Color models

different techniques

1. COLOR MODELS IN COMPUTER GRAPHICS Prepared by Prof. Rakesh Pandey

2. COLOR MODELS A color model is a specification of a 3D color co-ordinate system and a visible subset in the co-ordinate System within all colors in a particular color range lie. There are main four types of color models:- (1) RGB (2) CMY (3) YIQ (4) CIE

3. But these models are not easy to use because they does not relate directly to intuitive color notations of hue, saturation and brightness. Therefore another class of color model has been developed. These includes :- HSV HLS The human eye can perceive about 382000(!) different colors. The distance between two color in the color cube is not a measure. Cont…

4. Physical properties of light White light consists of a spectrum of all visible colors. All kinds of light can be described by the energy of each wavelength.

5. Cont… Most light we see is not just a single wavelength, but a combination of many wavelengths like below. This profile is often referred to as a spectrum, or spectral power distribution.

6. Additive color mixing When we combine Red(two-thirds), Green(two-third), Blue(one-third) then we can get WHITE color. But we can absorbs CMY color from this RGB color then we can get BLACK color

7. Subtractive color mixing When we combine CYAN, MAGENTA, YELLO then we can get BLACK color. But we can absorbs RGB color from this CMY color then we can get WHITE color.

8. Intuitive color concept Color mixing created by an artist. Shades, tints and tones in scene can be produced by mixing color pigments (hues) with white and black pigments. Shades: Add black pigment to pure color The more black pigment, the darker the shade Tints: Add white pigment to the original color Making it lighter as more white is added Tones: Produced by adding both black and white pigments

9. Additional color forms It is the presentation of different basics of achromatic and chromatic light :- Achromatic light: It can visual only two color combinations that is of black and white color pigments. Its attributes are intensity, luminance and brightness. Its visual as in black & white television or display monitors. Chromatic light: It consist of colored light and their combinations, which produce visual sensation. Its attributes are hue, saturation, lightness and brightness of different colors. Its visual as in color television or display.

10. RGB (Red,Green,Blue) color model The RGB color model used in color CRT monitors. In this model, Red,Green and Blue are added together to get the resultant color WHITE.

11. Cont… Each color point within the bounds of the cube is represented as the triple (R,G,B).where value for R,G,B are assigned in the range from 0 to 1. Here RGB color place together at 120 degree. RED + GREEN + BLUE = WHITE(contribute) All other colors are generated from these three primary colors.

12. CMY ( Cyan,Magenta,Yellow ) color model The CMY color model used in color printing devices. In this model, Cyan,Magenta and Yellow are added together to get the resultant color BLACK.

13. Cont… Each color point within the bounds of the cube is represented as the triple (C,M,Y).where value for C,M,Y are also assigned in the range from 0 to 1. Here CMY color also place together at 120 degree. CYAN+ MAGENTA + YELLO = BLACK(contribute) All other colors are generated from these three primary colors.

14. RGB vs CMY If the intensities are represented as 0≤r,g,b≤1 and 0≤c,m,y≤1 then the relation between RGB and CMY can be described as:

15. Color Models pros and cons RGB + Cartesian coordinate system + linear + hardware-based (easy to transform to video) + tristimulus -based - hard to use to pick and name colors - doesn’t cover gamut of perceivable colors - non-uniform: equal geometric distance => unequal perceptual distance

16. Color Models pros and cons CIE + covers gamut of perceived colors + based on human perception (matching experiments) + linear + contains all other spaces - non-uniform (but variations such as CIE Lab are closer to Munsell , which is uniform ) - xy -plot of chromaticity horseshoe diagram doesn’t show luminance

17. Color Models pros and cons HSV + easy to convert to RGB + easy to specify colors - nonlinear - doesn’t cover gamut of perceivable colors - nonuniform

18. CMYK (Cyan,Magenta,Yello,Black) color model For printing & art industry the CMY model is not enough. So,Fourth primary color K is added to CMY model. K stands for BLACK color . Used in electrostatic/ink-jet plotters that deposit pigment on paper Cyan , magenta, and yellow are complements of red, green, and blue

19. CMYK (Cyan,Magenta,Yello,Black) color model Subtractive primaries : colors are specified by what is subtracted from white light, rather than by what is added to blackness Cartesian coordinate system Subset is unit cube Subtractive primaries Cyan , magenta , and yellow are complements of red, green, and blue

20. YIQ (Luminance,Inphase,Quadrature) color model The YIQ color model is a recoding of RGB color model. YIQ color model is used for television broadcasting. The Y-channel contains luminance information(sufficient for Black & White television sets) And I-channel & Q-channel contains color information. For recoding RGB to YIQ , HSV & HLS color models are used. Inphase = RED – YELLOW Quadrature = BLUE – YELLOW

21. CIE (Chromaticity Diagram) color model To define a color in CIE model, provide weights for the X, Y and Z primaries, just as you would for an RGB display. X, Y and Z form a three dimensional color volume . We can ignore the dimension of luminance by normalizing with total light intensity, x+y+z = 1 . This gives chromaticity values: x’ = x/( x+y+z ) y’ = y/( x+y+z ) z’ = 1 - x’ - y’

22. CIE (Chromaticity Diagram) color model 1931 Hue of a color: found by inscribing a line from C (white) through the color to the edge of the diagram. The hue is the wavelength of the color at the intersection of the edge and the line . • Saturation of a color: found by taking the ratio of the distance of the color from C on the above line and the length of the whole line. • Complementary colors: can be mixed to produce white light (a non-spectral color !)

23. HSV (Hue,Saturation,Value) & HLS (Hue,Lightness,Saturation) color model HSV and HLS are the two most common cylindrical-coordinate representations of points in an RGB color model . HSV is described by a hexacone derived from the RGB cube.

24. Cont… Hue means color is mapped into 0-360’.The wavelength of Hue is 128. Saturation is defined into 0-1.In this the amount of WHITE color is 130. Value is defined into 0-1.In this the amount of BLACK color is 23

25. Comparison CMY CMYK YIQ HSV HLS RGB