1. COLOR MODELS Ramya Sarma-106111073 Anusha Holla-106111015

2. I NTRODUCTION Light can be characterized as chromatic and achromatic (void of color) Achromatic light has intensity as its only attribute Chromatic Light: Radiance : Total amount of energy that flows from the light source. Measured in watts (W) Luminance : amount of energy that an observer perceives from a light source. Measure in lumens (lm) Brightness: It is a subjective descriptor. It is impossible to measure

3. L IGHT VS. P IGMENTS Primary colors can be added to produced secondary colors Primary colors of light: Red, Blue and Green Secondary colors of light: Magenta (red plus blue) Cyan (green plus blue) Yellow (red plus green) Primary color is one that absorbs a primary color of light and reflects or absorbs the other two. Primary colors of light: Magenta, Cyan, Yellow Secondary colors of light: Red, Green, Blue LIGHT PIGMENTS

4. L IGHT VS. P IGMENTS

5. C OLOR M ODELS Purpose is to provide standard specification of colors A color system is a specification of coordinate system Each color is represented by a point in the coordinate system

6. T HE RGB C OLOR M ODEL Based on the cartesian coordinate system 24 bit color cube

7. T HE RGB C OLOR M ODEL

8. T HE CMY AND CMYK C OLOR M ODELS CMY model (Cyan, Magenta, Yellow) Cyan, Magenta and Yellow are the primary colors for light and secondary colors for pigments This model is used in color printers and copiers RGB to CMY color conversion is performed internally

9. T HE CMY AND CMYK C OLOR M ODELS

10. D ISADVANTAGES OF RGB AND CMY RGB and CMY Models are well suited for Hardware Implementation and for Color Generation, but not well suited for describing Colors in the terms that are practical for Human Interpretation. Hence HSI Model is used for color description

11. HSI M ODEL The HSI model uses three measures to describe colors: Hue: A color attribute that describes a pure color (pure yellow, orange or red) Saturation: Gives a measure of how much a pure color is diluted with white light Intensity: Brightness is nearly impossible to measure because it is so subjective. Instead we use intensity. Intensity is the same achromatic notion that we have seen in grey level images

12. Consider if we make the RGB cube stand on the black vertex and position the white vertex directly above it. The Line Joining Black and White is called the Intensity axis

13. C ALCULATING I NTENSITY Intensity component of any color can be determined by passing a plane perpendicular to the intensity axis and containing the color point. The intersection of the plane with the intensity axis gives us the intensity component of the color

14. C ALCULATING S ATURATION Saturation is the Perpendicular Distance of the color Point from the Intensity axis. Saturation(Purity) of the Color Increases as a function of distance from the Intensity axis. Saturation of the Point on Intensity axis is 0.

15. C ALCULATING H UE In a similar way we can extract the hue from the RGB color cube. Consider a plane defined by the three points cyan, black and white. All points contained in this plane have the same hue (cyan) as black and white cannot contribute hue information to a colour

16. P ROJECTION OF RGB C UBE Consider if we look straight down at the RGB cube as it was arranged previously We would see a hexagonal shape with each primary colour separated by 120° and secondary colours at 60° from the primaries

17. RGB-HSI R ELATION To the right we see a hexagonal shape and an arbitrary color point The hue is determined by an angle from a reference point, usually Red The saturation is the distance from the origin to the point The intensity is determined by how far up the vertical intensty axis this hexagonal plane sits (not apparent from this diagram)

18. C ONVERTING C OLORS FROM RGB T O HSI Given a color as R, G, and B its H, S, and I values are calculated as follows:

19. C ONVERTING C OLORS FROM HSI T O RGB Given a color as H, S, and I its R, G, and B values are calculated as follows: RG sector ( 0 <= H < 120° ) GB sector ( 120° <= H < 240° )

20. C ONVERTING C OLORS FROM HSI T O RGB ( CONT …) BR sector ( 240° <= H <= 360° )

21. C OLOR P ERCEPTION IN HSI M ODEL

22. RGB TO HSI E XAMPLE

23. A PPLICATIONS The Original purpose of HSL is in color selection tools.

24. R EFERENCES Digital Image Processing By Rafael C. Gonzalez, Richard E. Woods