB coordinates, if one allows those to become negative. However, a pixel of a color monitor cannot contribute less than no green light at all, for example, although that would be required to generate color impressions in the deepest red region.
Usually, one considers only non-negative RGB values, corresponding to colors which can physically be generated by mixing the used primary colors. Devices using RGB color profiles are often equipped with a color profile most often based on a definition from ICC, the International Color Consortium , which essentially defines more closely the used primary colors. One may, for example, use a colorimeter to generate a color profile of a computer monitor; the resulting file can then be used for properly adjusting the color values, e.
Also, a photo camera my embed its color profile into produced JPEG images files in order to clearly define what color space has been used. CMYK color spaces are based on a subtractive color model, as is appropriate in the context of printing, where colors are generated by mixing pigments or dyes.
Here, each segment absorbs light in some range of wavelengths. Although human vision is trichromatic, meaning that three different pigments would in principle be sufficient, a combination of four has been chosen to obtain a wider color gamut. Nevertheless, CMYK color gamuts are relatively narrow, with substantial deficiencies particularly in the blue—green region, but also for deep red color tones.
This reflects the fundamental difficulties of achieving such colors with pigments. We have mentioned before that it was important to make the separate the concept of chromaticity which defined how colorful a color is from the concept of a color's brightness. A typical example is the color white which has the same chromaticity than a gray color but doesn't have the same brightness than white. The xyY color space was developed in order to be able to separate these two properties and use only two components x and y to encode the color's chromaticity and keep the Y value from the XYZ tristimulus values to encode the color's brightness or value.
The idea is simple. It consists of normalizing the three components of a XYZ color by the sum of these components:. Once normalized, the values of the resulting x y and z components sum up to 1. Therefore we can find the value of any of the components if we know the values of the other two. It actually means that with only two components we can define the chromaticity of a color.
In the xyY color space, these two components are the x and y normalized values which we have just computed above we discard the z value which we can recompute if necessary from x and y as just showed. Finally, because it is important to keep track of the original color's brightness, we will also store the original Y value from the XYZ color next to the x and y values.
As we mentioned before, in the XYZ color space, Y represents the color's brightness. In the xyY color space, the xy values can be seen as a representation of the color's chromaticity while the Y values can be seen as a representation of the color's intensity or brightness value.
Because they are now defined using only two coordinates, the colors of the gamut can be plotted in a 2D coordinate system as showed in figure 1. Other color spaces such as the RGB color space which we will be taking about next, have their primaries red, blue, green as well as their white point defined with regard to that xy "chromaticity" coordinate system. Figure 4: in RGB color space, the subspace of interest is a cube in which RGB values are at three corners; cyan, magenta, and yellow are the three other corners, black is at their origin; and white is at the corner farthest from the origin.
The idea of the RGB color space is really to stick to the principle of human vision and represent colors as a simple sum of any quantities from 0 to 1 of the primary colors red, green and blue. As such it can be represented as a simple cube figure 4 where three of the vertices represent the primary colors. Moving in one direction along the vertices of this cube results in blending two of the primary colors together which leads when we reach the vertex of the cube opposite to two primary colors, to a secondary color either cyan, magenta or yellow.
Two of the cube vertices are special as they correspond to white when the three primary colors are mixed up together in full amount and black absence of any of the three primary colors. These vertices defines a diagonal along which all the colors are gray gray scale. Note here that we speak of color in terms of their chromaticity, not in terms of their possible brightness or value. Figure 5: the gamut of the RGB color space is defined by the colors contained within the triangle.
The RGB color space is one of the most basic color spaces in existence which is probably a reason for its popularity. It is also simple to understand and visualize as with the example of the cube.
However its gamut is much more limited than that of the XYZ color space as showed in figure 5. It can be represented as a triangle contained within the horseshoe shape of the XYZ's gamut where each of the triangle vertices represents a primary color expressed with regard to the XYZ color space more precisely these coordinates are expressed in the xyY color space which we have used before to draw the horseshoe shaped XYZ gamut.
The coordinates of the red, green and blue RGB colors expressed with regards to the xyY color space are summarized in the table on the right. Note that the coordinates of the white point are also defined the white point of a particular color space might vary depending on the viewing conditions it was designed for.
In the case of CIE RGB, the white point is defined as being the illuminant E which is, like the illuminant D65 which we talked about in the previous chapter, a predefined CIE illuminant that has the property of having a constant SPD across the visible spectrum an illuminant that gives equal weight to all wavelengths. We can plot these coordinates on our existing horseshoe shape and draw lines to connect the dots.
You can see that it defines a triangle. All colors contained within the limits of this triangle are the color which we can represent if we use the RGB color space. You can also see that the colors that we can represent with this model, are much more limited than those we can define using the XYZ color space see figure 5.
Figure 6: close up on a computer screen. Each pixel is made of a red, green and blue light which intensities vary to create colors. Most computer screens technology is based on a system similar to the RGB color model. Each pixel on the screen is made of three small lights, one red, one green and one blue, which contribution sums up to white when they are all three turned on at the same time when you look at the screen up close you can distinguish the three color but from the distance they blend into white.
However we will show in the next chapter that computer screens can also manipulate colors in a way that we are not always very much aware of. Several other models based on the same principle as the RGB color space have been proposed to enhance the original model or as an attempt to address particular issues.
For example the sRGB color model applies a gamma correction which we will explain in the next chapter to the original RGB values as a way of better representing the human vision response to variations of brightness which is non linear. Such models are said to be non linear because they modify the original RGB or XYZ linear tristimulus values into non linear values. This will be fully explained in the next chapter.
Remember from the beginning of this chapter that colors defined in a particular color space are usually defined by three coordinates that is at least the case of the RGB and XYZ color spaces which can be plotted in 3D space.
And when all the colors from the gamut that one particular color space can represent, are plotted in 3D space, they define a volume a cube in the case of the RGB model for example as illustrated in figure 4. The conversion of a color defined in one color space to another color space can simply be seen as moving a point in 3D space from one position to another. And generally such linear transformation is best handled by matrices. Keep in mind that this transformation do not change the color itself.
It is used to express the same color in different color spaces. Still wider is the Adobe Wide Gamut color space. This color space is not widely used and, because of the very wide gamut, justifies using a 16bit color depth per channel.
Bit depth will also be discussed further. An even wider color space is called PhotoPro which uses a spectral red but imaginary not present in the chromaticity diagram blue and green primaries.
The figure below exemplifies how the proportions of red, green and blue affect the look of a color. The RGB color spaces described above additively mix the primaries to obtain the desired color. Subtractive mixing uses mainly cyan, magenta and yellow as primaries and is generally used for white media i.
It is interesting to observe that taking any RGB color space triangle in the chromaticity diagram , the cyan, magenta, and yellow colors are situated roughly halfway between two RGB primaries on the sides of the triangle. CMY color spaces generally have a smaller gamut than the sRGB color space, particularly because that they do not contain very dark saturated colors. Subtracting fully saturated C, M and Y from white at the same time does not create black. This is the reason black is usually added as a primary.
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