Color in Image and Video

1. Color in Image and Video By Razia Nisar Noorani

2. Contents • Physical and perceptual aspects of color • Several color models

3. The Physics of Color • The electromagnetic spectrum • The spectrum of visible light • Human response to color

4. The electromagnetic spectrum, of which visible light is a very thin band

5. The spectrum of visible light

6. The spectrum as seen in nature

7. Human response to color

8. Color Models A color model is an abstract mathematical model describing the way colors can be represented as tuples of numbers, typically as three or four values or color components. When this model is associated with a precise description of how the components are to be interpreted (viewing conditions, etc.), the resulting set of colors is called color space.

9. Color Models • An artist’s color wheel: red, yellow, and blue (RYB) • Additive color: red, green, blue (RGB) • Subtractive color: cyan, magenta, yellow, and black (CMYK) • Hue, saturation, and brightness (HSB)

10. The artist’s model: red, yellow, and blue Primary colors are sets of colors that can be combined to make a useful range of colors.

11. The secondary colors

12. The tertiary colors

13. Additive color: things that emit light, especially monitors (RGB) For additive combination of colors, as in overlapping projected lights or in CRT displays, the primary colors normally used are red, green, and blue.

14. In additive color . . . • Red + Green = Yellow • Red + Blue = Magenta • Green + Blue = Cyan

15. Subtractive color: things that reflect (and selectively absorb) light (CMYK) For subtractive combination of colors, as in mixing of pigments or dyes, such as in printing, the primaries normally used are cyan, magenta, and yellow

16. In subtractive color . . . • Cyan subtracts red • Magenta subtracts green • Yellow subtracts blue • In photography, that’s it: all three together subtract all light, giving black • In print, the dyes/inks aren’t that good, and we need black ink too • Hence, four-color printing: CMYK • K from blacK; B already means Blue

17. HSB: Hue, Saturation, and Brightness • Hue: where a color lies around a color wheel: red, green, yellow, blue-green, etc. • Saturation: the “purity” of a color; a fully-saturated color has no white mixed with it, in paint terms • Brightness: light, dark, or in between?

18. The color cone: hue, saturation, and brightness in relation to each other

19. HSB: Hue, Saturation, and Brightness • White, black, and all grays are zero percent saturated • A color becomes more saturated as it moves away from gray to a pure color • A pure (fully-saturated) color, in RGB terms, in one that contains: • Only red, green, or blue, or • Only yellow (= red + green), or • Only magenta (= red + blue) • Only cyan (= blue + green)

20. More on saturation, continued • Note: the previous slide said nothing about the brightness of those pure colors • A saturated color can be a brilliant yellow, but • It can be a deep crimson, or midnight blue

21. Varying saturation, with brightness held constant

22. Varying brightness, with saturation held constant

23. Color models in computer • RGB • HSL

24. Color models in computer

25. Color models in computer

26. Color models in computer • For Monitors • RGB • HSL • For Printing • CMYK

27. HP 364 Yellow Ink Cartridge HP 364 Magenta Ink Cartridge HP 364 Cyan Ink Cartridge HP 364 Photo Black Ink Cartridge

28. The HP 28 tri-color inkjet cartridge provides brilliant ....

29. Contrast • Contrast is the difference in visual properties that makes an object (or its representation in an image) distinguishable from other objects and the background. • Contrast is determined by the difference in the color and brightness of the object and other objects within the same field of view.

30. Contrast • Contrasting colors in terms of a computer's representation of an image, means the "primary colors" or the colors with color components of 0 or 255 (Min and Max).  • Black, White, Red, Green, Blue, Cyan, Magenta, and Yellow are the high contrast colors.  • When all the colors in an image are around one single color, that image has low contrast. 

31. Contrast

32. Color Models in Video

33. Color Models in Video • Largely derive from older analog methods of coding color for TV. Luminance is separated from color information. • For example, a matrix transform method called YIQ is used to transmit TV signals in North America and Japan. • This coding also makes its way into VHS video tape coding in these countries since video tape technologies also use YIQ. • In Europe, video tape uses the PAL or SECAM codings, which are based on TV that uses a matrix transform called YUV. • Finally, digital video mostly uses a matrix transform called YCbCr that is closely related to YUV

34. Gamma Correction • Gamma correction provides displaying an image accurately on a computer screen. • Images which are not properly corrected can look either bleached out, or too dark. • Trying to reproduce colors accurately also requires some knowledge of gamma. • Varying the amount of gamma correction changes not only the brightness, but also the ratios of red to green to blue. Sample Input Graph of Correction L' = L ^ (1/2.5) Gamma Corrected Input Monitor Output

35. YUV Color Model • YUV codes a luminance signal (for gamma-corrected signals) equal to Y’. • Chrominance refers to the difference between a color and a reference white at the same luminance. U = B’ − Y’; V=R’−Y’ R' , G' , B' : gamma correction applied

36. YIQ Color Model • YIQ is used in NTSC color TV broadcasting.

37. Y component I component Q component

38. From RGB to YIQ: Y = 0.299 R + 0.587 G + 0.114 B I = 0.5957161349127745 R − 0.2744528378392564 G − 0.3212632970735180 B Q = 0.211456402120117 R − 0.5225910452916111 G + 0.3111346431714933 B From YIQ to RGB: R = Y + 0.9562948323208939905 I + 0.6210251254447287141 Q G = Y − 0.2721214740839773195 I − 0.6473809535176157223 Q B = Y − 1.106989908567128216 I + 1.704614975498829329 Q

39. YCbCr Color Model • The Rec. 601 standard for digital video uses another color space, YCbCr.