Light and Color

1. Light and Color

2. Topics • The Human Visual System • Displaying Intensity and Luminance • Display Using Fixed Intensities • Understanding Color • Display of Color • Color Models

3. Structure of the Human Eye Image taken from http://hyperphysics.phy-astr.gsu.edu/hbase/vision/eye.html

4. Main Parts of the Eye • Cornea

5. Main Parts of the Eye • Cornea • Provides most refraction

6. Main Parts of the Eye • Cornea • Iris

7. Main Parts of the Eye • Cornea • Iris • Opens and Closes to let in more/less light

8. Main Parts of the Eye • Cornea • Iris • Opens and Closes to let in more/less light • Hole is the pupil

9. Main Parts of the Eye • Cornea • Iris • Lens

10. Main Parts of the Eye • Cornea • Iris • Lens • Flexible - muscles adjust shape

11. Main Parts of the Eye • Cornea • Iris • Lens • Flexible - muscles adjust shape • Allows fine-detail focus

12. Main Parts of the Eye • Cornea • Iris • Lens • Retina

13. Main Parts of the Eye • Cornea • Iris • Lens • Retina • Layer of receptor cells at back of eye

14. Main Parts of the Eye • Cornea • Iris • Lens • Retina • Layer of receptor cells at back of eye • Center of focus is the fovea

15. Main Parts of the Eye • Cornea • Iris • Lens • Retina • Layer of receptor cells at back of eye • Center of focus is the fovea • Optic nerve collects information from retina, and brings to the back of the brain • Causes a blind spot! • Cephalopods (octopus) do not have blind spots

16. Focusing Light • For a point in focal plane, all light emitting from that point goes to same point on retina

17. Focusing Light • For a point not in focal plane, light gets spread across retina

18. Display Screens and Focus • For the usual (current) display systems, we maintain one focus plane • Even for stereo displays • Even if the image tries to simulate differing focus • This is different than nature…

19. Rods • Distinguish brightness only • Best response to blue-green light • Prevalent except at fovea (more peripheral) • About 100x more sensitive than cones • About 100 million in retina

20. Cones • Color response • Centered around fovea • About 147,000 cones/mm2 at fovea • 2mm away from fovea: 9,500 cones/mm2 • 6.3 - 6.8 million in retina

21. How We See • Individual receptors give response • Vision is limited by density of cells in part of brain • High detail, good color at center of vision • Peripheral vision can see dimmer light, but mainly black & white, and low resolution

22. Vision in the Brain • Signals are carried by optic nerve to brain • Brain processes to reconstruct image • Best understood part of brain function, but still many ill-understood parts • Many aspects of vision are “hard-wired” • Can lead to optical effects with significant graphics impact • Mach banding. • Filling in of “blind spot”

23. Topics • The Human Visual System • Displaying Intensity and Luminance • Display Using Fixed Intensities • Understanding Color • Display of Color • Color Models

24. Intensity and Luminance • Intensity/Luminance: How much light energy there is • The amount of energy carried by photons

25. Intensity and Luminance • Intensity/Luminance: How much light energy there is • The amount of energy carried by photons • Brightness: the perceived intensity • Eye does not respond to equal intensity changes equally • Eye notices the ratio of intensities

26. Brightness Levels • Intensity changes of 1->2, 2->4, 4->8 appear the same • Example: 3-way lightbulb • 50->100 seems like bigger change than 100 -> 150 • Display devices might limit the number of discrete intensity levels available

27. Brightness in Display Devices • Assume: • The maximum intensity of a display is 1.0 • The minimum is I0 • It can display n+1 intensity levels • Then, to get equal brightness increments, it should display levels: • I0, rI0, r2I0, …, rnI0=1.0

28. Brightness Levels • Human eye generally can notice r>1.01 • So, to have a “smooth” display, we need to make sure that r<1.01 • i.e. we need enough “levels” of display • For a given display, we need 1.01nI0=1.0 1.01n = 1.0/I0 n log 1.01 = - log I0 n = -log I0 / log 1.01 n = -log1.01I0

29. Gamma Correction • Designed to compensate for how humans perceive intensity of light • Iout = k Iing • Iin, Iout = intensity • k, g are device-specific terms • Typically, g = 2.0 to 2.5 (most displays use 2.2) • Combining colors/intensity is not linear! Must convert to linear space, combine, convert back

30. Original 1080p

31. YouTube 360p

32. Gamma Corrected 360p

33. Original 1080p

34. Gamma and non-CRT Displays • Gamma is meant to model CRTs only • Other displays (e.g. LCD) may have very different response curves between intensity and value sent • They do not match the gamma curve • Often manufacturers adjust responses to try to mimic CRT behavior • Rarely is any device’s response curve exactly what is desired/modeled • This can get much more complicated • Major effort just to compensate for gamma

35. Dynamic Range • 1/I0 is called dynamic range • The ratio of maximum to minimum intensity (remember, we set 1.0 = max) • Varies by display device • This is the maximum a device can possibly display vs. the minimum it can display.

36. Dynamic Range • 1/I0 is called dynamic range • The ratio of maximum to minimum intensity (remember, we set 1.0 = max) • Varies by display device • This is the maximum a device can possibly display vs. the minimum it can display. • Contrast: the maximum vs. minimum it can display at the same time. • i.e. for one image on screen, maximum vs. minimum.

37. Topics • The Human Visual System • Displaying Intensity and Luminance • Display Using Fixed Intensities • Understanding Color • Display of Color • Color Models

38. Limited Display Levels • We can’t always get a continuous range of display levels • Printing: either ink is there or not there • We can adjust amount of ink (i.e. how much space it takes), but not its intensity • Other display might limit the number of levels – e.g. 256 levels of intensity. • We need ways to mimic continuous colors with discrete levels

39. Example

40. Example

41. Halftoning/Dithering • Idea: Eyes integrate over an area • All light hitting one receptor cell is combined. • Eye only cares about the integrated information from an entire area • So, we can get varying intensity by filling in fractions of areas vs.

42. Halftoning • Subdivide image into blocks of pixels. • You will lose resolution! • e.g. a 100x100 region divided into 4x4 blocks of pixels can only display a 25x25 image. • The number of intensity levels is related to the number of pixels in a block • 2x2 = 5 intensity levels • 3x3 = 10 intensity levels • nxn = n2+1 intensity levels

43. Example: 2x2 block • Level 0: Intensities 0.0 – 0.2 • Level 1: Intensities 0.2 – 0.4 • Level 2: Intensities 0.4 – 0.6 • Level 3: Intensities 0.6 – 0.8 • Level 4: Intensities 0.8 – 1.0

44. Example Image

45. Example Image

46. Example Image Average Intensities over Blocks Halftone Image

47. 2x2 Halftone Example

48. 2x2 Halftone Example

49. 3x3 Halftone Example

50. Halftoning Patterns • The pattern you use to fill makes a difference. • Want a “random” pattern, so that artificial artifacts don’t appear • Brain is very good at recognizing some things, like lines