Color Models

Presented by: Venkata Satish Pappu (U41232937)

Contents

▪ Introduction

▪ Properties of Light

▪ Color Models

▪ Color Mixing

▪ Chromaticity

▪ RGB Color Model

Contents

▪ YIQ Color Model

▪ CMY Color Model

▪ CMYK Color Model

▪ HSV Color Model

▪ HLS Color Model

▪ Conclusion

▪ References

Introduction

A color model is an abstract mathematical model describing the way colors can be represented as tuples of numbers, typically as three or four values or color components. It is a specification of a color coordinate system and the subset of visible colors in this coordinate system.

It is an orderly system for creating a whole range of colors from a small set of primary colors. There are several established color models used in computer graphics. The different models include RGB, CMY, HSV, etc.

Introduction Cont..

Properties of Light

Light is electromagnetic radiation that has properties of waves. The electromagnetic spectrum can be divided into several bands based on the wavelength. As we have discussed before, visible light represents a narrow group of wavelengths between about 380 nm and 730 nm.

Properties of Light (Frequency and Wavelength)

Light waves also come in many frequencies. The frequency is the number of waves that pass a point in space during any time interval, usually one second. We measure it in units of cycles (waves) per second, or hertz. The frequency of visible light is referred to as color, and ranges from 430 trillion hertz, seen as red, to 750 trillion hertz, seen as violet. Again, the full range of frequencies extends beyond the visible portion, from less than 3 billion hertz, as in radio waves, to greater than 3 billion billion hertz (3 x 1019), as in gamma rays.

Properties of Light (Frequency and Wavelength)

Light waves come in many sizes. The size of a wave is measured as its wavelength, which is the distance between any two corresponding points on successive waves, usually peak to peak or trough to trough. The wavelengths of the light we can see range from 400 to 700 nanometers (or billionths of a meter). But the full range of wavelengths included in the definition of electromagnetic radiation extends from 0.1 nanometers, as in gamma rays, to centimeters and meters, as in radio waves.

Properties of Light (Period)

The period is the duration of time of one cycle in a repeating event, so the period is the reciprocal of the frequency.

Properties of Light (White Vs Dark)

Our eyes interpret these wavelengths as different colors. If only a single wavelength or limited range of wavelengths are present and enter our eyes, they are interpreted as a certain color. If a single wavelength is present we say that we have monochromatic light. If all wavelengths of visible light are present, our eyes interpret this as white light. If no wavelengths in the visible range are present, we interpret this as dark.

Properties of Light (White Vs Dark)

Properties of Light (Color of Objects)

In order to perceive a color, there are three essential components that must be present: Light, an illuminated object and an observer.

Properties of Light (Light and Color)

The frequency ( or mix of frequencies ) of the light determines the color. The amount of light(sheer quantity of photons ) is the intensity. Three independent quantities are used to describe any particular color: Hue, Saturation, and Lightness or Brightness or Intensity.

Properties of Light (Color - Hue)

The hue is determined by the dominant wavelength.(the apparent color of the light)

When we call an object "red," we are referring to its hue. Hue is determined by the dominant wavelength.

Properties of Light (Color - Saturation)

The saturation of a color ranges from neutral to brilliant. The circle on the right is a more vivid red than the circle on the left although both have the same hue.

The saturation is determined by the excitation purity , and depends on the amount of white light mixed with the hue. A pure hue is fully saturated, i.e. no white light mixed in. Hue and saturation together determine the chromaticity for a given color.

Properties of Light (Color – Brightness/Intensity)

Lightness or brightness refers to the amount of light the color reflects or transmits.

Finally, the intensity is determined by the actual amount of light, with more light corresponding to more intense colors ( the total light across all frequencies).

Properties of Light (Color and spectrum)

Color is the visual perceptual property corresponding in humans to the categories called red, blue, yellow, etc. Color derives from the spectrum of light (distribution of light power versus wavelength) interacting in the eye with the spectral sensitivities of the light receptors.

The familiar colors of the rainbow in the spectrum – named using the Latin word for appearance or apparition by Isaac Newton in 1671 – include all those colors that can be produced by visible light of a single wavelength only, the pure spectral or monochromatic colors. The table at right shows approximate frequencies (in terahertz) and wavelengths (in nanometers) for various pure spectral colors. The wavelengths listed are as measured in air or vacuum.

Properties of Light (Distribution of white light)

Properties of Light (Spectral Color)

A spectral color is a color that is evoked by a single wavelength of light in the visible spectrum, or by a relatively narrow band of wavelengths. Every wavelength of light is perceived as a spectral color, in a continuous spectrum; the colors of sufficiently close wavelengths are indistinguishable.

The spectrum is often divided up into named colors, though any division is somewhat arbitrary: the spectrum is continuous. Traditional colors include: red, orange, yellow, green, blue, and violet.

Properties of Light (Color frequency band)

Color Models

Color models are used to describe the color as accurately as possible. It uses the fact that colors can be described the by the combinations of three basic colors called primary colors.

It defines three primary colors X, Y and Z that can be used to describe all visible colors, as well as a standard white.

The range of colors that can be described by combinations of colors is called a color gamut.

Color Models – Cont..

Primary colors: These are colors that cannot be created through the mixing of other colors.

Primary colors are sets of colors that can be combined to make a useful range of colors. For human applications, three primary colors are typically used, since human color vision is usually trichromatic.

For additive combination of colors, as in overlapping projected lights or in electronic visual displays, the primary colors normally used are red, green, and blue. For a subtractive combination of colors, as in mixing of pigments or dyes for printing, the colors magenta, yellow, and cyan are normally used.

Color Models – Cont..

Above is the emission spectra that define the additive primary colors of a CRT color video display.

Color Models – Cont..

Complementary Colors: Complementary colors are pairs of colors which, when combined, cancel each other out. This means that when combined, they produce a grey-scale color like white or black. When placed next to each other, they create the strongest contrast for those particular two colors. Due to this striking color clash, the term opposite colors is often considered more appropriate than "complementary colors".

Color Models – Cont..

A traditional color star developed in 1867 by Charles Blanc. The traditional complementary colors used by 19th-century artists such as Van Gogh, Monet and Renoir are directly opposite each other.

Color Mixing

There are two types of color mixing: Additive and Subtractive. In both cases there are three primary colors, three secondary colors (colors made from 2 of the three primary colors in equal amounts), and one tertiary color made from all three primary colors. This point is a common source of confusion, as there are different sets of primary colors depending on whether you are working with additive or subtractive mixing.

The diagram displays the result of white light split by

the prism.

Color Mixing – Additive mixing

The additive mixing of colors is unintuitive as it does not correspond to the mixing of physical substances (such as paint) which would correspond to subtractive mixing. For instance, one can additively mix green, red and blue by shining green and red light together with blue light, which will result in not colored but white light. As in this example, one should always have the mixture of light in mind when considering additive color mixing as it is the only situation where it occurs. Despite being unintuitive, it is conceptually simpler than subtractive mixing. Two beams of light that are superimposed correspond to additive mixing.

Color Mixing – Additive mixing

Color Mixing – Subtractive mixing

The mixing of colored physical substances corresponds to subtractive color mixing, hence it corresponds to our intuition about mixing colors. To explain the mechanism, let us consider mixing red paint with yellow paint. The red paint is red because when the ambient light strikes it, the composition of the material is such that it absorbs all other colors in the visible spectrum except for red. The red light, not being absorbed, reflects off the paint and is what we see. This same mechanism describes the color of all material objects -- note that light is not a material object -- and so applies to the yellow paint as well.

Color Mixing – Subtractive mixing

Color Mixing – Functions

This is a sample mix function from the Sass:

mix($your_color,$another_color,20%)

The first part of the function is the overlay color, and the second is the background color. The number is the percentage of the first color to mix with the second color. A good way to think about the percentage is as the opacity of the first color. In the example, this would be the same as having the $your_color value with an opacity of 20% overlaying a background color of $another_color.

Color Mixing – Tints and Shades

In color theory, a tint is the mixture of a color with white, which increases lightness, and a shade is the mixture of a color with black, which reduces lightness. A tone is produced either by the mixture of a color with gray, or by both tinting and shading. Mixing a color with any neutral color (including black, gray and white) reduces the chroma, or colorfulness, while the hue remains unchanged.

In common language, the term "shade" can be generalized to furthermore encompass any varieties of a particular color, whether technically they are shades, tints, tones, or slightly different hues; while the term "tint" can be generalized to refer to the any lighter or darker variation of a color (e.g. Tinted windows)

Color Mixing – Tints and Shades

Some shades of blue

Color Matching

In order to define the perceptual 3D space in a "standard" way, a set of experiments can (and have been) carried by having observers try and match color of a given wavelength, lambda, by mixing three other pure wavelengths, such as R=700nm, G=546nm, and B=436nm in the following example. Note that the phosphorus of color TVs and other CRTs do not emit pure red, green, or blue light of a single wavelength, as is the case for this experiment.

Color Schemes

In color theory, a color scheme is the choice of colors used in design for a range of media. For example, the "Achromatic" use of a white background with black text is an example of a basic and commonly default color scheme in web design.

Following are the different color schemes available:

• Complementary color scheme.

• Analogous color scheme

• Triadic color scheme

• Rectangle color scheme

• Square color scheme

• Split complementary color scheme.

Color Schemes – Cont..

Triadic color scheme

Rectangle color scheme

Analogous color scheme

Complementary color scheme

CIE Color Models

CIE stands for Comission Internationale de l'Eclairage (International Commission on Illumination). The commission was founded in 1913 as an autonomous international board to provide a forum for the exchange of ideas and information and to set standards for all things related to lighting.

Once the RGB tristimulus values were obtained, they were found to be wanting in some regards. Due to gamut restraints, the RGB color model could not reproduce all spectral light without introducing the effect of negative RGB values (this was done by mixing red, green, or blue light with the test lamp as needed). CIE thought a system that used negative values would not be acceptable as an international standard. Accordingly, they translated the RGB tristimulus values into a different set of all positive tristimulus values, called XYZ, which formed the first CIE color model. From this first model, other models were derived in response to various concerns.

CIE Color Models – Cont..

CIEXYZ: The original CIE model using the chromaticity diagram adopted in 1931.

CIELUV: A model composed in 1960 and revised in 1976. This model uses an altered and elongated form of the original chromaticity diagram in an attempt to correct its non-uniformity.

CIELAB: A different approach developed by Richard Hunter in 1942 that defines colors along two polar axes for color (a and b) and a third for lightness (L).

CIEXYZ – Chromaticity Diagram

CIELUV – Chromaticity Diagram

CIELAB – Color axes

Chromaticity

Chromaticity is an objective specification of the quality of a color regardless of its luminance. Chromaticity consists of two independent parameters, often specified as hue (h) and colorfulness (s), where the latter is alternatively called saturation, chroma, intensity, or excitation purity. This number of parameters follows from trichromacy of vision of most humans, which is assumed by most models in color science.

CIE Chromaticity diagram

To measure and predict the appearance of a particular color, we need a way to link perception to numbers and formulas. Scientific color values were established earlier this century by the CIE group. CIE models for defining color space all rely on the same basic numbers.

CIE Chromaticity diagram – Cont..

In 1931, the CIE defined three standard primaries (X, Y, Z). The Y primary was intentionally chosen to be identical to the luminous-efficiency function of human eyes.

The below figure shows the amounts of X, Y and Z needed to exactly reproduce any visible color.

CIE Chromaticity coordinates

In 1931, the Commission Internationale de l'Éclairage (CIE) defined three standard primaries, called X, Y and Z, that can be added to form all visible colors. The primary Y was chosen so that its color matching function exactly matches the luminous-efficiency function for the human eye, given by the sum of the three curves2.

The chromaticity coordinates which describe the perceived color information are defined as:

The red chromaticity coordinate is given by x and the green chromaticity coordinate by y. The tristimulus values are linear in I() and thus the absolute intensity information has been lost in the calculation of the chromaticity coordinates {x,y}. All color distributions, I(), that appear to an observer as having the same color will have the same chromaticity coordinates.

CIE Chromaticity diagram – Cont..

Figure : The CIE Chromaticity Diagram showing all visible colors. x and y are the normalized amounts of the X and Y primaries present, and hence z = 1 - x - y gives the amount of the Z primary required.

Often it is convenient to work in a 2D color space. This is commonly done by projecting the 3D color space onto the plane X+Y+Z=1, yielding a CIE chromaticity diagram. The projection is defined as:

Giving the chromaticity diagram shown on the right.

CIE Chromaticity diagram – Cont..

CIE Chromaticity diagram – Cont..

The formulas for converting from the tristimulus values (X,Y,Z) to the well-known CRT colors (R,G,B) and back are given by:

As long as the position of a desired color (X,Y,Z) is inside the phosphor triangle in Figure , the values of R, G, and B as computed by eq. will be positive and can therefore be used to drive a CRT monitor.

RGB Color Model

The RGB color model is an additive color model in which red, green, and blue light are added together in various ways to reproduce a broad array of colors. The name of the model comes from the initials of the three additive primary colors, red, green, and blue.

The main purpose of the RGB color model is for the sensing, representation, and display of images in electronic systems, such as televisions and computers, though it has also been used in conventional photography. Before the electronic age, the RGB color model already had a solid theory behind it, based in human perception of colors.

RGB Color Model

Tristimulus Theory of Color

Important principle:

Any color spectra is perceived by sensors with three different response frequencies.

Tristimulus theory of color:

Color is inherently a three dimensional space.

Metamers:

If two colors produce the same tristimulus values, then they are visually indistinguishable.

RGB Color Cube

The additive color model used for computer graphics is represented by the RGB color cube, where R, G, and B represent the colors produced by red, green and blue phosphorus, respectively.

RGB Color Gamut

In color reproduction, including computer graphics and photography, the gamut, or color gamut is a certain complete subset of colors. The most common usage refers to the subset of colors which can be accurately represented in a given circumstance, such as within a given color space or by a certain output device.

YIQ Color Model

YIQ is the color space used by the NTSC color TV system, employed mainly in North and Central America, and Japan. I stands for in-phase, while Q stands for quadrature, referring to the components used in quadrature amplitude modulation.

YIQ Color Model

Figure: Image (a) shows a color test pattern, consisting of horizontal stripes of black, blue, green, cyan, red, magenta and yellow, a color ramp with constant intensity, maximal saturation, and hue changing linearly from red through green to blue, and a greyscale ramp from black to white. Image (b) shows the intensity for image (a). Note how much detail is lost. Image (c) shows the luminance. This third image accurately reflects the brightness variations perceived in the original image

Conversion – RGB to YIQ and YIQ to RGB

RGB to YIQ conversion is used in the NTSC encoder where the RGB inputs from the camera are converted to a luminance (Y) and two chrominance information (I,Q). In the NTSC encoder, these I,Q signals are modulated by a subcarrier and added to the Y signal.

Below are the conversion formulas for both models:

CMY Color Model and Conversion to RGB

This stands for cyan-magenta-yellow and is used for hardcopy devices. In contrast to color on the monitor, the color in printing acts subtractive and not additive. A printed color that looks red absorbs the other two components G and B and reflects R. Thus its (internal) color is G+B=CYAN. Similarly R+B=MAGENTA and R+G=YELLOW. Thus the C-M-Y coordinates are just the complements of the R-G-B coordinates:

Cyan, Magenta, and Yellow (CMY) are complementary colors of RGB. They can be used as Subtractive Primaries.

CMY model is mostly used in printing devices where the color pigments on the paper absorb certain colors (e.g., no red light reflected from cyan ink).

CMYK Color Model

The CMYK color model (process color, four color) is a subtractive color model, used in color printing, and is also used to describe the printing process itself. CMYK refers to the four inks used in some color printing: cyan, magenta, yellow, and key (black). Though it varies by print house, press operator, press manufacturer, and press run, ink is typically applied in the order of the abbreviation.

The "K" in CMYK stands for key because in four-color printing, cyan, magenta, and yellow printing plates are carefully keyed, or aligned, with the key of the black key plate. Some sources suggest that the "K" in CMYK comes from the last letter in "black" and was chosen because B already means blue. However, this explanation, although useful as a mnemonic, is incorrect. K is used as "Key", which was possibly chosen because black is often used as outline.

RGB – CMYK conversion

Since RGB and CMYK spaces are both device-dependent spaces, there is no simple or general conversion formula that converts between them. Conversions are generally done through color management systems, using color profiles that describe the spaces being converted. Nevertheless, the conversions cannot be exact, particularly where these spaces have different gamuts.

RGB – CMYK conversion

The R,G,B values are divided by 255 to change the range from 0..255 to 0..1:

R' = R/255, G' = G/255 and B' = B/255

The black key (K) color is calculated from the red (R'), green (G') and blue (B') colors:

K = 1-max(R', G', B')

The cyan color (C) is calculated from the red (R') and black (K) colors:

C = (1-R'-K) / (1-K)

The magenta color (M) is calculated from the green (G') and black (K) colors:

M = (1-G'-K) / (1-K)

The yellow color (Y) is calculated from the blue (B') and black (K) colors:

Y = (1-B'-K) / (1-K)

HSL and HSV Color Model

HSL and HSV are the two most common cylindrical-coordinate representations of points in an RGB color model. The two representations rearrange the geometry of RGB in an attempt to be more intuitive and perceptually relevant than the cartesian (cube) representation. Developed in the 1970s for computer graphics applications, HSL and HSV are used today in color pickers, in image editing software, and less commonly in image analysis and computer vision.

HSL stands for hue, saturation, and lightness, and is also often called HLS. HSV stands for hue, saturation, and value, and is also often called HSB (B for brightness).

HSL and HSV Color Model – Basic Principle

HSL and HSV are both cylindrical geometries (fig. 2), with hue, their angular dimension, starting at the red primary at 0°, passing through the green primary at 120° and the blue primary at 240°, and then wrapping back to red at 360°. In each geometry, the central vertical axis comprises the neutral, achromatic, or gray colors, ranging from black at lightness 0 or value 0, the bottom, to white at lightness 1 or value 1, the top. In both geometries, the additive primary and secondary colors – red, yellow, green, cyan, blue, and magenta – and linear mixtures between adjacent pairs of them, sometimes called pure colors, are arranged around the outside edge of the cylinder with saturation 1; in HSV these have value 1 while in HSL they have lightness ½. In HSV, mixing these pure colors with white – producing so-called tints – reduces saturation, while mixing them with black – producing shades – leaves saturation unchanged. In HSL, both tints and shades have full saturation, and only mixtures with both black and white – called tones – have saturation less than 1.

HSL and HSV Color Model – Basic Principle

HSL/HSV – RGB Conversion

To convert from HSL or HSV to RGB, we compute chroma, by multiplying saturation by the maximum chroma for a given lightness or value. Next, we find the point on one of the bottom three faces of the RGB cube which has the same hue and chroma as our color (and therefore projects onto the same point in the chromaticity plane). Finally, we add equal amounts of R, G, and B to reach the proper lightness or value.

HSV – RGB Conversion

HSL – RGB Conversion

Conclusion

Color models provide a standard way to specify a particular color, by defining a 3D coordinate system, and a subspace that contains all constructible colors within a particular model. Any color that can be specified using a model will correspond to a single point within the subspace it defines. Each color model is oriented towards either specific hardware (RGB,CMY,YIQ), or image processing applications (HSI).

References

http://www.techfak.unibielefeld.de/ags/wbski/lehre/digiSA/WS0607/3DVRCG/Vorlesung/8a.RT3DCGVR-color.pdf

http://web.eecs.utk.edu/~huangj/cs456/notes/456_color.pdf

https://en.wikipedia.org/wiki/Color_model

http://www.mat.univie.ac.at/~kriegl/Skripten/CG/node11.html

http://www.slideshare.net/mustafasalam167/color-model-29181025

http://www.pveducation.org/pvcdrom/properties-of-sunlight/properties-of-light

http://www.tulane.edu/~sanelson/eens211/proplight.htm

https://en.wikipedia.org/wiki/Color#Spectral_colors

https://en.wikipedia.org/wiki/Comparison_of_color_models_in_computer_graphics

References – Cont..

http://dba.med.sc.edu/price/irf/Adobe_tg/models/main.html

https://en.wikipedia.org/wiki/Chromaticity

https://en.wikipedia.org/wiki/Spectral_color

http://www.colorado.edu/physics/phys1230/phys1230_sm10/Lecture_Notes/class15_Colors_AddorSubtractiveColors_ColorVision_posted.pdf

http://science.howstuffworks.com/

https://en.wikipedia.org/wiki/Frequency

https://en.wikipedia.org/wiki/Complementary_colors

https://en.wikipedia.org/wiki/Primary_color

http://www.technologystudent.com/designpro/pricol1.htm

References – Cont..

https://en.wikipedia.org/wiki/RGB_color_model

http://www.willamette.edu/~gorr/classes/GeneralGraphics/Color/tristimulus.htm

https://en.wikipedia.org/wiki/CIE_1931_color_space

http://web.cse.ohio-state.edu/~parent/classes/581/

https://en.wikipedia.org/wiki/Gamut

https://en.wikipedia.org/wiki/YIQ#From_RGB_to_YIQ

https://www.eembc.org/techlit/datasheets/yiq_consumer.pdf

https://en.wikipedia.org/wiki/CMYK_color_model

http://www.mat.univie.ac.at/~kriegl/Skripten/CG/node13.html

http://dba.med.sc.edu/price/irf/Adobe_tg/models/rgbcmy.html

References – Cont..

http://www.rapidtables.com/convert/color/rgb-to-cmyk.htm

https://en.wikipedia.org/wiki/HSL_and_HSV#Converting_to_RGB

https://en.wikipedia.org/wiki/Color_mixing

http://seesparkbox.com/foundry/putting_the_fun_in_sass_color_functions

https://en.wikipedia.org/wiki/Tints_and_shades

http://www.tigercolor.com/color-lab/color-theory/color-theory-intro.htm

https://en.wikipedia.org/wiki/Color_scheme

http://dba.med.sc.edu/price/irf/Adobe_tg/models/

web.eecs.utk.edu

Thank You