Color and Graphics Displays

1. Color and Graphics Displays Jian Huang CS594

2. Physics • It’s all electromagnetic (EM) radiation • Different colors correspond to radiation of different wavelengths • Intensity of each wavelength specified by amplitude • Frequency = 2 pi/wavelength • We perceive EM radiation with in the 400-700 nm range, the tiny piece of spectrum between infra-red and ultraviolet

3. Visible Light

4. Color and Wavelength Most light we see is not just a single wavelength, but a combination of many wavelengths like below. This profile is often referred to as a spectrum, or spectral power distribution.

5. 3-Component Color • The de facto representation of color on screen display is RGB. (additive color) • Some printers use CMY(K), (subtractive color) • Why? • The color spectrum can be represented by 3 basis functions?

6. The Eye

7. Color is Human Sensation • Cone and rod receptors in the retina • Rod receptor is mostly for luminance perception • 3 different types of cone receptors in the fovea of retina, responsible for color representation. Each type is sensitive to different wavelengths

8. Cone Receptors • There are three types of cones, referred to as S, M, and L. They are roughly equivalent to blue, green, and red sensors, respectively. • Their peak sensitivities are located at approximately 430nm, 560nm, and 610nm for the "average" observer.

9. Limitation of Knowledge • We don’t know the precise light sensitivity on each person’s retina.

10. So, what is the standard color? • The basis of comparison is not math!! • The basis of comparison is human color matching experiments • 100% mathematically correct light object interaction need to be evaluated at more than 3 points in the spectrum

11. Main Color Spaces • CIE XYZ, xyY • RGB, CMYK • HSV (Munsell, HSL, IHS) • Lab, UVW, YUV, YCrCb, Luv,

12. Differences in Color Spaces • What is the use? For display, editing, computation, compression, …? • Several key (very often conflicting) features may be sought after: • Additive (RGB) or subtractive (CMYK) • Separation of luminance and chromaticity • Equal distance between colors are equally perceivable

13. CIE Standard • CIE: International Commission on Illumination (Comission Internationale de l’Eclairage). • Human perception based standard (1931), established with color matching experiment • Standard observer: a composite of a group of 15 to 20 people

14. CIE Experiment

15. CIE Experiment Result • Three pure light source: R = 700 nm, G = 546 nm, B = 436 nm.

16. CIE Color Space • 3 hypothetical light sources, X, Y, and Z, which yield positive matching curves • Y: roughly corresponds to luminous efficiency characteristic of human eye

17. CIE Color Space

18. CIE xyY Space • Irregular 3D volume shape is difficult to understand • Chromaticity diagram (the same color of the varying intensity, Y, should all end up at the same point)

19. Color Gamut • The range of color representation of a display device

20. RGB (monitors) • The de facto standard

21. The RGB Cube • RGB color space is perceptually non-linear • RGB space is a subset of the colors human can perceive • Con: what is ‘bloody red’ in RGB?

22. CMY(K): printing • Cyan, Magenta, Yellow (Black) – CMY(K) • A subtractive color model dye color absorbs reflects cyan red blue and green magenta green blue and red yellow blue red and green black all none

23. RGB and CMY • Converting between RGB and CMY

24. RGB and CMY

25. HSV • This color model is based on polar coordinates, not Cartesian coordinates. • HSV is a non-linearly transformed (skewed) version of RGB cube • Hue: quantity that distinguishes color family, say red from yellow, green from blue • Saturation (Chroma): color intensity (strong to weak). Intensity of distinctive hue, or degree of color sensation from that of white or grey • Value (luminance): light color or dark color

26. HSV Hexcone • Intuitive interface to color

27. Lab: photoshop • Photoshop uses this model to get more control over color • It’s named CIE Lab model (refined from the original CIE model • Liminance: L • Chrominance: a – ranges from green to red and b ranges from blue to yellow

28. Luv and UVW • A color model for which, a unit change in luminance and chrominance are uniformly perceptible U = 13 W* (u - uo ); V = 13 W* (v - vo); W = 25 ( 100 Y ) 1/3 - 17 where Y , u and v can be calculated from : X = O.607 Rn + 0.174 Gn + 0.200Bn Y = 0.299 Rn + 0.587 Gn + 0.114Bn Z = 0.066 Gn + 1.116 Bn x = X / ( X + Y + Z ) y = Y / ( X + Y + Z ) z = Z / ( X + Y + Z ) u = 4x / ( -2x + 12y + 3 ) v = 6y / ( -2x + 12y + 3 ) • Luv is derived from UVW and Lab, with all components guaranteed to be positive

29. Yuv and YCrCb: digital video • Initially, for PAL analog video, it is now also used in CCIR 601 standard for digital video • Y (luminance) is the CIE Y primary. Y = 0.299R + 0.587G + 0.114B • Chrominance is defined as the difference between a color and a reference white at the same luminance. It can be represented by U and V -- the color differences. U = B – Y; V = R - Y • YCrCb is a scaled and shifted version of YUV and used in JPEG and MPEG (all components are positive) Cb = (B - Y) / 1.772 + 0.5; Cr = (R - Y) / 1.402 + 0.5

30. Examples (RGB, HSV, Luv)

31. Color Matching on Monitors • Use CIE XYZ space as the standard • Use a simple linear conversion • Color matching on printer is more difficult, approximation is needed (CMYK)

32. Gamut Mapping • Negative RGB: add white (maintains hue, de-saturate) • >1 RGB, scale down (in what space?) • Not a trivial question (sometimes known as tone mapping)

33. Tone mapping • Real scene: large range of luminance (from 10 -6 to 10 6 cd/m2 ) • Limitation of the display1-100 cd/m2 • cd : candela, unit for measuring intensity of flux of light

34. Gamma Correction • The phosphor dots are not a linear system (voltage vs. intensity)

35. Gamma correction • Without gamma correction, how will (0,255,127) look like? • Normally gamma is within 1.7 and 2.8 • Who is responsible for Gamma correction? • SGI does it for you • PC/Mac etc, you should do it yourself

36. No gamma correction

37. Gamma corrected to 1.7

38. Residual Gamma or System Gamma • Systems such as SGI monitor has a gamma of 2.4, but they only gamma correct for 1.7. • The residue gamma is 2.4/1.7 = 1.4, why? • Depends on how you see it? Bright screen, dark room causes changes in your eye transfer function too. • What about web pages? Which screen do you intend for?

39. CRT Display • Cathode Ray Tubes (CRTs) • Most common display device • Evacuated glass bottle • Electrons attracted to focusing anode cylinder • Vertical and Horizontal deflection plates • Beam strikes phosphor coating on front of tube

40. Vector Display • Oscilloscopes were some of the 1st computer displays, used by both analog and digital computers • Computation results used to drive the vertical and horizontal axis (x,y), intensity could also be controlled (z) • Used mostly for line drawings, called vector, calligraphic display • Display list had to be constantly updated

41. Raster Display • TV boom made it cheap • Entire screen painted 30 times/ sec • Screen is traversed 60 times/ sec • Even/ Odd lines on alternate scans, ‘interlace’.

42. Color CRT • Requires precision geometry • Patterned phosphors on CRT face • Aligned metal shadow mask • Three electron guns • Less bright than monochrome CRTs

43. Pro/Con for Raster CRT Display • Disadvantages • Requires screen- sized memory array (frame buffer) • Discrete spatial sampling (pixels) • Moire patterns: when shadow- mask and dot- pitch frequencies mismatch • Convergence (varying angles of approach distance of e-beam across CRT face) • Limit on practical size (< 40 inches) • Spurious X- ray radiation • Occupies a large volume • Advantages • Allows solids to be displayed • Leverages low- cost CRT H/W • Whole Screen is constantly updated

44. LCD Displays • Liquid Crystal Display • Organic molecules that remain in crystalline structure without external force, but re-aligns themselves like liquid under external force • So LCDs realigns themselves to EM field and changes their own polarizations

45. Passive LCD • LCD slowly transit between states. • In scanned displays, with a large number of pixels, the percentage of the time that LCDs are excited is very small. • Crystals spend most of their time in intermediate states, being neither "On" or "Off". • These displays are not very sharp and are prone to ghosting.

46. Active Matrix LCD • E field is retained by a capacitor so that the crystal remains in a constant state. • Transistor switches are used to transfer charge into the capacitors during scanning. • The capacitors can hold the charge for significantly longer than the refresh period • Crisp display with no shadows. • More expensive to produce.

47. Plasma Display • Basically fluorescent tubes • High- voltage discharge excites gas mixture (He, Xe), upon relaxation UV light is emitted, UV light excites phosphors • Large view angle • Large format display • Less efficient than CRT, more power • Large pixels: 1mm (0.2 mm for CRT) • Phosphors depletion

48. Raster Displays • Display synchronized with CRT sweep • Special memory for screen update • Pixels are the discrete elements displayed • Generally, updates are visible

49. Double Buffer • Adds a second frame buffer • Swaps during vertical blanking • Updates are invisible • Costly

50. Memory Rasterizer • Maintains a copy of the screen (or some part of it) in memory • Relies on a fast copy • Updates are nearly invisible