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Color Models

Color Models. by Patrick Kraft Hochschule Ravensburg-Weingarten Technik | Wirtschaft | Sozialwesen Doggenriedstraße 88250 Weingarten. Contents. Introduction Physical Aspects of Colored Light Physiological Characteristics of the Human Visual System Tristimulus Color Spaces

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Color Models

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  1. Color Models by Patrick Kraft Hochschule Ravensburg-WeingartenTechnik | Wirtschaft | SozialwesenDoggenriedstraße88250 Weingarten

  2. Contents • Introduction • Physical Aspects of Colored Light • Physiological Characteristics of the Human Visual System • Tristimulus Color Spaces • Polar Coordinate Color Models • Conclusion

  3. Introduction Vision sensor / RGB camera is designed for the reproduction of Color for the human eye. We first need to understand the human vision to use that kind of vision sensor. Than we are able to convert RGB camera colors into an appropriate model for image processing.

  4. Physical Aspects of Colored Light

  5. Visible Light

  6. Light and surfaces Radiance = reflectance*radiance

  7. Physiological Characteristics of the Human Visual System

  8. Color vision Relative spectral sensivity curves of the three different types of cones Normalized spectral sensivity curves of the three different types of cones Colored light stimulates the cones and the rods cones: s, m, l-cones

  9. There are three types ofcones. short-wavelength (s) cones 420 nm middle-wavelength (m) cones 534 nm long-wavelength (l) cones 564 nm Rods are most sensitive at 498 nm and used for night vision.

  10. Tristimulus Color Spaces

  11. Monochromatic Color Response Every base color causes a certain activity on all three cones

  12. Tristimulus space / values We then define a given colour as or just

  13. CIE 1931 Standard RGB Color Space Because of the linear algebraic properties stated by Grassmann’s laws, it is possible to represent colour stimuli by vectors in a three-dimensional space, called the tristimulus space. CIE defined the CIE 1931 Standard RGB Colorimetric System with the monochromatic primaries:

  14. Metamerism There are several different spectra that can appear as the same color to the observer. Without metamerism there would be no color image reproduction on paper or screen Metamerism implies that two objects which appear to have exactly the same color, may have very different colors under different lighting conditions.

  15. Bayer Array A pattern of red, green, and blue filters on the camera image sensor. Interpolation of surrounding colors to get the right color at each pixel

  16. Brightness There are twice as many green filter elements in the array as red and blue elements because: the human eye is more sensitive to green light than both red and blue light. Therefore green colour accuracy is more important. The human eye is also more sensitive to changes in brightness than colour. Luminance = 0.35*R + 0.55*G + 0.1*B

  17. Polar Coordinate Color Models Polar Coordinate Color Spacesallows movements in color space which correspond more closely to human perception

  18. These spaces use a cylindrical (3D-polar) coordinate system to encode the following three psycho-visual coordinates: • Hue Wavelength of the pure colour observed in the signal. Normalized -> Rainbow color More the 400 hues can be seen by the human eye. • Saturation Purity, a pure colour has 100% saturation , the white and grey have 0% saturation. About 20 saturation levels are visible per hue. 3. Brightness Amount of light emitted. Distinguishes the greylevels. The human eye perceives about 100 levels.

  19. HSB / HSV Color Model • Transformation from RGB <> HSV by • Travis • Foley and Van Dam • Gonzalez und Woods Cone: Mathematically incorrect Cylinder: Mathematically correct

  20. HSL / HLS Color Model Hue , Saturation, Lightness (also Luminance or Luminosity) Cylinder: Mathematically correct Cone: Mathematically incorrect

  21. IHLS Color Model Improved Hue, Luminance and Saturation (IHLS) colour model

  22. Saturation weighted histogram Saturation weighted IHLShistogram Saturation weighted HSV histogram

  23. Advantages and Disadvantages of the IHLS Color Model Advantages Use of real luminance: Luminance = 0.35*R + 0.55*G + 0.1*B Saturation can be used as a continous increasing/decreasing factor on hue histograms to eleminate spikes (i.e. no jumps at black/white) Disadvantages expansive calculation (trigonometry) but with shader technology faster to computate

  24. Conclusion It is application dependent, which color model to use There are more interesting color models used in colorimetry which are worth to study

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