Light and Color

COLLEGE OF ENGINEERING UNIVERSITY OF PORTO

COMPUTER GRAPHICS AND INTERFACES /GRAPHICS SYSTEMS JGB / AAS 2004

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Light and Color

Graphics Systems /

Computer Graphics and Interfaces



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Light Chromatic

In perceptual terms evaluate chromatic light by the following numbers:

1. Hue (Hue): distinguish between various colors like red, green, yellow, etc..

2. Saturation: refers to the distance to the gray color of equal intensity

Ex: green is saturated, but color pink or azure is little.

Very little saturated colors include white.

3. Intensity (Lightness): reflected intensity

4. Brightness (Brightness): emitted intensity (lamp, sun, ...)



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Light Chromatic

The artists specify the color as different tints,shadesAnd Tones an pure pigment (Or saturated).

Grays (Gray): result of joining black and white pigment

The colors obtained by the mixture referred to are the same color Hue (Pure color) with different intensity and saturation.

Tints (Dyed): result from joining white pigment to a pure pigment (less saturated)

Shades (Shaded): result of joining black pigment to a pure pigment (decreases intensity)

Tones (Tones): result from joining white and black pigment to a pure pigment



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Light Chromatic

HSV Model: translates the color definition used by the artists. Some programs allow the specification of color through this diagram.



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Light Chromatic

The above specifications are subjective it is necessary to resort to physical objectively quantify the color.

Electromagnetic spectrum: the colors represent a narrow band of the spectrum- Lowest Frequency: Red with λ = 700 nm- Higher Frequency: Violet with λ = 400 nm- Frequency and wavelength: c = λ . fwhere c is the speed of light (3 * 1010 cm / s)



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Light Chromatic

Colorimetry: Branch of physics that studies the color

- Dominant Wavelength: Dominant color (hue)

- Luminance: Light intensity (Brightness)

- Excitation puritySaturation

Color + Saturation = Cromatância (Chromaticity),color definition regardless of the intensity of light

E

fRed Violet

Energy Distribution of a white light source:

All frequencies are present.



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Light Chromatic

ED - Dominant Energy

EW - Energy for white

Higher ED- EW, The purer the color will be issued.

EW = 0 , 100% purity

EW = ED , 0% purity (white)

Luminance = Area under the curve of the total energy emitted

E

fRed Violet

Energy Distribution of a light source with dominant wavelength near red:

ED

EW



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Light Chromatic - human eye response

Rods (rods): Are sensitive to low light levels and do not distinguish color

Cones: Located mainly in fovea are extremely sensitive to color.

45000

90000

135000

Distribution of rods and cones in the retina

Number of rods and cones per mm2



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Theory of tri-stimulus:

There are 3 types of cones, whose response to light is maximum in Blue, Red and Green, respectively.

Due to the light absorption characteristics of the human eye, the colors are represented as a combination of so-called primary colors of R (red), G (green) and B (blue).

Sensitive cones to light:65% to red light33% for green light2% to blue light

In the graph, there is less response of the eye to blue light.

Response obtained for each type of cone



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Luminous efficiency function:

- Answer the eye at a constant light luminance is varied when the dominant frequency:

Experiments show that this curve is obtained by summing the curves of the previous figure.



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Required amount of the 3 primary to achieve all wavelengths of the visible spectrum colors with constant luminance:

- Thus, in CRT monitors, based on the sum of the 3 primary colors, we can not represent all the visible colors.

- Negative values in the figure mean that we can not get all the colors of the sum of the primary colors R, G, B.



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Chromatic Light - Model CIE

- The representation of color by mixing three primary colors is desirable, but the need for negative weights is not convenient.

- To overcome this difficulty, in 1931, the Commission Internationale de l'Eclairage (CIE) defined 3 primary colors imaginary X, Y and Z to replace red, green and blue.

Primary colors:- Imaginary, X, Y andZ- Y coincides with the

luminous-efficiency function

X = C X + Y Y + Z Z

The color is obtained with the X, Y and Z weights of primaries X, Y and Z



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Chromatic Light - Model CIE

- The figure shows the conical volume that contains the visible colors.

The values Cromatância are defined by normalizing against X + Y + Z, ie luminance

x = X / (X + Y + Z) y = Y / (X + Y + Z) z = Z / (X + Y + Z)

The values Cromatância x, y, z:- Depend on the dominant wavelength and saturation- Not depend on the luminance

Note that:

x + y + z = 1, or x, y, z in the plane X + Y + Z = 1 as shown in Fig.



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Light Chromatic - CIE Diagram

- x, y and Y define a color altogether:

X = (x / y) * Y Y Y = Z = (1 - x - y) / Y * Y

Y usually indicates luminance

The projection of the X + Y + Z = 1 plane (X, Y) corresponds to the CIE diagram Cromatância



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- x, y define a color, not counting the luminance.

- Drawing x and y for all visible colors, gives the CIE diagram Cromatância

- For each point of the picture area has been several colors with the same cromatância but with different luminances.

- The colors are 100% pure on the boundary curve

- The central point corresponds to the color white

CIE diagram cromatância



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Determining the dominant wavelength:

- Adding two colors, the resulting color is on the line joining the two colors added.

- Color The can be seen as B + CSoon B is the dominant wavelength.

Determination of the purity of color:

- AC / BC purity expressed (in percent) of color The.

- The closer The of C, Whiter light will be included in The and this color is less pure.



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Complementary colors:

- Are two colors which together give rise to white

- Ex: Colors D and E.

Not spectral colors:

- Colors which can not be defined by a dominant wavelength. Ex: color F.

- In this case, the dominant wavelength is defined as the complement of the wavelength where the line joining F and C intersects in B (In this example 555 nm).

- The purity of the color is defined by FC / GC.

- Non-spectral colors are purple and magenta



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Range of colors:

- The colors of the segment IJ are obtained by the mixture of colors I and J.

- Joining the color K, One obtains all the colors situated in the triangle of FIG.

Coverage of colors in the CIE model

- No triangle covers all colors CIE Diagram

- Ie no set of 3 colors is sufficient to produce all the colors of the CIE Diagram.

Adding R, G, and B can not be any color.

The diagram is also used to compare the range of colors available in devices such as monitors and printers.



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Light ChromaticColor models in Raster Systems

The aim of color model is to provide a format in which colors can be coded clearly. There are models oriented User Interface and Hardware.

Model for color monitors– RGB additive model

Color model for printers– Subtractive CMY model

Model facilitating user interface– Intuitive model HSV



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Light ChromaticRGB color model

Model used in CRT– Additive model– Primary colors of R, G, B– Black = (0, 0, 0)– White = (1, 1, 1)

RGB Cube– The diagonal (0, 0, 0) to (1, 1, 1)

represents the gray levels, with an equal contribution of the three primary colors.

– CRT's phosphors with different feature sets of different colors



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Light ChromaticCMY color model

Model used in printers– Primary colors: Cyan,Magenta and Yellow

Are the complementary colors of R, G and B respectively. – The colors are specified by what is removed or subtracted from the white

lightInstead of what is added to black (Such as RGB).– The cube is identical to the RGB model, but now the white is on the rise.

White = (0, 0, 0), and Black = (1, 1, 1)

The white paper reflects white light has components red,green and blue

If the surface is covered with ink cyanThere is no reflected light red surface, i.e. the color cyan subtracted color red reflected by the white paper.

So, in terms of the 3 primary colors RGB, cyan = white - red, That is to say blue + green



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Light Chromatic CMY color model

Transformation RGB to CMY:

B

G

R

Y

M

C

1

1

1

Transforming CMY to RGB:

Y

M

C

B

G

R

1

1

1

In RGB, Cyan = (0,1,1)

In CMY, cyan = (1,0,0)



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Light Chromatic HSV model

Oriented model for the user

– HSV (Hue, Saturation, Value)

– Top: V = 1, where the colors brighter

– Angle around the vertical axis:

H = 0 ° corresponds to red

H = 120, corresponds to the green

– Complementary colors: between them an angle of 180

– Saturation: ranges from 0 (center) to 1 (in the periphery)



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Examples:

– S = 0 and V = 1 white

– S = 0 and V = 0 .. 1 gray

– H = 0 and S = 1 and V = 1 pure red

– V = 1 and S = 1 pure colors (border at the top)

– V = 1 red and white joining (decrement S)

– S = 1 and joining shaded black (decrement V)

– S and V varying shades

Corresponds to the model of the artists referred to in the beginning.



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• The top of the pyramid of the HSV model corresponds to the projection of the RGB cube seen along its diagonal from white to black.

• Varying V by 1 to 0

The RGB cube appears smaller

The diagonal of the RGB cube corresponding to the V axis

• It is this correspondence that are based conversion algorithms between the two RGB systems HSV RGB and HSV



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Example of interaction for interactive color specification:



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Light Chromatic Exercise

Light and Color

Documents

interfacescomputer graphics

efredvioletcomputer

pure pigmentcomputer

color hue pure color

chromatic light

color definition

color pink

specification of color