Лекция 2 КГиД

1. Lecture 2

Color Models in Computer
Graphics
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2.

• We see objects because they emit or reflect light.
• Light is electromagnetic radiation.
• When light rays reach the retina of the eye, they create the
sensation of color.
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3.

• Like the Sun and other light sources, a monitor emits
light.
• Paper, on which an image is printed, reflects light.
• Since color can be created through emission and
reflection, there are two main ways to describe color:
additive and subtractive color models.
• Color model is a mathematical way of describing
colors and representing them in digital form.
• There are several color models, but none of them is
perfect.
• The basic idea is similar in all models: each model
uses several basic components, and each component
contributes to creating a particular color.
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4.

• The basic components of a color model are called
channels.
• The colors that can be represented by a color model
are called its color gamut. These colors form the color
space of the model.
• Different color models have different color gamuts.
• The most well-known color models are:
• RGB
• CMYK
• Lab
• HSB

5. Additive Color Model

• Colors produced by light emission are described using
the additive color model.
• Each pixel on a color screen is made of three very
small color elements: red, green, and blue.
• Because these elements are very small, our eyes
combine the three colors into one.
• By combining red, green, and blue light, we can create
other colors:
• Red + Green = Yellow
• Red + Blue = Magenta
• Green + Blue = Cyan
• Red + Green + Blue = White
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6. Additive Color Model

• By changing the intensity of the colored points, we
can create a wide range of colors and shades.
• The additive color model combines three primary
colors: red, green, and blue.
• If the intensity of all three colors reaches 100%, the
result is white.
• If there is no light from any of the three colors, the
result is black.
• The additive color model used in computer monitors
is called RGB:
• R — Red
G — Green
B — Blue
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7. RGB Color Model

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8. Subtractive Color Model

• Reflected light is described using the subtractive
color model.
• Paper does not produce light. It reflects and absorbs
light. The human eye sees the light reflected from the
surface of the paper.
• Therefore, the subtractive color model is used for
printing graphic images.
• White paper, when illuminated by white light, reflects
all colors. Colored paper absorbs some colors and
reflects the others.
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9. Subtractive Color Model

• The primary colors in the subtractive color model are
cyan, magenta, and yellow.
• Each of these colors absorbs, or subtracts, certain
colors from the white light that falls on the printed
page.
• That is why it is called the subtractive color model.
The word “subtractive” comes from the English word
“subtract,” which means “to take away” or “to
subtract.”
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10. Subtractive Color Model

• In the subtractive color model, the three primary
colors can be combined to create other colors:
• Cyan + Magenta + Yellow = Black
• Cyan + Magenta = Blue
• Yellow + Magenta = Red
• Yellow + Cyan = Green
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11. Subtractive Color Model

• By mixing the primary colors in different proportions
on white paper, we can create a wide variety of colors
and shades.
• White is produced when none of the three primary
colors is present.
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12. Subtractive Color Model

• A high percentage of cyan, magenta, and yellow
produces black.
• However, in real printing, mixing all three primary
colors usually produces a dark brown or muddy color
because of the properties of printing inks.
• Therefore, an additional black ink is used when
printing images.
• The subtractive color model is called CMYK:
• C — Cyan
• M — Magenta
• Y — Yellow
• K — blacK
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13.

Converting One Color Model to Another
The RGB model works with emitted light, while
CMYK works with reflected light.
When we need to print an image created on a
monitor, special software converts the image from
RGB to CMYK.
The number of colors that can be reproduced in
printing is much smaller than the number of colors
that can be displayed on a monitor.
Therefore, a color that we see on the screen cannot
always be reproduced exactly in print.
Usually, colors look brighter on a screen than on
paper.
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14.

Color Gamut
A color gamut is the range of colors that can be
created or displayed by a color model.
The widest gamut is the natural color gamut,
which includes all colors that the human eye can
distinguish.
Compared with it, the RGB color gamut is
smaller, while the CMYK color gamut is even
smaller than RGB.
Some graphics editors, such as Adobe
Photoshop and CorelDRAW, show which colors
cannot be reproduced accurately in print.
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