Definition of rgb. RGB and CMYK Color Models: An Accessible Guide

Good day, dear readers, acquaintances, visitors, passing individuals and other strange creatures! Today we will talk about a slightly specific, but undoubtedly important thing for any user, namely this thing: the representation of color in a computer.

Whatever one may say, sooner or later everyone will be faced with the practical need to understand what a color model is, and simply this knowledge is useful from the point of view of broadening one’s horizons and awareness of what and how it works in a computer and what it consists of, both software and and from a physical point of view.

What is a color model

In general color model- this is some abstract thing in which color is represented as a collection of numbers. And each such model has its own characteristics and disadvantages. Essentially, it’s like with a language, for example, if a color is the word “house,” then in different languages ​​it will be written and sounded differently, but the meaning of the word will be the same everywhere. It's the same with color.

We will look at the most basic models. Their 5 . As a rule, several different models are used simultaneously, because some are best used visually, while others are best used numerically.

RGB

This is the most common color representation model. In it, any color is considered as shades of three primary (or basic) colors: red, green (Green) and blue (Blue). There are two types of this model: eight-bit representation where the color is specified by numbers from 0 before 255 (for example color will correspond to blue, and - yellow), and sixteen-bit, which is most often used in graphic editors and html, where the color is specified by numbers from 0 before ff(green - # 00ff00, blue - # 0000ff, yellow - # ffff00).

The difference in ideas is that in eight-bit form, a separate scale is used for each base color, and in sixteen-bit color is immediately introduced. In other words, eight-bit presentation - three scales with each primary color, sixteen-bit- one scale with three colors.

The peculiarity of this model is that here a new color is obtained by adding shades of primary colors, i.e. "mixing".

In the picture above you can see how the colors mix with each other to form new colors (yellow - [ 255,255,0 ], purple - [ 255,0,255 ], blue - [ 0,255,255 ] and white [ 255,255,255 ]).

Moreover, this model is most often used in numerical form, and not in visual form (when the color is set by entering its value in the corresponding field, and not selected with the mouse). Other models are used to visually adjust color. Because visually the model RGB is a three-dimensional cube, which, as you can see in the picture above, is not very convenient to use :)

So this is the most common model among web designers (we send our warm regards css) and programmers.

The disadvantage of this model is that it depends on the hardware, in other words, the same picture will look different on different monitors (because monitors use a so-called phosphor - a substance that converts the energy it absorbs into light radiation, and therefore Depending on the quality of this substance, the basic colors will be determined).

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CMYK

This is also a very common model, but many may not have heard anything about it at all :)

And all due to the fact that it is used exclusively for printing. It stands for Cyan, Magenta, Yellow, Black(or Key Color), i.e. Cyan, Magenta, Yellow And Black(or key color).

The use of this model in printing is due to the fact that mixing three shades for each new color is too expensive and dirty, because when one color is first applied to paper, then another on top of it, and then a third color on top of them, firstly, the paper gets very wet (if inkjet printing), and secondly, it is not at all a fact that you will get exactly the shade that you wanted. Yes, that's how physics works :)

The most attentive may have noticed that there are three colors in the picture, and black is obtained by mixing these three. So, why was he taken out separately? Again, the reason is that, firstly, mixing three colors is expensive in terms of using toner (special powder for a printer cartridge, which is used instead of ink in laser printers), and secondly, the paper gets very wet, which increases drying time, thirdly, the colors may not actually mix properly, but may be more faded, for example. The picture below shows this model in reality

Thus, the result will not be black, but dirty gray or dirty brown.

That’s why (and not only) they introduced black color, so as not to stain the paper, not to spend money on toners, and in general to make life easier :)

The following animation very clearly illustrates the whole point (opens by clicking, weight approx. 14 Mb):

The color in this model is specified by numbers from 0 before 100 , where these numbers are often called "parts" or "portions" of the selected color. For example, khaki color is obtained by mixing 30 pieces of blue paint, 45 - purple, 80 - yellow and 5 - black, i.e. khaki color will be .

The difficulties of this model lie in the fact that in harsh realities (or in real harsh conditions) color depends not so much on numerical data as on the characteristics of the paper, the ink in the toner, the method of applying this ink, etc. So the numerical values ​​will clearly indicate the color on the monitor, but they will not show the actual picture on paper.

HSV (HSB) and HSL

I combined these two color models because... they are similar in principle.

3D implementation HSL(left) and HSV(on the right) of the models is presented in the form of a cylinder below, but in practice it is not used in software (software), because .. because it is three-dimensional :)

HSV (or HSB) means Hue, Saturation, Value(may also be called Brightness), Where:

• Hue- color tone, i.e. color shade.
• Saturation- saturation. The higher this parameter, the “purer” the color will be, and the lower, the closer it will be to gray.
• Value(Brightness) - value (brightness) of color. The higher the value, the brighter the color will be (but not whiter). And the lower, the darker (0% - black)

HSL - Hue, Saturation, Lightness

• Hue- You already know
• Saturation- similar
• Lightness- this is the lightness of the color (not to be confused with brightness). The higher the parameter, the lighter the color (100% - white), and the lower, the darker (0% - black).

A more common model is HSV, it is often used together with the model RGB, Where HSV is shown visually, and numerical values ​​are specified in RGB. :

Here RGB- the model is circled in red and the shade values ​​are given by numbers from 0 before 255 , or you can immediately specify the color in hexadecimal form. And circled in blue HSV model (visual part in left rectangle, numeric - in right). You can also often specify opacity (called alpha channel).

This model is most often used in simple (or non-professional) image processing, because Using it, it’s convenient to adjust the basic parameters of photos without resorting to a bunch of different filters or individual settings.
For example, in everyone’s favorite (or cursed) Photoshop, both models are present, only one of them is in the color selection editor, and the other is in the settings window Hue/Saturation

Showing in red here RGB- model, blue - H.S.B., green - CMYK and blue Lab(more on her a little later), as can be seen in the picture :)
A HSL- The model is in a window like this:

Flaw HSB- model is that it also depends on the hardware. It simply does not correspond to the perception of the human eye, because... It perceives colors with different brightness (for example, blue is perceived by us as darker than red), but in this model all colors have the same brightness. U HSL similar problems :)

They wanted to avoid such shortcomings, so one well-known company CIE(International Illumination Commission - Commission Internationale de l'Eclairage) came up with a new model designed to be independent of hardware. And they named her Lab(no, this is not an abbreviation for Laboratory).

Lab or L,a,b

This model is one of the standard ones, although it is little known to the average user.

It is deciphered as follows:

• L - Luminance- illumination (this is a combination of brightness and intensity)
• a- one of the components of color, changes from green to red
• b- the second of the color components, changes from blue to yellow

The figure shows the component ranges a And b For illumination 25% (left) and 75% (right)

The brightness in this model is separated from the colors, so it is convenient to use it to adjust contrast, sharpness and other light indicators without touching the colors :)

However, this model is not at all obvious to use and is quite difficult to use in practice. Therefore, it is used mainly in image processing and for converting them from one color model to another without loss (yes, this is the only model that does this without loss), but for ordinary mortal suffering users, as a rule, it is enough HSL And HSV plus filters.

Well, as an example of how the model works HSV, HSL And Lab here is a picture from Wikipedia (clickable)

We perceive the world around us through various factors, one of which is color. A person opens his eyes and sees different colors, and if you need to tell another person about these colors, then you can say something like “his pants are like ripe lemons” or “her eyes are like a clear sky” and the person basically understands what color the pants are and eyes, even if he doesn't see them.

That is, transmitting information about color from person to person is not difficult. And if it is not people who must operate with color information, but some technical devices, then the “eyes like a clear sky” option will not work. We need some other description of color that is understandable to these devices (monitors, printers, cameras, etc.). This is exactly what color models are for.

Types of color models

There are many color models, the most commonly used ones can be divided into three groups:

• hardware dependent— color models of this group describe color in relation to a specific color-reproducing device (for example, a monitor), - RGB, CMYK
• hardware independent- this group of color models in order to give unambiguous information about color - XYZ, Lab
• psychological- these models are based on the characteristics of human perception - HSB, HSV, HSL

Let's take a look at some frequently used color models separately.

This color model describes the color of a light source (this could include, for example, a monitor or TV screen). From a huge variety of colors, three colors were identified as the main (primary) ones: red ( B ed), green ( G reen), blue ( B lue). The first letters of the names of the primary colors form the name of the RGB color model.

When two primary colors are mixed, the resulting color lightens: red and green make yellow, green and blue make cyan, and blue and red make purple. If you mix all three primary colors, white is formed. Such colors are called additive.

This model can be represented as a three-dimensional coordinate system, where each one reflects the value of one of the primary colors in the range from zero to maximum. The result is a cube containing all the colors that form the RGB color space.

Important points and lines of the RGB model

• Origin of coordinates: at this point the values ​​of all primary colors are zero, there is no radiation, i.e. it is a black point.
• At the point closest to the viewer, all components have a maximum value, this means maximum luminescence - a white point.
• On the line connecting these points (along the diagonal of the cube), there are shades of gray: from black to white. This range is otherwise called the gray scale.
• Three vertices of the cube give pure original colors, the other three reflect double mixtures of the original colors.

The advantage of this model is that it describes all 16 million colors, but the disadvantage is that during printing some (the brightest and most saturated) of these colors will be lost.

Since RGB is a hardware-dependent model, the same picture on different monitors may differ in color, for example, because the screens of these monitors are made using different technologies or the monitors are configured differently.

If the previous model describes luminous colors, then CMYK, on ​​the contrary, describes reflected colors. They are also called subtractive (“subtractive”) because they remain after subtracting the main additive ones. Since we have three colors for subtraction, there will also be three primary subtractive colors: blue ( C yan), purple ( M agenta), yellow ( Y ellow).

The three primary colors of the CMYK model are called the printing triad. When printing with these inks, the red, green and blue components are absorbed. In a CMYK image, each pixel has a percentage value of process inks.

When we mix two subtractive paints, the resulting color is darkened, but if we mix three, the result should be black. When all colors are set to zero, we get white. And when the values ​​of all components are equal, we get a gray color.

In fact, it turns out that if we mix three colors at maximum values, instead of a deep black color, we end up with a dirty dark brown color. This is because printing inks are not perfect and cannot reflect the entire color range.

To compensate for this problem, a fourth black color was added to this triad, which added the last letter to the name of the color model WITH - C yan (Blue), M - M agenta (Purple), Y - Y ellow (Yellow), TO- blac K(Black). All paints are usually designated by the initial letter of the name, but black was designated by the last letter. Why? .

Like RGB, CMYK is also a hardware-dependent model. The final result depends on the paint, the type of paper, the printing machine, and the features of the printing technology. Therefore, the same image in different printing houses may be printed differently.

HSB color model

If the above-described models are combined into one, the result can be depicted in the form of a color wheel, where the primary colors of the RGB and CMY models are located in the following relationship: each color is opposite the complementary color that complements it and between the colors with which it is formed.

To strengthen a color, you need to weaken the color opposite (complementary). For example, to enhance yellow, you need to weaken blue.

To describe the color in this model there are three parameters H ue (hue) - shows the position of the color on the color wheel and is indicated by the angle value from 0 to 360 degrees, S aturation - determines the purity of the color (decreasing saturation is similar to adding white to the original color), B rightness (brightness) - shows the lightness or shading of a color (decreasing the brightness is similar to adding black paint). The first letters in the names of these parameters give the name of the color model.

The HSB model agrees well with human perception: hue is the wavelength of light, saturation is the intensity of the wave, and brightness is the amount of light.

The disadvantage of the HSB model is the need to convert it into RGB for display on a monitor screen or in CMYK for print.

This model was created by the International Commission on Illumination in order to overcome the shortcomings of previous models. It was necessary to create a hardware-independent model to determine color independent of device parameters.

In the Lab model, color is represented by three parameters:

• L- lightness
• a- chromatic component ranging from green to red
• b- chromatic component ranging from blue to yellow

When transferring a color from a model to Lab, all colors are preserved, since Lab space is the largest. Therefore, this space is used as an intermediary when converting color from one model to another.

Grayscale color model

The simplest and most understandable space is used to display a black and white image. Color in this model is described by just one parameter. The parameter value can be in gradations (from 0 to 256) or as a percentage (from 0% to 100%). The minimum value corresponds to white, and the maximum value corresponds to black.

Index colors

It’s unlikely that a pre-printer will have to work with index colors, but it won’t hurt to know what they are.

So, once upon a time, at the dawn of computer technology, computers could display no more than 256 colors on the screen at the same time, and before that 64 and 16 colors. Based on these conditions, an index method of color coding was invented. Each color contained in the image received a serial number; this number was used to describe the color of all pixels that have the corresponding color. But different images have different sets of colors, and therefore each picture had to store its own set of colors (the set of colors was called a color table).

Modern computers (even the simplest ones) are capable of displaying 16.8 million colors on the screen, so there is no particular need to use index colors. But with the development of the Internet, this model is being used again. This is because such a file can be much smaller in size.

HEX/HTML

HEX color is nothing but a hexadecimal representation of RGB.

Colors are represented as three groups of hexadecimal digits, where each group is responsible for its own color: #112233, where 11 is red, 22 is green, 33 is blue. All values ​​must be between 00 and FF.

Many applications allow a shortened form of hexadecimal color notation. If each of the three groups contains the same characters, for example #112233, then they can be written as #123.

1. h1 ( color: #ff0000; ) /* red */
2. h2 ( color: #00ff00; ) /* green */
3. h3 ( color: #0000ff; ) /* blue */
4. h4 ( color: #00f; ) /* same blue, shorthand */

RGB

The RGB (Red, Green, Blue) color space consists of all possible colors that can be created by mixing red, green, and blue. This model is popular in photography, television, and computer graphics.

RGB values ​​are specified as an integer from 0 to 255. For example, rgb(0,0,255) is displayed as blue because the blue parameter is set to its highest value (255) and the others are set to 0.

Some applications (particularly web browsers) support percentage recording of RGB values ​​(from 0% to 100%).

1. h1 ( color: rgb(255, 0, 0); ) /* red */
2. h2 ( color: rgb(0, 255, 0); ) /* green */
3. h3 ( color: rgb(0, 0, 255); ) /* blue */
4. h4 ( color: rgb(0%, 0%, 100%); ) /* same blue, percentage entry */

RGB color values ​​are supported in all major browsers.

RGBA

Recently, modern browsers have learned to work with the RGBA color model - an extension of RGB with support for an alpha channel, which determines the opacity of an object.

The RGBA color value is specified as: rgba(red, green, blue, alpha). The alpha parameter is a number ranging from 0.0 (fully transparent) to 1.0 (fully opaque).

1. h1 ( color: rgb(0, 0, 255); ) /* blue in regular RGB */
2. h2 ( color: rgba(0, 0, 255, 1); ) /* the same blue in RGBA, because opacity: 100% */
3. h3 ( color: rgba(0, 0, 255, 0.5); ) /* opacity: 50% */
4. h4 ( color: rgba(0, 0, 255, .155); ) /* opacity: 15.5% */
5. h5 ( color: rgba(0, 0, 255, 0); ) /* completely transparent */

RGBA is supported in IE9+, Firefox 3+, Chrome, Safari, and Opera 10+.

HSL

The HSL color model is a representation of the RGB model in a cylindrical coordinate system. HSL represents colors in a more intuitive and human-readable way than typical RGB. The model is often used in graphics applications, color palettes, and image analysis.

HSL stands for Hue (color/hue), Saturation (saturation), Lightness/Luminance (lightness/lightness/luminosity, not to be confused with brightness).

Hue specifies the color's position on the color wheel (from 0 to 360). Saturation is the percentage value of the saturation (from 0% to 100%). Lightness is a percentage of lightness (from 0% to 100%).

1. h1 ( color: hsl(120, 100%, 50%); ) /* green */
2. h2 ( color: hsl(120, 100%, 75%); ) /* light green */
3. h3 ( color: hsl(120, 100%, 25%); ) /* dark green */
4. h4 ( color: hsl(120, 60%, 70%); ) /* pastel green */

HSL is supported in IE9+, Firefox, Chrome, Safari, and Opera 10+.

HSLA

Similar to RGB/RGBA, HSL has an HSLA mode with alpha channel support to indicate the opacity of an object.

The HSLA color value is specified as: hsla(hue, saturation, lightness, alpha). The alpha parameter is a number ranging from 0.0 (fully transparent) to 1.0 (fully opaque).

1. h1 ( color: hsl(120, 100%, 50%); ) /* green in normal HSL */
2. h2 ( color: hsla(120, 100%, 50%, 1); ) /* the same green in HSLA, because opacity: 100% */
3. h3 ( color: hsla(120, 100%, 50%, 0.5); ) /* opacity: 50% */
4. h4 ( color: hsla(120, 100%, 50%, .155); ) /* opacity: 15.5% */
5. h5 ( color: hsla(120, 100%, 50%, 0); ) /* completely transparent */

CMYK

The CMYK color model is often associated with color printing and printing. CMYK (unlike RGB) is a subtractive model, meaning that higher values ​​are associated with darker colors.

Colors are determined by the ratio of cyan (Cyan), magenta (Magenta), yellow (Yellow), with the addition of black (Key/blacK).

Each of the numbers that define a color in CMYK represents the percentage of ink of a given color that makes up the color combination, or more precisely, the size of the screen dot that is output on the phototypesetting machine on film of that color (or directly on the printing plate in the case of CTP).

For example, to obtain the PANTONE 7526 color, you would mix 9 parts cyan, 83 parts magenta, 100 parts yellow, and 46 parts black. This can be denoted as follows: (9,83,100,46). Sometimes the following designations are used: C9M83Y100K46, or (9%, 83%, 100%, 46%), or (0.09/0.83/1.0/0.46).

HSB/HSV

HSB (also known as HSV) is similar to HSL, but they are two different color models. They are both based on cylindrical geometry, but HSB/HSV is based on the "hexcone" model, while HSL is based on the "bi-hexcone" model. Artists often prefer to use this model, it is generally accepted that the HSB/HSV device is closer to the natural perception of colors. In particular, the HSB color model is used in Adobe Photoshop.

HSB/HSV stands for Hue (color/hue), Saturation (saturation), Brightness/Value (brightness/value).

Hue specifies the color's position on the color wheel (from 0 to 360). Saturation is the percentage value of the saturation (from 0% to 100%). Brightness is a percentage of brightness (from 0% to 100%).

XYZ

The XYZ color model (CIE 1931 XYZ) is a purely mathematical space. Unlike RGB, CMYK, and other models, in XYZ the principal components are “imaginary,” meaning you cannot associate X, Y, and Z with any set of colors to mix. XYZ is the master model for almost all other color models used in technical fields.

LAB

The LAB color model (CIELAB, “CIE 1976 L*a*b*”) is calculated from the CIE XYZ space. Lab's design goal was to create a color space in which color changes would be more linear in terms of human perception (compared to XYZ), that is, so that the same change in color coordinate values ​​in different regions of the color space would produce the same sensation of color change.

RGB color model(from the English Red, Green, Blue - red, green, blue) - an additive color model that describes a method of color synthesis for color reproduction. In the Russian tradition it is sometimes referred to as KZS.

Story
In 1861, the English physicist James Clark Maxwell came up with a proposal to use a method for obtaining a color image, which is known as additive color fusion. The additive (summative) color rendering system means that the colors in this model are added to the black color. Additive color shift can be interpreted as the process of combining light streams of different colors before they reach the eye.
Additive color models (from the English add - add) are color models in which a luminous flux with a spectral distribution, visually perceived as the desired color, is created based on the operation of proportional mixing of light emitted by three sources. Mixing schemes can be different, one of them is presented in
The additive color model assumes that each light source has its own constant spectral distribution, and its intensity is adjustable.
There are two types of additive color model: hardware dependent and perceptual. In the device-dependent model, the color space depends on the characteristics of the image output device (monitor, projector). Because of this, the same image presented based on such a model will be visually perceived slightly differently when played on different devices.
The perceptual model is built taking into account the characteristics of the observer's vision, and not the technical characteristics of the device.
In 1931, the International Commission on Illumination (CIE) standardized the color system and also completed work that created a mathematical model of human vision. The CIE 1931 XYZ color space was adopted and remains the basic model to this day.

Mechanism of flower formation
When a person perceives color, it is they that are directly perceived by the eye. The remaining colors are a mixture of three basic colors in different proportions. The color model is shown here . R+G=Y (Yellow); G+B=C (Cyan - blue); B+R=M (Magenta - purple). The sum of all three primary colors in equal parts gives white (White) color R+G+B=W (White - white). For example, on the screen of a monitor with a cathode ray tube, as well as a similar TV, an image is created by illuminating a phosphor with a beam of electrons. With this effect, the phosphor begins to emit light. Depending on the composition of the phosphor, this light has one color or another.
Intermediate shades are obtained due to the fact that different colored grains are located close to each other. At the same time, their images in the eye merge, and the colors form some mixed shade. If grains of one color are illuminated differently than the others, then the mixed color will not be a shade of gray, but will acquire color. This method of color formation is reminiscent of illuminating a white screen in complete darkness with multi-colored spotlights. If we encode the color of one image point with three bits, each of which will indicate the presence (1) or absence (0) of the corresponding system component, RGB 1 bit for each RGB component, then we will get all eight different colors . In practice, to store information about the color of each point of a color image in the RGB model, 3 bytes (i.e. 24 bits) are usually allocated, 1 byte (i.e. 8 bits) for the color value of each component. Thus, each RGB component can take a value in the range from 0 to 255 (total 2 to the 8th power = 256 values). Therefore, you can mix colors in different proportions, changing the brightness of each component. Thus, you can get 256 x 256 x 256 = 16,777,216 colors. RGB coordinates varying in the range from 0 to 255 form a color cube. . Any color is located inside this cube and is described by its own set of coordinates, showing in what proportions the red, green and blue components are mixed in it. The ability to display no less than 16.7 million shades is a full-color image type that is sometimes called True Color (true or true colors). because the human eye is still unable to discern greater diversity. The maximum brightness of all three basic components corresponds to white, the minimum to black. Therefore, white color has the code (255,255,255) in decimal, and FFFFFF in hexadecimal. Black color encodes (0,0,0) or 000000, respectively. All shades of gray are formed by mixing three components of the same brightness. For example, (200,200,200) or C8C8C8 produces a light gray color, while (100,100,100) or 646464 produces a dark gray color. The darker the shade of gray you want to achieve, the lower the number you need to enter in each text field. Black color is formed when the intensity of all three components is zero, and white - when their intensity is maximum.

Restrictions
The RGB color model has three fundamental disadvantages: The first is insufficient color gamut. Regardless of the size of the color space of the RGB color model, it is impossible to reproduce many colors perceived by the eye (for example, spectrally pure blue and orange). Such colors in the RGB color formula have negative intensities of the base color, and it is very difficult to implement not addition, but subtraction of base colors in the technical implementation of the additive model. This shortcoming is eliminated in the perceptual additive model.
The second disadvantage of the RGB color model is the impossibility of uniform color reproduction on different devices (hardware dependency) due to the fact that the basic colors of this model depend on the technical parameters of the image output devices. Therefore, strictly speaking, there is no single RGB color space; the regions of reproduced colors are different for each output device. Moreover, even comparing these spaces numerically is only possible using other color models. The third drawback is the correlation of color channels (as the brightness of one channel increases, others reduce it).

Advantages
A lot of computer equipment works using the RGB model, in addition, this model is very simple, its “genetic” relationship with the equipment (scanner and monitor), wide color gamut (the ability to display a variety of colors close to the capabilities of human vision) this explains its wide distribution .
The main advantages of the RGB color model are its simplicity, clarity, and the fact that any point in its color space corresponds to a visually perceived color.
Due to the simplicity of this model, it can be easily implemented in hardware. In particular, in monitors, microscopic particles of three types of phosphor serve as controlled light sources with different spectral distributions. They are clearly visible through a magnifying glass, but when viewing the monitor with the naked eye, due to the phenomenon of visual closure, a continuous image is visible.
The intensity of light radiation in monitors based on cathode ray tubes is controlled using three electron guns that excite the glow of phosphors. The availability of many image processing procedures (filters) in raster graphics programs, the small (compared to the CMYK model) volume occupied by the image in the computer’s RAM and on disk.

Application
The RGB color model is widely used in computer graphics for the reason that the main output device (monitor) works in this system. The image on the monitor is formed from individual luminous points of red, green and blue colors. By looking at the screen of a working monitor through a magnifying glass, you can see individual colored dots - and it’s even easier to see this on a TV screen, since its dots are much larger.
Widely used in the development of electronic (multimedia) and printed publications.
Illustrations made using raster graphics are rarely created manually using computer programs. Most often, scanned illustrations prepared by the artist on paper or photographs are used for this purpose.
Recently, digital photo and video cameras have found widespread use for inputting raster images into a computer. Accordingly, most graphic editors designed for working with raster illustrations are focused not so much on creating images, but on processing them. On the Internet, raster illustrations are used in cases where it is necessary to convey the full range of shades of a color image.

Sources used
1. Domasev M.V. Color, color management, color calculations and measurements. St. Petersburg: Peter 2009
2. Petrov M. N. Computer graphics. Textbook for universities. St. Petersburg: Peter 2002
3. ru.wikipedia.org/wiki/Color model.
4. darkroomphoto.ru
5. bourabai.kz/graphics/0104.htm
6.litpedia.ru
7. youtube.com/watch?v=sA9s8HL-7ZM

Very often, people who are not directly involved in print design have questions: “What is CMYK?”, “What is Pantone?” and "why can't you use anything other than CMYK?"

In this article we will try to understand a little what color spaces are. CMYK, RGB, LAB, HSB and how to use paints Pantone in layouts.

Color model

CMY(K), RGB, Lab, HSB is a color model. Color model- a term denoting an abstract model for describing the representation of colors as tuples of numbers, usually three or four values, called color components or color coordinates. Together with the method for interpreting this data, the set of colors in a color model defines a color space.

RGB- abbreviation of English words Red, Green, Blue- red, green, blue. Additive (Add, English - add) color model, usually used to display images on monitor screens and other electronic devices. As the name implies, it consists of blue, red and green colors, which form all the intermediate ones. Has a large color gamut.

The main thing to understand is that the additive color model assumes that the entire color palette is made up of luminous points. That is, on paper, for example, it is impossible to display color in the RGB color model, since paper absorbs color and does not glow on its own. The final color can be obtained by adding percentages from each of the key colors to the original black (non-luminous) surface.

CMYK - Cyan, Magenta, Yellow, Key color- subtractive (subtract, English - subtract) color formation scheme used in printing for standard process printing. It has a smaller color gamut compared to RGB.

CMYK is called a subtractive model because paper and other printed materials are surfaces that reflect light. It is more convenient to calculate how much light was reflected from a particular surface rather than how much was absorbed. Thus, if we subtract three primary colors - RGB - from white, we get three additional CMY colors. "Subtractive" means "subtractive" - ​​the primary colors are subtracted from white.

Key Color(black) is used in this color model as a replacement for mixing equal parts of the CMY triad colors. The fact is that only in the ideal case, when mixing the colors of the triad, a pure black color is obtained. In practice, it will turn out, rather, dirty brown - as a result of external conditions, the conditions of paint absorption by the material and the imperfection of dyes. In addition, the risk of non-registration in elements printed in black, as well as waterlogging of the material (paper), increases.

In color space Lab the value of lightness is separated from the value of the chromatic component of color (hue, saturation). Lightness is specified by the L coordinate (varies from 0 to 100, that is, from the darkest to the lightest), the chromatic component is specified by two Cartesian coordinates a and b. The first denotes the color position in the range from green to purple, the second - from blue to yellow.

Unlike RGB or CMYK color spaces, which are essentially a set of hardware data for reproducing color on paper or on a monitor screen (color may depend on the type of printing machine, brand of ink, humidity in production, or the manufacturer of the monitor and its settings) ,Lab uniquely identifies the color. Therefore, Lab has found widespread use in image processing software as an intermediate color space through which data is converted between other color spaces (for example, from the RGB of a scanner to the CMYK of a printing process). At the same time, the special properties of Lab made editing in this space a powerful color correction tool.

Due to the nature of the color definition in Lab, it is possible to separately influence the brightness, contrast of the image and its color. In many cases, this allows for faster image processing, for example during prepress. Lab provides the ability to selectively influence individual colors in an image, enhancing color contrast, and the capabilities that this color space provides for combating noise in digital photographs are also irreplaceable.

H.S.B.- a model that is, in principle, an analogue of RGB, it is based on its colors, but differs in the coordinate system.

Any color in this model is characterized by Hue, Saturation and Brightness. Tone is the actual color. Saturation is the percentage of white paint added to the color. Brightness is the percentage of added black paint. So, HSB is a three-channel color model. Any color in HSB is obtained by adding black or white to the main spectrum, i.e. actually gray paint. The HSB model is not a rigorous mathematical model. The description of colors in it does not correspond to the colors perceived by the eye. The fact is that the eye perceives colors as having different brightnesses. For example, spectral green has greater brightness than spectral blue. In HSB, all colors in the main spectrum (hue channel) are considered to have 100% brightness. This is actually not true.

Although the HSB model is declared as hardware-independent, in fact it is based on RGB. In any case, HSB is converted to RGB for display on the monitor and to CMYK for printing, and any conversion is not without losses.

Standard paint set

In the standard case, printing is carried out using cyan, magenta, yellow and black inks, which, in fact, makes up the CMYK palette. Layouts prepared for printing must be in this space, since in the process of preparing photo forms, the raster processor unambiguously interprets any color as a CMYK component. Accordingly, an RGB pattern that looks very beautiful and bright on the screen will look completely different on the final product, but rather gray and pale. The CMYK color gamut is smaller than RGB, so all images prepared for printing require color correction and correct conversion to the CMYK color space!. In particular, if you use Adobe Photoshop to process raster images, you should use the Convert to Profile command from the Edit menu.

Printing with additional inks

Due to the fact that the CMYK color gamut is not enough to reproduce very bright, “poisonous” colors, in some cases CMYK printing + additional (SPOT) paints. Additional colors are usually called Pantone, although this is not entirely true (the Pantone catalog describes all colors, both included in CMYK and not contained in it) - it is correct to call such colors SPOT (spot), in contrast to spot colors, that is, CMYK.

Physically, this means that instead of four printing units with standard CMYK colors, more are used. If there are only four printing sections, an additional run is organized, during which additional colors are imprinted into the finished product.

There are presses with five printing units, so all colors are printed in one pass, which undoubtedly improves the quality of color registration in the finished product. When printing in 4 CMYK sections and additionally running through a printing machine with spot inks, color matching may suffer. This will be especially noticeable on machines with less than 4 printing sections - you’ve probably seen advertising leaflets more than once, where a yellow frame may protrude slightly beyond the edges of, for example, beautiful bright red letters, which is nothing more than yellow paint from the layout this beautiful red color.

Preparation of layouts for printing

If you are preparing a layout for printing in a printing house and you have not agreed on the possibility of printing with additional (SPOT) inks, prepare the layout in the CMYK color space, no matter how attractive the colors in the Pantone palettes may seem to you. The fact is that to simulate Pantone colors on screen, colors that fall outside the CMYK color space are used. Accordingly, all your SPOT inks will be automatically converted to CMYK and the result will not be at all what you expect.

If your layout (with an agreement to use a triad) still contains non-CMYK paints, be prepared for the layout to be returned to you and asked to be remade.