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EE 7700

EE 7700. Color. References. On Color: Wikipedia, Gonzalez, Poynton, many others… On HDR: Slides and papers by Debevec, Ward, Pattaniak, Nayar, Durand, et al… http://people.csail.mit.edu/fredo/PUBLI/Siggraph2002/. Color. Color is a perceptual property.

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EE 7700

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  1. EE 7700 Color

  2. References • On Color: Wikipedia, Gonzalez, Poynton, many others… • On HDR: Slides and papers by Debevec, Ward, Pattaniak, Nayar, Durand, et al… • http://people.csail.mit.edu/fredo/PUBLI/Siggraph2002/

  3. Color • Color is a perceptual property. • It comes from the spectrum of light (energy distribution of light versus wavelength) interacting with the spectral sensitivities of the light receptors (photoreceptors) in the eye.

  4. Human Visual System • Human visual system is sensitive to a narrow range of the electromagnetic spectrum. (Approximately from 380nm to 740nm.)

  5. Human Visual System • The diameter of the eyeball is around 22mm. • Retina is a thin layer of neural cells that lines the back of the eyeball. • Retina contains photoreceptors (rods and cons) that respond to light. • Fovea is the most sensitive part of the retina; it is responsible for our sharp central vision. • Some birds (such as hawks) have more than one fovea. (two). • The axons (coming from receptors) exit the eye at the optic disc (blind spot), forming the optic nerve. • There are 1.2million axons in the optic nerve. • There are 130million photoreceptors.  A large amount of pre-processing is done within the retina. 10% of the axons are devoted to the fovea area.

  6. 0.5mm

  7. Human Visual System • There are two classes of receptors: cones and rods. • Cones: • Sensitive to color (there are three cone types in humans) • Produces high-resolution vision • 6-7 million cone receptors, located primarily in the central portion of the retina • Rods: • Not involved in color vision • 75-150 million rod receptors, distributed over retina • Sensitive to low levels of illumination. Not effective in bright light. • Produces lower-resolution vision

  8. Human Visual System • There are three types of cones in humans 65% sensitive to Long-wavelength 33% sensitive to Medium 2% sensitive to Small • A side note: • Humans and some monkeys have three types of cones (trichromatic vision); most other mammals have two types of cones (dichromatic vision). • Marine mammals have one type of cone. • Most birds and fish have four types. • Lacking one or more type of cones result in color blindness. Human lens and cornea are increasingly absorvative to smaller wavelengths, which sets wavelength sensitivity limit to around 380nm. Humans lacking lens reported to see ultraviolet.

  9. Human Visual System • Light is reduced to three color components by the eye. • These values are called tristimulus values. • The set of all possible tristimulus values determines the human color space. • It is estimated that humans can distinguish around 10million colors. The mechanisms of color vision within the retina are explained well in terms of tristimulus values. The way the values sent out of eye is little different: A dominant theory says that color is sent out of the eye in three opponent channels: a red-green channel, a blue-yellow channel and a black-white "luminance" channel. These channels are constructed from the tristimulus values.

  10. Human Visual System • Color constancy (Chromatic adaptation): The perceived color of objects remains relatively constant under varying illumination conditions. This helps us identify objects. • A red apple appears red in sunlight, at sunset, in florescent illumination, etc. Of course, this works only if the illumination contains a range of wavelengths. The HVS determines the approximate composition of the illuminating light, and then discounted to obtain the objects “true color” or reflectance.

  11. Human Visual System Which square is darker? A or B?

  12. Human Visual System

  13. Human Visual System

  14. A Color Blindness Test 5 3 5 2 56 8 26 6

  15. Human Visual System • Colors consisting of a single wavelength are called pure spectral or monochromatic colors. • Most light sources are mixtures of various wavelengths of light. If they produce a similar stimulus in the eye, a non-monochromatic light source can be perceived as a monochromatic light. • For a non-monochromatic light source, we may talk about the dominant wavelength (or color), which identifies the single wavelength of light that produces the most similar sensation. • Of course, there are many color perceptions that cannot be identified by pure spectral colors, such as pink, tan, magenta, achromatic colors (black, gray, white).

  16. Human Visual System • Two different light spectra that have the same effect on the three color receptors will be perceived as the same color. • Most human color perceptions can be generated by a mixture of three colors, called primaries. • This is used to reproduce color in photography, printing, TV, etc.

  17. CIE • In 1931, the Commission Internationale de l’Eclairage (CIE) established standards for color representation. Subjects were shown color patches and asked to match the color by adjusting three monochromatic colors. Based on the experiments, they defined the color-matching-functions:

  18. Tristimulus • Let X, Y, and Z be the tristimulus values. • A color can be specified by its trichromatic coefficients, defined as X ratio Y ratio Z ratio Two trichromatic coefficients are enough to specify a color. (x + y + z = 1)

  19. CIE Chromaticity Diagram Input light spectrum y x

  20. CIE Chromaticity Diagram Input light spectrum y x

  21. CIE Chromaticity Diagram Input light spectrum y 700nm Boundary 380nm x

  22. CIE Chromaticity Diagram Input light spectrum Boundary

  23. CIE Chromaticity Diagram Light composition

  24. CIE Chromaticity Diagram Light composition Light composition

  25. CIE Chromaticity Diagram • The CIE chromaticity diagram shows the human color space as a function of x and y. • Boundary indicates the pure spectrum colors. (Full saturation.) • Inside the boundary shows mixture of spectrum colors. Boundary

  26. CIE Chromaticity Diagram • The CIE chromaticity diagram is helpful to determine the range of colors that can be obtained from any given colors in the diagram. Gamut: The range of colors that can be produced by the given primaries. Source: http://hyperphysics.phy-astr.gsu.edu/hbase/vision/visioncon.html#c1 http://www.brucelindbloom.com/index.html?Eqn_ChromAdapt.html

  27. CIE Chromaticity Diagram R’G’B’: Gamma corrected values Green: Corresponding RGB with gamma 1.8 Orange: … with gamma 2.2 Green: ColorMatch primaries, D50 Orange: sRGB primaries, D65

  28. Mixtures of Light • The primary colors (primaries) can be added to produce the secondary colors of light. Example: Color TV displays use this additive nature of colors. An electron gun hits red, green, blue phosphors (with different energies) in a small region to produce different shades of color.

  29. Mixtures of Light • In printing, subtractive primaries are used: • Cyan absorbs only Red. • Magenta absorbs only Green. • Yellow absorbs only Blue. M Y C In printing, dark colors may be obtained by addition of black ink. Such color systems are known as CMYK systems.

  30. Color Space • A color space relates numbers to actual colors; it contains all realizable color combinations. • A color space could be device-dependent or device-independent. B An RGB color space has three components: Red, Green, and Blue. But, it does not specify the exact color unless Red, Green, and Blue are defined. R G The sRGB is a device-independent color space. It was created in 1996 by HP and Microsoft for use on monitors and printers. It is the most commonly used color space.

  31. Color Space The Adobe RGB is developed by Adobe in 1998. It was designed for printers; it has a wider gamut than sRGB.

  32. Color Space • HSV color space defines color in terms of Hue, Saturation, and Value. • Hue is the color type (such as, red, blue, yellow). (0-360 degrees) • Saturation is the purity of the color. (0-100%) • Value is the brightness of the color. (0-100%) • HSV is not device-independent. It is defined in terms of RGB intensities. • It is commonly used in computer graphics applications.

  33. Color Space • YUV color space defines color in terms of one luminance (brightness) and two chrominance (color) components. • YUV is created from RGB components. YCbCr YUV

  34. Color Space Profile Connection Space Output device Input device Color space conversion • International Color Consortium (ICC) was established in 1993 to create an open color management system. • The system involves three things: color profiles, color spaces, and color space conversion. • The color profile keeps track of what colors are produces for a particular device’s RGB or CMYK numbers, and maps these colors as a subset of the “profile connection space”.

  35. Color Space Profile Connection Space Output device Input device Color space conversion When there is gamut mismatch, There should be color rendering.

  36. for • otherwise CIELAB (CIE L*a*b*) • It was found that CIExyz is not a perceptually uniform color space: The minimum distance between two discernable colors differs in different parts of the CIExyz diagram. • Perceptually linear means that a change of the same amount in a color value should produce a change of about the same visual importance. When storing colors in limited precision values, this can improve the reproduction of tones. • L*a*b* color space was defined in 1976. Conversion from XYZ to L*a*b* is Xn, Yn and Zn are the CIE XYZ values of the reference white point.

  37. White Point • A white point is the reference point to define the color “white”. • Primaries plus the white point (indicating power ratio of primaries) should be given. • Depending on the application, different definitions of white are needed to get acceptable results. For example, photographs taken indoors may be lit by incandescent light, which are relatively orange compared to daylight. Defining “white” as daylight will give unacceptable results when attempting to color-correct a photograph. A list of common white points:

  38. star light moon light office light day light search light 10-6 10-2 101 102 104 108 High Dynamic Range (HDR) Imaging • The range of radiances is more than 10^12 candela/m2 100 Range of human eye at an instant is around 10^4:1 (4log units) Human eye can adapt to see much wider range. Candela is the unit of luminous intensity (power emitted by a light source in a particular direction, with wavelengths weighted by the sensitivity of the human eye.

  39. star light moon light office light day light search light 10-6 10-2 101 102 104 108 HDR • The range of radiances is more than 10^12 candela/m2 100 Range of Typical Displays: from ~1 to ~100 cd/m2 0 255

  40. Sensitivity of Eye Cone dominated Gain rod cone log Gain 1000 cd/m^2 -6 -2 6 0 -4 2 4 log La

  41. Gain rod cone log Gain -6 -2 6 0 -4 2 4 log La Sensitivity of Eye Rod dominated 0.04 cd/m^2

  42. Sensitivity of Eye

  43. HDR • The range of image capture devices is also low

  44. HDR • The range of image capture devices is also low

  45. HDR • HDR image rendered to be displayed on a LDR display.

  46. HDR Problems: • How to capture an HDR image with LDR cameras? • How to display an HDR image on LDR displays?

  47. Capture multiple images with varying exposure. • Combine them to produce an HDR image.

  48. Creating HDR from Multiple Pictures Measured intensity, z t1 t2 Irradiance, E t2 t1

  49. Creating HDR from Multiple Pictures Measured intensity, z t1 z1 t2 t2 t1 z2 E Irradiance, E z1 = t1 * E z2 = t2 * E Estimates: Take a weighted sum of E1 and E2: E1=z1/t1 E2=z2/t2 w2 w1 E=( w1*E1 + w2*E2 ) / (w1+w2) E

  50. Creating HDR from Multiple Pictures Measured intensity, z t1 z1 t2 t2 t1 z2 E Irradiance, E z1 = t1 * E z2 = t2 * E Estimates: Take a weighted sum of E1 and E2: E1=z1/t1 E2=z2/t2 w E=( w(z1)*E1 + w(z2)*E2 ) / (w(z1)+w(z2)) z

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