MAE152 Computer Graphics for Scientists and Engineers

1. MAE152Computer Graphics for Scientists and Engineers Colors In Computer Graphics

2. How much red was she with anger ?! • glColor3d (1.0, 0.78, 0.78); • May be! It’s YOUR perception. • Can color be quantified ? • Can a color be uniquely defined ? • Is there a “common understanding” about colors ?

3. Why “color management”

4. Source, Object, Observer

5. Reflected light = color of object

7. The Eye

8. Rods and Cones • Rods: sensitive to color intensity (black and white sensitivity in dark) • Cones: three types – S, M and L

9. From left to right, the curves above show the sensitivity of the S, M, and L cones to various wavelengths of visible light

10. CIE Color Matching Experiment Basis for industrial color standards and “pointwise” color models.

11. Color Matching Experiment Image courtesy Bill Freeman

12. CIE Experiment Result • Three pure light source: R = 700.0 nm, G = 546.1 nm, B = 435.8 nm.

13. Color Matching Experiment

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

15. The CIE 1931 Standard Observer represents the color perception of a "normal" person. The curves show the intensity of X, Y, and Z values (akin to cone response) for a given wavelength

16. Principle of Tri-chromaticity

17. Grassman’s Laws

18. Color Spaces • Use color matching functions to define a coordinate system for color. • Each color can be assigned a triple of coordinates with respect to some color space (e.g. RGB). • Devices (monitors, printers, projectors) and computers can communicate colors precisely.

19. 3D Tri-chromatic Space

20. CIE Chart A qualitative rendering of the CIE (x,y) space. The blobby region represents visible colors. There are sets of (x, y) coordinates that don’t represent real colors, because the primaries are not real lights (so that the color matching functions could be positive everywhere). Slide courtesy Forsyth and Ponce

21. A plot of the CIE (x,y) space: The spectral locus (the colors of monochromatic lights) and the black-body locus (the colors of heated black-bodies) is shown. The range of typical incandescent lighting is also plotted. Slide courtesy Forsyth and Ponce

22. Some Colour Gamuts

24. Undisplayable Colours • Suppose XYZ colour computed, but not displayable? • Terminology • Dominant wavelength • Saturation

26. A Maxwell Triangle, with white in the centre

27. Maxwell Triangle, showing where the spectral cyan matches

28. The spectral locus and the resulting RGB colour matching functions

29. The CIE tristimulus values and there relation to the spectral locus.

30. Colour might not be displayable • Falls outside of the triangle (its chromaticity not displayable on this device) • Might desaturate it, move it along line QW until inside gamut (so dominant wavelength invariant) • Colour with luminance outside of displayable range. • Clip vector through the origin to the RGB cube (chrominance invariant)

31. RGB Cube Mapped to XYZ Space

32. Market for Display Technologies

33. Cathode Ray Tube

34. Color Shadow Mask CRT

35. Color CRT Phosphor PatternVersus Spot Size

36. 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’.

37. 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

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

39. Combining Colors Additive (RGB) Shining colored lightson a white ball Subtractive (CMYK) Mixing paint colors andilluminating with white light

40. Additive and subtractive color system (r,g,b) RGB = (1,1,1) – (c,m,y)CMY

41. Maping (r,g,b) = (x,y,z) 2.739 -1.110 .138-1.145 2.029 -.333-.424 0.033 1.105

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

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

44. Color Depth = 1 is black and 0 is white

45. Color Index scheme

46. Color Index Example

47. True Color

48. True Color Example

49. The RGB Cube