What colors are included in the rgb model. RGB, CMYK, XYZ and other image color schemes

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.

The origins of the RGB color model

In the middle of the 19th century, the English physicist James Clerk 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 shown in Figure 1.

Figure 1. Scheme of mixing light fluxes in an additive color model

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.

RGB is used in computer monitors, televisions, scanners, digital cameras and other light-emitting technical devices.

From a monitor screen, a person perceives color as the sum of the radiation of three basic colors: red, green and blue. This color rendering system is called RGB, after the first letters of the English color names (Red - red, Green - green, Blue - blue).

Mechanism for forming colors of the RGB model

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. Figure 2 shows the RGB color model.

Figure 2 - RGB color model

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 the color White.

R+G+B=W (White)

For example, on a monitor screen with a cathode ray tube (as well as a similar TV), an image is created by illuminating a phosphor with an electron beam. With this effect, the phosphor begins to emit light. Depending on the composition of the phosphor, this light has one color or another. To form a full-color image, a phosphor with a glow of three colors is used - red, green and blue. By themselves, the phosphor grains of different colors make it possible to obtain only pure colors (pure red, pure green and pure blue).

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. By adjusting the brightness of the grains, the resulting blended tone can be adjusted. For example, at maximum brightness of all three types of grains, a white color will be obtained, in the absence of illumination, black, and at intermediate values, various shades of gray will be obtained. 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 component of the RGB system, 1 bit for each RGB component, then we will get all eight different colors (Table 1).

Table 1 - Presence of 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 (Figure 3).

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.

Figure 3 - Color cube

Each color can be assigned a code using decimal and hexadecimal representations of the code. Decimal notation is a trio of decimal numbers separated by commas. The first number corresponds to the brightness of the red component, the second to the green, and the third to the blue.

Hexadecimal representation is three two-digit hexadecimal numbers, each corresponding to the brightness of a base color. The first number (first pair of numbers) corresponds to the brightness of the red color, the second number (second pair of numbers) - green, the third (third pair of numbers) - blue.

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.

Lesson objectives:

• Educational: Provide fundamental knowledge of the physical models of object color perception RGB and CMY(K). Explain the interaction of color coordinates of these models.
• Developmental : develop the ability to present research results in a given format
• Educational: develop the skills to independently complete a task, develop aesthetic taste, show a creative attitude to work

Lesson objectives:

• Repeat: the purpose and main functions of a graphic editor, principles of image formation in raster and vector graphics
• Learn to identify primary colors using color models
• Check your understanding of the material. Analyze identified errors.

As a result of studying the topic, students should:

know:

• physical models of object color perception RGB and CMY(K)
• ratio of RGB and CMY models

be able to:

• identify colors according to a given color scheme

Equipment: PC, PowerPoint program, multimedia projector, interactive whiteboard, handouts, presentation “Color Models”.

During the classes

Lesson Plan

1. Organizational moment (2 min)
2. Frontal survey (3 min)
3. Explanation of new material (19 min)
4. View presentation (8 min)
5. Checking your understanding of the material (10 min)
6. Summing up the lesson (1 min).
7. Homework (2 min)

LESSON 45 min

1. Organizational moment ( 2 minutes).

• Checking those present
• Magazine design
• Introducing students to the topic of the lesson

2. Frontal survey (3 min).

Students must answer the following questions:

a) appointment of a graphic editor

Graphics editor - a program (or software package) that allows you to create and edit images using a computer.

b) principles of image formation in raster and vector graphics

In raster graphics, an image is represented by a two-dimensional array of dots (raster elements), the color and brightness of each of which is set independently. Pixel is the basic element of all raster images. Vector graphics describe an image using mathematical formulas.

c) Explanation of new material ( 19 min )

Teacher: It is believed that our human eye is capable of distinguishing about 16 million shades of color. A natural question arises: how to explain to a computer that one object is red and the other is pink? What is the difference between them, so clearly visible to us by eye? To formally describe color, several color models and corresponding encoding methods have been invented.

Let's write down the definition in our notebook:

The method of dividing a color shade into its component components is called a color model.

Today we will look at the RGB and CMY(K) models.

Copy this in your notebook.

RGB color model(abbreviation of English words R ed, G reen, B lue - red, green, blue) - additive color model.

Is used for emitted light , i.e. when preparing screen documents.

The choice of primary colors is determined by the physiology of color perception by the retina of the human eye.

Any color can be represented as a combination of 3 primary colors R ed (red), G reen (green), B lue (blue). These colors are called color components.

Additive the model is called because the colors are obtained by adding (English addition) to black.

Write down the primary colors in your notebook. (Students copy the material from the board)

Teacher: The word additive (addition) emphasizes that color is obtained by adding points of three basic colors, each with its own brightness. The brightness of each base color can take values ​​from 0 to 255 (256 values), so the model can encode 2563 or about 16.7 million colors. These triplets of base points (luminous points) are located very close to each other, so that each triple merges for us into a large point of a certain color. The brighter the color dot (red, green, blue), the more of that color will be added to the resulting (triple) dot.

Look at the board and the material given.

The RGB model is displayed on the interactive board (a similar diagram is in the handout for each student). The teacher continues to explain and shows on the diagram.

The image in this color model consists of three channels.

• Pure red can be defined as (255,0,0) - R ed
• Pure Green (0.255.0) - G reen
• Pure bright blue color (0,0,255) – B lue

In the diagram you see that when mixing primary colors (the primary colors are red, green and blue), we get

• when blue (B) and red (R) are mixed, we get purple or lilac (M magenta)
• when mixing green (G) and red (R) - yellow (Y yellow)
• when mixing green (G) and blue (B) - cyan (C cyan)
• when mixing all three color components we get white color (W)

• If the brightness of all three basic colors is minimal (equal to zero), it turns out black dot (Black - (0,0,0))
• If the brightness of all three colors is maximum (255), adding them together gives white dot (White - (255,255,255)
• If the brightness of each base color is the same, we get gray dot (the higher the brightness value, the brighter).

A dot of some beautiful, rich color is obtained when mixing one (or two) colors is much less than two (one) others. For example, the color lilac is obtained if we take the maximum of red and blue colors and we won't take the green one , and yellow color is achieved by mixing red and green.

Graphic information input devices (scanner, digital camera) and output devices (monitor) work in this model.

Color model RGB has a wider color gamut in many tones (can represent richer colors) than the typical CMYK color gamut, so sometimes images that look great in RGB will fade significantly and fade out in the CMYK model we'll look at now.

Color model CMY ( K)

Colored, non-luminous objects absorb part of the white light spectrum that illuminates them and reflect the remaining radiation. Depending on the region of the spectrum in which absorption occurs, objects reflect different colors (are colored in them).

The name of the model and basic colors are already written on the board.

CMY ( K )
C yan M agenta Y ellow Black K
Cyan Magenta Yellow Black

Copy this in your notebook.

Colors that use white light by subtracting certain parts of the spectrum from it are called subtractive ("subtractive") . To describe them it is used subtractive model CMY (C is Cyan, M is Magenta, Y is Yellow). In this model, primary colors are formed by subtracting the primary additive colors of the RGB model from white.

If we subtract the three RGB primary colors from white, we get the three complementary CMY colors.

In this case, there will be three main subtractive colors:

• blue (white minus red)
• magenta (white minus green)
• yellow (white minus blue)

Color model CMY ( K ) used when working with reflected color (when printing) .

When two subtractive (subtractive) components are mixed, the resulting color is darkened (more light is absorbed, more paint is applied). Thus:

• when mixing the maximum values ​​of all three components, the color should be black
• in the complete absence of paint (zero values ​​of the components), the result will be white (white paper)
• shifting equal values ​​of the three components will produce shades of gray.

This model is the main printing model. Purple, cyan, yellow colors make up the so-called printing triad , and when printed with these inks, most of the visible color spectrum can be reproduced on paper.

However, real paints have impurities, their color may not be ideal, and a mixture of three primary colors that should produce black results instead in a vague dirty brown (look at the material issued). In addition, to obtain intense blacks, you need to put a large amount of each color of paint on the paper. This will cause the paper to become waterlogged and print quality will decrease. In addition, using a large amount of paint is uneconomical.

To improve the quality of the print, add basic printing inks (and model) added black paint. It was she who added the last letter to the name of the CMYK model, although not quite usually. The black component is abbreviated to the letter K, since this paint is the main, key ( K ey) in the process of color printing (or blac K).

As with the RGB model, the amount of each component can be expressed as a percentage or in gradations from 0 to 255.

Printing with four colors corresponding to CMYK is also called printing process colors.

Color in CMYK depends not only on the spectral characteristics of the dyes and the method of their application, but also on their quantity, paper characteristics and other factors. In fact, CMYK numbers are just a set of hardware data for the phototypesetting machine and do not uniquely define color.

Color circle

When processing images, it is necessary to clearly understand the interaction of color coordinates of the additive RGB system and the subtractive CMYK system. Without knowledge of these patterns, it is difficult to assess the quality of color, prescribe corrective operations, and it is simply wise to use the simplest tools designed to work with color.

If these two models are represented in the form unified model , then it will work out truncated a variant of the color wheel in which the colors are located in the order known from school (only without the derivative orange color): red (R), yellow (Y), green (G), cyan (C), blue (B) - purple (lilac , purple) M - Magenta

EVERY HUNTER WANTS TO KNOW WHERE THE PHEASANT SITS
or
HOW ONCE JEAN THE BELLER KNOCKED A LANTERN WITH HIS HEAD
or
EVERY DESIGNER WANTS TO KNOW WHERE TO DOWNLOAD PHOTOSHOP

Let's consider the simplest and most popular model, called the color wheel. It contains the coordinates of the main color systems RGB and CMYK at the same distance from each other.

Pairs of flowers located at the ends of the same diameter (at an angle of 180 degrees) are called
On the color wheel, the primary colors of the RGB and CMY models are in the following relationship: each color is located opposite its complementary color; at the same time it is at an equal distance between the colors with which it is obtained.

Complimentary colors are:

• green and purple,
• blue and yellow,
• blue and red.

Complementary colors are in some ways mutually exclusive. Adding any paint on the color wheel compensates for the additional paint, as if diluting it in the resulting color.

For example, to change the color ratio towards green tones, you should reduce the content of magenta, which is complementary to green.

This statement can be expressed in the form of the following brief formulas:

The teacher writes on the board:

Now write down the remaining 5 formulas in your notebook:

100%Magenta = 0Green

100%Yellow = 0Blue

0%Magenta = 255Green

0%Yellow = 255Blue.

Listen and write down the following sentence in your notebook:

Cyan is the opposite of red because cyan dyes absorb red and reflect blue and green. Blue is the absence of red.

The teacher asks 5 students to change the wording of the sentence for the remaining 5 colors.

Here is a summary of the basic and derivative rules of color synthesis using the circular model (see handout):

• Each subtractive (additive) color is located between two additive (subtractive) ones.
• Adding any two RGB (CMY) colors produces the CMY (RGB) color that lies between them. For example, mixing green and blue produces cyan, and mixing yellow and magenta produces red.

Write down in your notebook all possible relationships of this type (6 formulas)

Red + Green = Yellow

Blue + Green = Cyan

Red + Blue = Magenta

Cyan+ Magenta = Blue

Cyan + Yellow = Green

Magenta + Yellow = Red.

• Superimposing red and green at maximum intensity produces pure yellow. Decreasing the intensity of red shifts the resultant towards green hues, and reducing the contribution of green makes the color orange.
• Mixing blue and red in maximum proportions gives the color violet. Decreasing blue shifts the color toward pink, while decreasing red shifts the color toward purple.
• Green and blue colors form cyan. There are about 65 thousand different shades of blue that can be synthesized by mixing these color coordinates in different proportions.
• Layering cyan and magenta at maximum density produces a deep blue color.
• Purple and yellow dyes produce red. The higher the density of the components, the higher its brightness. Reducing the intensity of magenta gives the color an orange tint, reducing the proportion of the yellow component gives a pink color; Yellow and blue produce a bright green color. A decrease in the share of yellow gives rise to emerald, and a decrease in the contribution of blue gives rise to light green.
• Lightening or darkening a color of extreme saturation entails a decrease in its saturation.

Let's write in our notebook:

The color attachment can be increased and decreased by adjusting its inputs complimentary colors or adjacent colors.

4. View the presentation ( 8 min)

Now we will watch the presentation to consolidate the material we have covered and find out what awaits us in the next lessons.

5. Checking the mastery of the material ( 10 min)

Please answer questions on a new topic:

1. List the basic colors of the RGB and CMY(K) models.

• RGB color model - Red, Green, Blue - red, green, blue
• Color model CMY- C is Cyan (Blue), M is Magenta (Purple), Y is Yellow (Yellow)

2. What color model is used for the emitted color?

3. Why is it called additive?

The additive model is called because colors are obtained by adding (English addition) to black

4. What does the letter K mean in the CMYK color model?

The black component, since this paint is the main, key ( K ey) in the color printing process (or blac K).

5. What is the color wheel model used for?

To understand the interaction of color coordinates between the additive RGB system and the subtractive CMYK system.

6. What colors are called complementary?

Pairs of colors located at ends of the same diameter on the color wheel (at an angle of 180 degrees) are called complimentary or additional.

• List complimentary colors.
• green and purple
• blue and yellow
• blue and red.

6. Summing up the lesson ( 1 min).

Our lesson is coming to an end. Today you learned about the RGB and CMY(K) color models, the base colors of these models, the interaction of color coordinates of the additive RGB system and the subtractive CMYK system. We will continue our acquaintance with color models in the next lesson.

7. Homework ( 2 minutes)

Write down your homework:

1. Using the Color Wheel model, repeat the basic formulas for obtaining color
2. Profile school “Text information processing technology. Technology for processing graphic and multimedia information” A.V. Mogilev, L.V. Listratova St. Petersburg: BHV-Petersburg, 2010 p. 8.2.
3. Computer graphics lessons. CorelDRAW. Training course L. Levkovets St. Petersburg: Peter, 2006 level 2

When printing color computer maps in one way or another, the problem inevitably arises of ensuring accuracy in transmitting the original colors. This problem occurs for a variety of reasons.

Firstly, scanners And monitors work in an additive color model RGB, based on the rules of color addition, and printing is carried out in a subtractive model CMYK, in which the rules for subtracting colors apply.

Secondly, the methods of transmitting images on a computer monitor and on paper are different.

Third, the reproduction process occurs in stages and is carried out on several devices, such as scanner, monitor, phototypesetting machine, which requires their adjustment in order to minimize color distortion throughout the entire technological cycle - process calibration.

RGB model.

RGB color model(Fig. 1) ( R-Red- red, G-Green- green, B - Blue- blue) is used to describe colors visible in transmitted or direct light. It is adequate to the color perception of the human eye. Therefore, the construction of images on monitor screens, scanners, digital cameras and other optical devices corresponds to the RGB model. In a computer RGB model, each primary color can have 256 brightness levels, which corresponds to 8-bit mode.

Rice. 1. RGB color model

Model CMY (CMYK)

CMY color model(Fig. 2) C-Cyan- blue, M - Magenta- purple, Y-Yellow- yellow, used to describe colors visible in reflected light (for example, the color of paint applied to paper). In theory, the sum of CMY colors at maximum intensity should produce pure black. In real practice, due to the imperfection of the coloring pigments of the paint and the initial instability to blue color during color separation, the sum of cyan, magenta and yellow paints gives a dirty brown color. Therefore, a fourth dye is also used in printing - black - blacK, which produces a rich, uniform black color. It is used for printing text and designing other important details, as well as for adjusting the overall tonal range of images. Color saturation in CMYK model measured as a percentage, so each color has 100 gradations of brightness.

The main task of the reproduction process is to convert the image from the model RGB into the model CMYK. This transformation is carried out using special software filters, taking into account all future printing settings: process ink system, dot gain coefficient, black color generation method, ink balance and others. Thus, color separation is a complex process on which the quality of the final image largely depends. But even with optimal conversion from RGB V CMYK inevitably there is a loss of some shades. This is due to the different nature of these color models. It should also be noted that the models RGB And CMYK cannot convey the full spectrum of colors visible to the human eye.

Rice. 2. CMY color model

Model HSB.

Color can be characterized using other visual components. Yes, in the model H.S.B. the basic color space is constructed according to three coordinates: color tone (Hue) ; saturation (Saturation) ; brightness (Brightness) . These parameters can be represented as three coordinates, which can be used to graphically determine the position of a visible color in color space.

Rice. 3. HSB color model

On the central vertical axis postponed brightness(Fig. 3), and on horizontal - saturation. The color tone corresponds to the angle at which saturation axis moves away from luminance axis. In the area of ​​the outer radius there are saturated, bright color tones, which, as they approach the center, mix and become less saturated. As you move along the vertical axis, colors of different hue and saturation become either lighter or darker.

In the center, where all the color tones mix, a neutral gray color is formed.

This color model fits well with human perception: Color tone is the equivalent wavelength of light, saturation- wave intensity, and brightness characterizes the amount of light.

CIE system.

Color space can be used to describe the range of colors that are perceived by an observer or reproduced by a device. This range is called scale. This 3D format is also very convenient for comparing two or more colors. Three-dimensional color models and and three-digit color systems, such as RGB, CMY And H.S.B., are called three-coordinate colorimetric data.

Any measurement system requires a repeatable set of standard scales. For colorimetric measurements, the RGB color model is used as standard cannot be used because it unique- this space depends on the specific device. Therefore, there was a need to create universal color system. Such a system is CIE. To obtain a set of standard colorimetric scales, in 1931 International Commission on Illumination- Commission Internationale de l'Eclairage (CIE) - approved several standard color spaces describing the visible spectrum. With these systems, the color spaces of individual observers and devices can be compared against each other based on repeatable standards.

CIE color systems are similar to the other three-dimensional models discussed above in that they also use three coordinates to determine the position of a color in color space. However, unlike the CIE spaces described above - that is, CIE XYZ, CIE L*a*b* and CIE L*u*v* - are device independent, meaning the range of colors that can be defined in these spaces is not limited to pictorial the capabilities of a particular device or the visual experience of a particular observer.

CIE XYZ.

The main CIE color space is the CIE XYZ space. It is built on the basis of the visual capabilities of the so-called standard observer, that is, a hypothetical viewer, whose capabilities were carefully studied and recorded during long-term studies of human vision conducted by the CIE commission. This system has three primary colors (red, green and blue) standardized along the wavelength and have fixed coordinates in the xy coordinate plane.

Based on the data obtained as a result of the research, a xyY color diagram was constructed - a chromatic diagram (Fig. 11).

All shades visible to the human eye are located inside a closed curve. The primary colors of the RGB model form the vertices of the triangle. This triangle contains the colors displayed on the monitor. The CMYK colors that can be reproduced in printing are enclosed within a polygon. The third coordinate, Y, is perpendicular to any point on the curve and displays the gradations of brightness of a particular color.

CIE Lab model

This model is created as an improved CIE model and is also hardware independent. The idea behind the Lab model is that each step in increasing the numerical value of one channel corresponds to the same visual perception as the other steps.

In the model Lab:

Magnitude L characterizes lightness (Lightness) (from 0 to 100%);

Index A defines color range on the color wheel from green to red (- 120 (green) to +120 (red));

Index b defines range from blue (-120) to yellow (+120).

At the center of the wheel, the color saturation is 0.

Lab's color gamut completely includes the color gamuts of all other color models and the human eye. Publishing programs use the Lab model as an intermediate when converting RGB to CMYK.

This is one of the most common and frequently used models. It is used in devices that emit light, such as monitors, spotlights, filters and other similar devices.

In the RGB model, derived colors are obtained by adding or mixing base, primary colors, called color coordinates. The coordinates are red (Red), green (Green) and blue (Blue). The RGB model got its name from the first letters of the English names of color coordinates.

Each of the above components can range from 0 to 255, forming different colors and thus providing access to all 16 million (the total number of colors represented by this model is 256 * 256 * 256 = 16,777,216.).

This model additive. The word additive (addition) emphasizes that color is obtained by adding points of three basic colors, each with its own brightness. The brightness of each base color can take values ​​from 0 to 255 (256 values), so the model can encode 256 3 or about 16.7 million colors. These triplets of base points (luminous points) are located very close to each other, so that each triple merges for us into a large point of a certain color. The brighter the color dot (red, green, blue), the more of that color will be added to the resulting (triple) dot.

When working with the Adobe PhotoShop graphic editor, we can choose a color, relying not only on what we see, but, if necessary, specify a digital value, thereby sometimes, especially when color correction, controlling the work process.

This color model is considered additive, that is, when Increasing the brightness of individual components will increase the brightness of the resulting color: If you mix all three colors with maximum intensity, the result will be white; on the contrary, in the absence of all colors the result is black.

Table 1

The meanings of some colors in the RGB model

The model is hardware-dependent, since the values ​​of the basic colors (as well as the white point) are determined by the quality of the phosphor used in the monitor. As a result, the same image looks different on different monitors.

The properties of the RGB model are well described by the so-called color cube (see Fig. 3). This is a fragment of three-dimensional space, the coordinates of which are red, green and blue. Each point inside the cube corresponds to a certain color and is described by three projections - color coordinates: the content of red, green and blue. Adding all the primary colors of maximum brightness gives the color white; the starting point of the cube means zero contributions of the primary colors and corresponds to the color black.

If color coordinates are mixed in equal proportions, the result is a gray color of varying saturation. The points corresponding to the gray color lie on the diagonal of the cube. Mixing red and green produces yellow, red and blue produce magenta, and green and blue produce cyan.

Rice. 3.

Color coordinates: red, green, and blue are sometimes called primary or additive colors. The colors cyan, magenta, and yellow, which are obtained as a result of pairwise mixing of primary colors, are called secondary. Since addition is the basic operation of color synthesis, the RGB model is sometimes called additive (from the Latin additivus, which means added).

The principle of adding colors is often depicted in the form of a flat circular diagram (see Fig. 4), which, although it does not provide new information about the model, compared to a spatial image, is easier to perceive and easier to remember.

Rice. 4.

Many technical devices work on the principle of color addition: monitors, televisions, scanners, overhead projectors, digital cameras, etc. If you look through a magnifying glass at the monitor screen, you can see a regular grid, at the nodes of which there are red, green and blue phosphor grain dots . When excited by a beam of electrons, they emit basic colors of varying intensities. The addition of radiation from closely spaced grains is perceived by the human eye as color at a given point on the screen.

In computer technology, the intensity of primary colors is usually measured by integers in the range from 0 to 255. Zero means the absence of a given color component, the number 255 means its maximum intensity. Since primary colors can be mixed without restriction, it is easy to calculate the total number of colors that an additive model produces. It is equal to 256 * 256 * 256 = 16,777,216, or more than 16.7 million colors. This number seems huge, but in reality the model produces only a small part of the color spectrum.

Any natural color can be broken down into its red, green and blue components and their intensity measured. But the reverse transition is not always possible. It has been experimentally and theoretically proven that the range of colors in the RGB model is narrower than many colors in the visible spectrum. To obtain the part of the spectrum lying between blue and green, emitters with a negative red intensity are required, which, of course, do not exist in nature. The range of colors a model or device can reproduce is called color gamut. One of the serious disadvantages of the additive model, as paradoxical as it may sound, is its narrow color gamut.

It seems that this set of color coordinates uniquely defines a light green color on any device that works on the principle of adding base colors. In reality, things are much more complicated. The color reproduced by the device depends on many external factors, which often cannot be taken into account.

Display screens are coated with phosphors that differ in chemical and spectral composition. Monitors of the same brand have different wear and lighting conditions. Even one monitor produces different colors when warmed up and immediately after turning on. By calibrating devices and using color management systems, you can try to approximate the color gamuts of different devices. This is discussed in more detail in the next chapter.

It is impossible not to mention another drawback of this color model. From the point of view of a practicing designer or computer artist, it is non-intuitive. Operating in its environment, it can be difficult to answer the simplest questions related to color synthesis. For example, how should the color coordinates be changed to make the current color a little brighter or less saturated? To answer this simple question correctly requires a lot of experience with this color system.