Color Features

1. Color Features

2. Color Features Color Histogram Color Spaces RGB color model XYZ color model HSV color model LAB color model Color Perception Color match

3. Color Fundamentals Colors: different wavelengths of light Color Spectrum: In 1666, Sir Isaac Newton discovered that when a beam of sunlight passes trough a glass prism, the emerging beam of light is a continuous spectrum of colors ranging from violet to red.

4. Color Fundamentals (Cont'd) If the light is achromatic (void of color or grayscale), its only attribute is its intensity or amount. Chromatic light spans the electromagnetic spectrum from approximately 400 nm to 700 nm.

5. Color Histogram Distribution of colors in an image

6. Color Histogram A common practice is to use the hue, saturation, value (HSV) color space because it separates out the hue channel from the saturation and brightness channels, and the hue channel is relatively reliable to identify different objects. The color histogram of an object is a vector with U bins/elements. Assume that there is a hue-to-bin mapping b(q) that maps a hue value q to its corresponding bin index. Hence there is a hue value associated with each bin and hu represents the number of pixels on the object with similar hue values to that of the u-th bin.

7. Color Histogram In practice, hu is found as follows according to the hue-to-bin mapping b(q) where?? is the Kronecker delta function and n = 1 ,�, N is the pixel index over the image region of the object. The histogram representation of color distribution is computationally efficient.

8. Color Spaces: CIE-RGB An additive color system: starts from darkness and mixes Red, Green, and Blue to produce other colors. The RGB color space is a linear color space that formally uses single wavelength primaries

9. Color Spaces: CIE-RGB (Cont'd) RGB Cube

10. RGB vs. CMY

11. Color Spaces: CMY Cyan (blue+green), Magenta (blue+red), and Yellow (red+green) are the primary colors of pigments. Most devices that deposit colored pigment on paper, such as color printer and copiers, require CMY data. A subtractive color system: starts with white and subtracts (absorbs) to produce basis colors Cyan (white-red), Magenta (white-green), and Yellow (white-blue) Combine basis colors to produce other colors.

12. Color Spaces: CIE-XYZ Commission International d�Eclairage 1931 CIE-XYZ is a popular standard in which the color matching functions were chosen to be everywhere positive. However, the primary sources are physically unrealizable, since for some wavelengths the value of their spectral radiance is negative. CIE-XYZ is a convenient coordinate system for colorimetric calculation.

13. Color Spaces: CIE-XYZ luminance component Y: When an SPD is integrated using this curve as a weighting function, the result is CIE luminance, denoted� Y. The luminous efficiency of the Standard Observer is defined numerically, is everywhere positive, and peaks at about 555� nm. two additional components X and Z. The spectral weighting curves of X and Z have been standardized by the CIE based on statistics from experiments involving human observers.

15. Color Spaces: CIE-XYZ

18. Why use black ink in a color printer? Printing black by overlaying cyan, yellow and magenta ink in offset printing has three major problems. First, colored ink is expensive. Replacing colored ink by black ink - which is primarily carbon - makes economic sense. Second, printing three ink layers causes the printed paper to become quite wet. If three inks can be replaced by one, the ink will dry more quickly, the press can be run faster, and the job will be less expensive. Third, if black is printed by combining three inks, and mechanical tolerances cause the three inks to be printed slightly out of register, then black edges will suffer colored tinges. Vision is most demanding of spatial detail in black and white areas. Printing black with a single ink minimizes the visibility of registration errors.

19. CIE RGB v.s. CIE-XYZ

20. Nonlinear Color Spaces Color coordinates in a linear space may not necessarily encode properties that are common in language or important in applications. For example, useful color terms include Hue: the color attribute that describes a pure color (the property of a color that varies in passing from red, yellow, green, cyan, blue, magenta, red). It is the attribute of a visual sensation according to which an area appears to be similar to one of the perceived colors, red, yellow, green and blue, or a combination of two of them. If the dominant wavelength of an SPD shifts, the hue of the associated color will shift Saturation: the colorfulness of an area judged in proportion to its brightness, i.e. purity of color, percentage of color. Saturation runs from neutral gray through pastel to saturated colors. Roughly speaking, the more an SPD is concentrated at one wavelength, the more saturated will be the associated color. You can desaturate a color by adding light that contains power at all wavelengths. Value (lightness or brightness): the property that varies in pass from black to white.

21. Nonlinear Color Spaces: HSV Given an image in RGB color format, the HSV coordinates can be obtained as follows:

22. HSV Hexcone

23. Nonlinear Color Spaces: HSV A nonlinear transformation can convert RGB to HSV.

24. Nonlinear Color Spaces: HSV (Example: Hue)

25. Nonlinear Color Spaces: HSV (Example: Saturation)

26. Nonlinear Color Spaces: HSV (Example: Value)

27. Uniform Color Spaces It is important to know whether a color difference would be noticeable to a human viewer. One can determine just noticeable differences by modifying a color shown to an observer until they can only just tell it has been changed. These difference are plotted on a color space, and form the boundary of a region of colors that are indistinguishable from the original color. Usually, ellipses are fitted to the just noticeable differences.

28. Uniform Color Spaces: MacAdam Ellipses

29. Uniform Color Spaces: UV Color Coordinate Therefore, we can see that the size of a difference in (x,y) coordinates is a poor indicator of the significance of a difference in color (why?). A uniform color space is one in which the distance in coordinate space is a fair guide to the significance of the difference between two colors. A more uniform space (U,V) can be obtained from XYZ space.

30. Uniform Color Spaces: CIE-LAB CIE-LAB is almost universally the most popular uniform color space. Coordinates of a color in LAB are obtained as a nonlinear mapping of the XYZ coordinates.

31. The Principle of Trichromacy Experimental facts: Three primaries will work for most people if we allow subtractive matching Exceptional people can match with two or only one primary. This could be caused by a variety of deficiencies. Most people make the same matches.

32. Colors Colors: perception of different spectral power distribution (SPD) by human visual system. Spectral colors can be one-to-one correlated with light wavelength. The perception of light with multiple wavelengths is more complicated. It is found that many different combinations of light wavelengths can produce the same perception of color. Two colors look the same when their SPD difference falls into the null space of the cone responses.

33. Color Matching: Grassman's Laws (1) If we mix two test lights, then mixing the matches will match the result. If Then

34. Color Matching: Grassman's Laws (2) If two test lights can be matched with the same set of weights, then they will match each other. If Then

35. Color Matching: Grassman's Laws (3) Color matching is (approximately) linear If Then for non-negative k.

36. Grassman�s Laws Colour matching is (approximately) linear symmetry: U=V <=>V=U transitivity: U=V and V=W => U=W proportionality: U=V <=> tU=tV additivity: if any two (or more) of the statements U=V, W=X, (U+W)=(V+X) are true, then so is the third These statements are as true as any biological law. They mean that color matching under these conditions is linear.

38. Color Perception There are three different types of cones in the HVS retina with spectral absorption,